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Article

The Petrogenesis and Geological Implications of the Sanggeda Gabbros, Southern Tibet: Insights from the Amphibole Crystal Population

by
Xiumin Jiang
,
Zhaohua Luo
* and
Xiao Liu
School of Earth Sciences and Resources, China University of Geosciences, Xueyuan Road 29, Haidian District, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(18), 8435; https://doi.org/10.3390/app14188435
Submission received: 14 August 2024 / Revised: 13 September 2024 / Accepted: 13 September 2024 / Published: 19 September 2024
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Abstract

:
Amphibole is an important mineral during the differentiation of arc magmas but rarely as a phenocryst in arc lavas or eruptive pyroclastic rocks. The Sanggeda complex, intruded into the ophiolite of the Indus–Yarlung Zangbo Suture Zone (IYZSZ), Zedong, southern Tibet, mainly consists of amphibole-rich, fine-grained, and porphyritic gabbros. The complex provides an opportunity to study the differentiation of arc magmas through amphibole crystals. Four distinct amphibole crystal populations can be recognized according to petrographic observations, EMPA, and LA–ICP–MS analysis. The first ones (Type 1) are fined-grained and euhedral, are crystallized during ascent, and are the product of the shallow emplacement of host magma. The second ones (Type 2) are euhedral, with slight negative Eu and Sr anomalies, and crystallize from an evolved magma that previously experienced plagioclase fractionation. Type 3 amphiboles have similar morphological characteristics to Type 2 but are without Eu and Sr anomalies. Type 4 crystals are shown as pseudomorphs, formed by the reaction–replacement between the clinopyroxene and melt. Type 1 crystals are autocrysts. Other amphiboles within host magma, whether presented as phenocrysts or cumulate nodules, are antecrysts. Based on the amphibole crystal population developed in the complex, in this study, a trans-crustal magma plumbing system is proposed, containing at least three magma reservoirs located at different crust depths: the shallow emplaced crust (~4.8 km), the mid-crust (~12.9 km), and the lower crust (~21.8–24.9 km). Early amphibole crystallization is an effective process to generate silicic residual melts. Gravity could help in that sense. Precursor amphibole and clinopyroxene can efficiently delaminate back into the mantle and promote the generation of silicic continental crust.

1. Introduction

The continental crust is characterized by an andesitic to dacitic bulk composition [1,2,3], but its origin is still a controversial issue ([4,5] and references therein). It is widely accepted that, at least since the Phanerozoic, the growth of the continental crust has occurred at convergent margins based on the obvious trace element similarity between the continental crust and arc magmas (enriched in Pb and depleted in Nb, Ta, and Ti). According to this, the differentiation of arc magmas should play an important role in the formation of the continental crust [3,6,7]. However, compared with the andesitic composition of the continental crust, the partial melting of the sub-arc mantle wedge produces mainly basaltic melts [2,8]. Therefore, deciphering the evolution of arc magmas is significant in understanding the formation of the continental crust.
Minerals record the petrogenesis and evolution of igneous rocks, such as phenocryst assemblages in lavas or minerals in plutons [9]. However, crystals in magma are rarely composed of a single crystal population but are composed of a combination of phenocrysts, xenocrysts, antecrysts, and microcrysts [10]. The information recorded by these crystals provides a window for timescales of magmatism, contamination, and magma mixing or recharging, degassing, and magma plumbing histories [11].
Amphibole crystallization plays a critical role during arc magma differentiation [12,13,14], responsible for the chemical diversity of igneous rocks in plutons. But amphiboles rarely present as a phenocryst phase in arc lavas and eruptive pyroclastic rocks due to their instability at low pressures [15,16], which is regarded as a cryptic fractional crystallization phase [12,13,17]. However, without the significant nucleation and growth of amphibole grains, this cryptic fractionation will produce evolved melts with La/Yb and Dy/Yb “amphibole sponge” signatures. Some minerals, such as amphiboles and garnets, rarely exist as phenocrysts in arc lavas and can also participate in controlling the differentiation trend of magma [18]. At the same time, crystal assemblage in volcanic rocks may not match the liquid line of descent of the magma in which phenocrysts are often not equilibrated with the matrix [19]. The stability condition of amphiboles in volcanic rocks sometimes is more rigorous compared with other common phenocrysts (e.g., olivine, pyroxene, etc.). Therefore, it is more common to find abundant amphiboles in arc root complexes, cumulates, and cognate xenoliths in arc lava. Natural samples for revealing the mechanism of amphibole differentiation in magmas have been reported in the Adamello Batholith, Central Alps, Italy [20,21]; the Chelan Complex, Washington Cascades [22]; and the Cuijiu Complex and Daggyai Co pluton, Eastern Gangdese Batholith, southern Tibet [23,24,25].
Most ultramafic–mafic arc complexes are considered cumulates formed through fractional crystallization [17,22,26]. Amphibole-rich cumulates are formed by nucleation, growth, and settling from parent liquids [12] or reactions between early crystallized phases (e.g., olivine and clinopyroxene) and interstitial melts in a closed system [19,27]. There are two genetic models for cumulates with textural and chemical heterogeneities. The first model argues that the amphibole cumulates are products of polybaric fractional crystallization of the same parent melt in a closed system; that is, the primitive calc-alkaline basaltic magma crystallizes the cumulus phases in the deep crust and then rises to the shallow crust along with continuous crystallization, resulting in the textural heterogeneity of intrusive rocks [23,28,29,30]. The other model proposes that the cumulates form through a rock–melt reaction in an open system. The precursory olivine and clinopyroxene react with the permeable melt unbalanced with later nonequilibrium liquid to form amphiboles [14,20,21,31].
The amphibole-bearing Sanggeda mafic intrusion is heterogeneous in texture, appearing as cumulate, fine-grained, and porphyritic textures in the same rock, with abundant unbalanced crystals. The intrusion was emplaced into the ophiolite of the Indus–Yarlung Zangbo Suture Zone (IYZSZ) during the Paleogene. At the same time, the Ganggese magmatic arc activity developed in an intense episode [32]. To determine the petrogenesis of the Sanggeda intrusion and explore the role of amphiboles during the evolution of magmas, in this study, detailed petrographic, mineralogical, and geochemical studies are carried out. Combining mineral compositions with textures, four different types of amphiboles can be distinguished. A trans-crustal magma plumbing system with multiple magma chambers can be revealed based on the intrusion of the Sanggeda gabbros through the amphibole crystal population.

2. Geological Setting and Field Relationships

The Tibetan Plateau consists of several northward accretionary blocks, from north to south: the Qaidam, Songpan–Ganze, Qiangtang, Lhasa, and Himalaya terranes [33]. The northern part of the Lhasa terrane is bounded by the Bangong–Nujiang Suture Zone (BNSZ), and the IYZSZ is to the south (Figure 1a). The Lhasa terrane is divided into northern, central, and southern parts from north to south by the Shiquan River–Nam Tso Mélange Zone (SNMZ) and the Luobadui–Milashan Fault (LMF) (Figure 1a) based on the different sedimentary sequences and basements [34].
The IYZSZ, extending approximately 2000 km from east to west, is the remnant of the Neo-Tethys oceanic crust and mantle, formed during the India–Eurasia collision. It represents the final suture zone between the Indian plate and Eurasia, marking the closure of the Neo-Tethys in the Paleogene [35,36,37]. The Lhasa terrane to the north is mainly composed of Paleozoic to Cretaceous marine sedimentary strata. Subsequently, it was intruded and transformed by large-scale calc-alkaline intrusive and volcanic rocks, known as the Gangdese arc.
Abundant zircon U–Pb geochronological data have shown that the age of the Gangdese magmatic belt ranges from Late Triassic (220 Ma) to late Miocene (10 Ma) [32,38], due to the northward subduction of the Paleo- to Neo-Tethys oceanic lithosphere [34,39,40,41].
The study area is located in western Zedong, southern Tibet, which is part of the eastern section of the IYZSZ. It is mainly composed of a suite of island arc rock assemblage (K2) and ophiolite. The ophiolite extends approximately 20 km from east to west, with Late Triassic (T3) flysch sedimentary facies to the south, representing the passive continental margin of the northern Indian plate (Figure 1b). The Sanggeda mafic intrusion consists of fine-grained hornblende gabbro and porphyritic hornblende gabbro, both containing centimeter-sized clots of amphiboles or hornblendite enclaves. The country rocks surrounding the gabbros are Middle Cretaceous volcanic rocks and Zedong ophiolite, while the main body of the intrusion remains unexposed (Figure 1c). Based on the outcrops, it is inferred that the intrusion extends several kilometers from Sanggeda Village in the south to the southern foothill of Jiasha in the north, and approximately 1 km from east to west (Figure 1b). The gabbros display a faulted contact with the Middle Cretaceous andesites (Figure 2a) but contain andesitic xenoliths and roof pendants, indicating an intrusive contact with the volcanic rocks, primarily andesite. Additionally, clear intrusive contact relationships with Zedong ophiolite can be observed (Figure 2b,g,h). The contact margin along the intrusion side exhibits a fine-grained texture, while the ophiolite side shows strong serpentinization and schistosity (Figure 2b,g). The contact boundary between the fine-grained hornblende gabbro and porphyritic hornblende gabbro is sinuous (Figure 2c). Hornblendite cumulate enclaves are observed in both fine-grained and porphyritic gabbros (Figure 2d–f).

3. Sampling Strategy, Analytical Methods, and Results

3.1. Sampling Strategy

The samples (eight amphibole grains and one clinopyroxene grain from two polished thin sections, D17-B1 and D17-B2) in this study are the same as the ones used for the whole-rock analysis in Jing’s master thesis [42], taken from two outcrops of the geological cross-section AA’. The samples used for the amphibole crystal population study in this text come from the southern outcrop (Figure 1b), including amphiboles in fine-grained hornblende gabbro, porphyritic hornblende gabbro, and hornblendite cumulate enclaves.

3.2. Analytical Methods

Polished thin sections were prepared in the grinding room at the China University of Geosciences (Beijing, China). First, a detailed petrographic study was carried out to identify the crystal population of different types. To investigate the compositional variations within crystal interiors, as well as discrepancies between grains, representative grains of different amphibole types were selected for an electron probe profile analysis. In situ trace element compositions were also analyzed.
The major element compositions of amphibole and clinopyroxene were determined by an electron probe microanalysis (EPMA) at the Mineral Laser Microprobe Analysis Laboratory, China University of Geosciences (Beijing, China), using an EPMA–1720 (beam current of 10 nA at 15 kV with a beam diameter of 5 μm) electron microprobe. Corrections for inter-elemental effects were made using a ZAF (Z: atomic number; A: absorption; F: fluorescence) procedure. Analytical uncertainties were generally <2%.
Trace element compositions of amphibole and clinopyroxene were determined by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) at the Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (Guangzhou, Chian). The LA–ICP–MS system consists of a GeoLas 2005 laser (Applied Spectra, West Sacramento, CA, USA) coupled to an Agilent 7500a ICP–MS (Agilent Technologies, Santa Clara, CA, USA). A detailed description of the method is provided in [43]. Trace element concentrations were calibrated using multiple reference materials (BCR–2G, BHVO–2G, and BIR–1G) as external standards, with Ca for internal standardization [44]. The off-line selection and integration of background and analyte signals, time-drift corrections, and quantitative calibrations were undertaken using the software ICPMSDataCal 11 [43,44]. The analytical precision for most elements was <3%.
An Excel 2022 spreadsheet was used to calculate the chemical formulas and crystallization conditions of amphiboles.

3.3. Petrology and Mineralogical Characteristics

The Sanggeda mafic intrusion is enriched in large ion lithophile elements (LILEs), such as Rb and Ba, and in high-field-strength elements (HFSEs) like Th, U, and Pb, while being relatively depleted in HFSEs such as Nb, Ta, and Zr [42], a typical characteristic of arc magmas. The fine-grained equigranular (50–100 μm in size) gabbro and porphyritic gabbro are relatively fresh. Contacts between them are either sharp or gradual, lacking chilled edges (Figure 2c).
The proportion of amphiboles in gabbro varies from 5% to 30%. The gabbro paragenesis consists of plagioclase, amphibole, and clinopyroxene. Most plagioclase has undergone significant saussuritization, leaving only pseudomorphs. Granoblastic hornblendite enclaves (Figure 2d), with grain sizes ranging from 1 mm to 5 mm and some up to 1 cm, are commonly observed in the intrusion. The interstitial material exhibits a cryptocrystalline or microcrystalline texture.
Here, we use the term “phenocryst” to describe the crystals dispersed in the matrix of the Sanggeda intrusion, without implying they are in equilibrium with the host magma (see Section 4.1). “Matrix” refers the fine-grained material in porphyritic hornblende gabbro.
Based on crystal habits, mineral characteristics, and mineral chemistry, the amphibole crystal populations in the intrusion are divided into four categories, referred to as amphibole Types 1–4 (see Table 1 for details). Especially for amphibole Type 2, the mineral chemistry (see the analysis result in the following section) is a key feature distinguishing it from other types. The suffixes P and C denote crystals present as phenocrysts and cumulates, respectively.
Some clinopyroxene grains in cumulate enclaves (Figure 3d) exhibit a cracked morphology. They are subhedral, with a total volume fraction of approximately 4%, and have sizes similar to amphibole in cumulate, about 1–2 mm.

3.4. Major Element Composition: EPMA Results

The analytical results of the major element composition are presented in Supplementary Data Tables S1 and S2.

3.4.1. Amphibole

Although new classifications and nomenclature schemes for amphibole minerals have been developed in recent years based on more accurate measurements [45,46,47], the nomenclature scheme for the calcic amphibole group has remained largely unchanged. Cations of amphibole were calculated based on a 23-oxygen-atom basis, following the procedure outlined in [45] (see Table S1 of the electronic supplement). Since the valence state of Fe has not been measured, the Fe3+/Σ Fe ratio was calculated by constraining the sum of specific cations to a particular value and assuming electroneutrality for the entire formula [47]. The various cation sums and constraints proposed for this calculation are discussed in detail in [45,47,48,49].
The amphiboles analyzed in this study are all calcic ((Ca + Na)B ≥ 1.0, typically, CaB ≥ 1.5). See Figure 4a,b for further classification. Amphiboles in the fine-grained hornblende gabbro (Type 1: D17-B1-02) are classified as magnesiohornblende, while others are categorized as magnesio-hastingsite (VIAl ≥ Fe3+) or pargasite (VIAl < Fe3+). The magnesium number (Mg# = 100 Mg/(Mg + Fe2+) varies between 59.39 and 72.15, and SiO2 contents range from 38.60 to 48.48 wt.% and Al2O3 from 7.57 to 15.24 wt.%, with TiO2 (0.55–2.98 wt.%), FeOT (9.86–14.70 wt.%), MgO (11.27–14.84 wt.%), CaO (10.84–13.83 wt.%), Na2O (0.73–2.48 wt.%), and K2O (0.33–1.61 wt.%) [45].
Figure 5 shows the compositional profiles of the amphiboles analyzed in this study. At the rims, amphiboles exhibit similar concentrations of Mg#, SiO2, and TiO2 (Figure 5). Some positive or negative peaks are caused by cleavage or fractures. The Type 2 amphibole shows a slight decrease in Mg#, SiO2, and TiO2, nearly constant Al2O3, and a slight increase in K2O toward the rim (Figure 5a). The Type 3C grain shows a progressively decreasing Mg# and K2O content from core to rim, while SiO2, TiO2, and Al2O3 contents remain uniform (Figure 5b). The Type 4 amphibole shows a lower Mg# in the core, which then increases toward the edge of the crystal (Figure 5c,d). The Type 4P amphibole shows a large variation in Mg# at the edge, consistent with the narrow zoning observed at the edge of the amphibole under the microscope.
Representative amphibole analyses were plotted in bivariate diagrams against Mg# (Figure 6). The Mg# of the Type 1 amphibole (approximately 65–68) is at an intermediate level, with higher SiO2 and lower Al2O3 and K2O contents compared to other types of amphiboles. The Type 2 amphibole has higher TiO2 and Na2O contents, lower Al2O3, CaO, and K2O contents, and similar SiO2 contents. The Type 3 and 4 amphiboles show a wide Mg# range (about 59–72) but relatively similar major element compositions.

3.4.2. Clinopyroxene

The selected clinopyroxene sample is classified as diopside (En41.7–42.8Wo50.5–51.6Fs6.4–7.8) (Figure 4c and Supplementary Table S2) according to the criteria of [43]. It contains SiO2 concentrations ranging from 50.25 to 51.68 wt.%, slightly elevated Mg# (80.87–83.69), MgO (13.92–14.71 wt.%), TiO2 (0.40–0.63 wt.%), and CaO (23.44–23.92 wt.%), with slightly lower FeOT (5.11–5.88 wt.%) and Cr2O3 (0.12–0.31 wt.%).

3.5. Trace Element Composition: LA–ICP–MS Result

Amphibole (Type 2 to Type 4) and clinopyroxene were analyzed using LA–ICP–MS for trace element concentrations. To ensure more accurate results, crystals with larger grain sizes were selected and, for those with obvious zoning, divided into core–(mantle)–rim regions. The detailed results are provided in Supplementary Tables S3 and S4.

3.5.1. Amphibole

The Rare Earth Element (REE) concentrations in the amphiboles are enriched by 5 to 80 times relative to the chondrite. The chondrite-normalized REE patterns are convex-upward (Figure 7a,c,e,g and Table S3 of the Supplementary Materials) except at the edges of Type 2 (Figure 7a) and Type 4P (Figure 7e) amphiboles. The MREEs are especially enriched compared to the light REEs (LREEs) and heavy REEs (HREEs). This trend weakens for more enriched La and Ce concentrations in Type 2 (Figure 7a) and Type 4C (Figure 7g). Slightly negative Eu anomalies (Eu/Eu* = 2EuN/[SmN + GdN] = 0.75–1.22, where the subscript N denotes normalization to the chondrite composition of [46]) may or may not be observed. Notably, the edges of the samples usually show slightly negative Eu anomalies, with the Type 2 amphibole (D17-B1-01) showing the largest anomaly (0.75) (Figure 7a).
The Type 4P amphibole (D17-B1-05) exhibits a more complex core–mantle–rim texture (Figure 7e). The core and mantle share similar chondrite-normalized REE patterns, but the core has higher total contents. Compared to the core and mantle, the edge is enriched in LREEs and shows intermediate concentrations of MREEs and HREEs (Figure 7, Type 4P, Figure 7e).
The samples show primitive mantle-normalized multi-element patterns that are enriched in Ba, Pb, and Sr but depleted in Th, U, Zr, and Hf (Figure 7b,d,f,h). Th and U contents are higher at the edge than the core for most samples. The Type 2 amphibole (D17-B1-01, Figure 7b) exhibits a distinctive negative Sr anomaly (Sr/Sr* = 2SrPM/[CePM + NdPM], where the subscript PM denotes primitive mantle normalization [46]), unlike other grains. Unlike samples with high Th and U concentrations at the edge, amphibole Type 3C (D17-B2-01, Figure 7d) shows consistency between the rim and core.

3.5.2. Clinopyroxene

The clinopyroxene analyzed in this study exhibits bell-shaped chondrite-normalized REE patterns (Figure 7i), similar to the amphiboles (Figure 7g and Supplementary Table S4). Compared to the core, the rim of the clinopyroxene has higher REE concentrations, but both share the same convex-upward chondrite-normalized REE patterns, with a slight negative Eu anomaly (0.83–1.02). Th and U concentrations are higher, while Ba, Nb, Ta, and Zr concentrations are lower (Figure 7j).

4. Discussion

4.1. Genesis of Amphibole Crystal Population

The whole-rock study in Jing’s master thesis [42] is provided in Supplementary Table S5. The Sanggeda intrusion primarily consists of fine-grained hornblende gabbro and porphyritic hornblende gabbro. They formed contemporaneously, as indicated by the gradual transition between the two lithologies and the absence of chilled edges. These lithologies exhibit similar REE patterns to the lower continental crust [3,42] (Figure 8). The shallow-level emplacement and rapid cooling of magma resulted in the fine-grained and porphyritic textures of the intrusion. Precursory crystal populations can be easily distinguished from those crystalized from the host magma.
Compared to single fractional crystallization or rock-melt reactions, the amphibole crystal population in the Sanggeda intrusion exhibits a more complex genesis and sourcing.

4.1.1. The Source of Amphibole Crystal Population: Autocryst and Antecryst

The amphibole crystal population plays a crucial role in the formation of the Sanggeda gabbros. The complex texture and composition of the amphiboles indicate that the intrusion is not the result of a single magma chamber undergoing continuous evolution. Equilibrium diagrams have been employed [52] to access the mineral-melt equilibrium between different amphibole types and the host magma. The Mg# of the amphiboles is plotted against the Mg# of the melt (calculated as molar ration: 100MgO/(MgO + FeO)). The curves, calculated based on the iron-magnesium exchange coefficient for amphibole ( Kd Fe – Mg Amp – melt = 0.28 ± 0.11) [53], represent the range where mineral and melt compositions are in equilibrium. All amphiboles plot below the curves (Figure 9), suggesting that they may not be in equilibrium with the melt.
The fine-grained amphibole (Type 1) is considered an autocryst, euhedral and homogeneous, primarily present in the matrix, and is the product of host magma crystallization. The disequilibrium of the Type 1 amphibole shown in the diagram may result from the incorporation of numerous mafic minerals (e.g., amphibole phenocrysts) during magma recharge, which increases the Mg# of the magma components [54]. Other amphiboles (Type 2 to Type 4) plotted below the curves show clear disequilibrium with the melt. Considering their mineral characteristics, habits, and mineral chemistry, the dissolution phases developed at the edge of some amphibole grains suggest they are “foreign” crystals. Due to the rapid ascent and cooling of the host magma in the shallower crust, they are well preserved. The similar Mg# (Figure 5, 66–68) at the rims and the higher Th and U concentrations compared to the interior (Figure 7), especially for amphiboles as phenocrysts, may relate to the influence of the host magma during ascent, while internal compositional discrepancies originate from the magma reservoirs where the crystals formed. Despite noticeable differences in total REE concentrations among the various amphibole types, they exhibit similar REE patterns, indicating potential genetic relationships. These amphiboles did not crystallize from the host magma but are recycled crystals from earlier-stage melts. Therefore, it is more reasonable to consider these crystals as antecrysts from the same magma system rather than captured materials from unrelated wall rocks.

4.1.2. Crystallization Conditions for Amphibole

The amphibole physicochemical crystallization conditions calculated using the method proposed by Ridolfi [55] yield reliable temperature and oxygen fugacity [56,57], though the barometry and water mass fraction are occasionally debated, as noted by [56]. However, despite these uncertainties, the H2O variation trend remains a useful reference. In the case of amphibole barometry, although plagioclase and hornblende are now observed together in the host magma, they may not have formed simultaneously. The absence of negative Eu anomalies in Type 3 and Type 4 amphiboles, along with the scarcity of plagioclase grains in the cumulate, suggests that plagioclase is likely a late-stage phase. Additionally, the high water content in the basaltic melt suppresses the crystallization of plagioclase [58]. Therefore, pressure calculations based on plagioclase and hornblende [57,59] may not be applicable here. Pressure is calculated using a single-phase empirical thermobarometer, the extended calibration of the Larocque and Canil [17] barometer, as published by Krawczynski [60] (Supplementary Table S1).
The results indicate that the amphiboles typically contain high water content in the melts (H2Omelt, 4.4–7.4 wt.%, Table 2). The crystallization conditions for the Type 1 amphibole are significantly different from other types (Figure 10). The crystallization temperature (T) of Type 1 ranges from 847 ± 22 °C to 881 ± 22 °C, the pressure (P) ranges from 93 MPa to 176 MPa, and the calculated continental depth is 3.5–6.7 km, with an average depth of 4.8 km, the shallowest among all amphibole types, consistent with the host magma’s shallow crustal intrusion. The oxygen fugacity (∆NNO) ranges from 1.0 to 1.3 log units above the nickel-nickel oxide (NNO) buffer, and H2Omelt values range from 5.1 to 5.6 wt.% (Table 2).
The Type 2 amphibole (D17-B1-01) shows higher T (919 ± 22 °C–1018 ± 22 °C) and P (139 MPa–514 MPa) compared to the Type 1 amphibole, but lower than the other types. The calculated crystallization depth ranges from 5.3 to 19.4 km, with an average of 12.9 km. The logfO2 ranges from –10.8 to –9.3 and the ∆NNO from +0.3 to +1.0, with H2Omelt values ranging from 4.4 to 5.9 wt.% (Table 2).
The wide range of temperatures (912–1051 °C, 954–1062 °C) and pressures (290–804 MPa, 123–872 MPa) (Figure 10, Table 2) for the Type 3 and Type 4 amphiboles indicates varying conditions during crystallization, which corresponds well with the crystal’s zoning texture. The calculated crystallization depth ranges from 4.6 to 33.0 km, with an average depth of 21.8–24.9 km.

4.1.3. The Crystallization Process of Amphibole Crystal Population

As noted earlier, the Type 1 amphibole is an autocryst with Mg# ranging from 65 to 68, formed directly from the host magma after intrusion into the shallow crust.
The Type 2 idiomorphic amphibole, with typical cleavages, is characterized by negative Eu and Sr anomalies (Figure 7), indicating prior plagioclase crystallization differentiation in the parent magma. It formed in a shallower reservoir (9.5–19.7 km) with lower P and T and higher fO2 (Figure 10) compared to the Type 3 and Type 4 amphiboles. This euhedral crystal is nearly homogeneous, with only narrow chemical zoning at the rim. The homogeneous interior formed during the early stage may record the initial stable crystallization conditions (Figure 5). The narrow zoning at the rim exhibits low REE contents and elevated Th and U contents, indicating overgrowth after capture by the host magma. The Type 3 amphibole is also euhedral, with typical cleavages. As the upper temperature limit for amphiboles is typically around 1050 °C [12], the average crystallization temperature of 1010–1030 °C suggests that this amphibole is in a liquidus phase, formed during the cooling of the parent melt. Before the amphibole formation, most of the olivine and clinopyroxene crystallized at higher temperatures and separated from the residual melt via fractional crystallization, with some retaining as free crystals. This process provided more space for amphibole crystallization. This may explain why amphiboles in the Sanggeda gabbros are mostly euhedral, unlike the interstitial forms found in other gabbros worldwide [19,23,61]. This is further supported by the coexistence of clinopyroxene and the Type 4 amphibole. The high Cr content (1199–1600 ppm, Table S4), Mg# (80.87–83.69, Table S2), and low trace element concentrations (Figure 7) of clinopyroxene suggest formation from a primitive parent melt. This also accounts for the high water content (4.9–7.4 wt.%) in the parental melt of the amphiboles. The crystallization of anhydrous minerals (olivine and clinopyroxene) causes relative enrichment of water in the melt, as mafic arc magmas typically contain around 4 wt.% water [62,63]. High water content in the basaltic melt suppresses the crystallization of plagioclase [63,64,65] and promotes the stability of clinopyroxene compared to orthopyroxene [66,67]. This aligns with the observation that plagioclase and orthopyroxene are rare in the cumulate and that amphiboles or clinopyroxene shows no significant negative anomaly of Eu. The Mg# in the Type 3C amphibole decreases gradually, indicating that the magma system stabilizes during crystallization without melt recharging. The high compositional consistency between the rim and core Type 3C amphibole may be due to limited influence from the host magma, as it is surrounded by cumulates.
The Type 4 amphibole appears as a clinopyroxene pseudomorph (Figure 3b,c), indicating a close relationship with clinopyroxene. Amphiboles formed by the reaction–replacement of clinopyroxene typically retain remnants of precursor clinopyroxene [19,23,24,61]. However, no relics of clinopyroxene are observed in the Type 4 amphibole. This may result from extensive and prolonged reactions between the precursor clinopyroxene and water-rich magma [19,68,69], consistent with the high water fugacity (5.1–7.4 wt.%). Additionally, most clinopyroxene, which separates from the magma via fractional crystallization at an early stage, may promote this reaction. The difference in Mg# between the Type 4P and Type 4C amphiboles suggests that their precursor clinopyroxene may originate from a heterogeneous magma chamber or different magma reservoirs. Amphibole Type 4P (D17-B1-05), with a complex core–mantle–rim texture, records evidence of corrosion and regrowth post-formation, consistent with its chemical characteristics. The Type 4C amphibole experienced less influence until it was incorporated into the host magma as a cumulate enclave (Figure 7). The accumulation of clinopyroxene and Type 3C and Type 4C amphiboles suggests a magma chamber at lower crust depth (9.1–28.0 km), where the Type 3 amphibole crystalized.
In conclusion, the amphibole crystal population, particularly zoned antecryst, crystallized from magma reservoirs at varying depths [11,28]. Fractional crystallization processes play an important role in the formation of amphibole Types 2–4. The fractionation of plagioclase accounts for the negative Eu and Sr anomaly characteristics of the Type 2 amphibole. Meanwhile, the fractional crystallization of anhydrous minerals promotes the formation of amphibole Types 3–4, though the water enrichment in the melt provides more growth space for idiomorphic amphiboles and precursor clinopyroxene for amphibole Type 4. The cumulate enclaves of hornblendite represent products of fractional crystallization in the magma chamber. The rim of the antecryst, like the REE composition, is slightly modified after being trapped by the host magma. However, the residence time appears to be relatively short, given the scarcity of antecrysts with significant resorption and recrystallization rims.

4.2. Petrogenesis of the Sanggeda Intrusion: A Trans-Crustal Magma Plumbing System

The amphibole crystal population in the Sanggeda gabbros suggests that the formation of the intrusion required a complex magma assembly process, gathering dispersed crystals, crystal mushes, and cumulates from magma reservoirs at varying depths. Amphiboles may undergo reabsorption while ascending through the lithosphere, particularly near the surface, as they move outside their stability field [70,71,72]. This partially explains why amphiboles are rare in arc lavas but abundant in cumulate xenoliths and arc plutons, a phenomenon linked to “cryptic amphibole fractionation” [12]. A mechanism must exist for the Sanggeda intrusion to transport such a large number of amphibole crystals to the shallow crust with minimal resorption. The trans-crustal magma plumbing system (Figure 11a) may be the appropriate model [10,71,72,73] to resolve these contradictions.
In the lower crustal magma chamber, olivine and pyroxene likely crystallize first from the primitive magma, with most accumulating and separating from the residual melt, leaving only a small fraction in the melt (Figure 11b). This fractional crystallization is beneficial for the formation of amphibole Types 3–4. Crystallization of anhydrous minerals progressively enriches water in the melt. This promotes the stability of clinopyroxene over orthopyroxene while suppressing plagioclase crystallization. The Type 3 amphibole will precipitate when the H2O-saturated magma encounters the amphibole stability curve [19], forming an idiomorphic morphology due to ample growth space, as mentioned earlier. The Type 4 amphibole forms via a rock-melt reaction, in which precursor clinopyroxene reacts with residual or injected melts. During this process, clinopyroxene is entirely consumed (Figure 5c,d).
The abundance of crystals increases magma viscosity, forming a crystal mush that hinders further magma ascent in the deep crust. Interstitial melt is extracted through compaction or buoyancy, while the cumulates delaminate back to the mantle (Figure 11b). This process contributes to the formation of SiO2-rich continental crust materials.
Finally, cumulate enclaves and antecrysts, such as amphiboles, clinopyroxenes, and plagioclases, are trapped in the ascending host magma [74]. Due to their high density and size, antecrysts may not always reach the surface. Depending on the availability of antecrysts in the magmatic system, antecryst-bearing or antecryst-free magmas are formed. The Type 1 amphibole crystalizes during ascent in the trans-crustal magma plumbing system, emplacing in the shallower crust (Figure 11c).

5. Conclusions

A comprehensive study of the amphibole crystal population in the Sanggeda intrusion, Zedong, southern Tibet, has led to the following conclusions:
(1)
The amphibole crystal population from the intrusion can be divided into four categories: the fined-grained euhedral amphiboles (Type 1) that crystallized from the host magma autocryst; euhedral amphiboles (Type 2) that crystallized from magma after plagioclase fractionation; euhedral amphiboles (Type 3) formed in a lower crust magma chamber; and amphiboles as clinopyroxene pseudomorphs, formed by reaction–replacement of clinopyroxene (Type 4). Amphibole Types 2–4 are classified as antecrysts.
(2)
The heterogeneity of the intrusion is attributed to the combination of polybaric fractional crystallization and rock–melt reactions. A trans-crustal magma plumbing system with multiple magma chambers is proposed, through which amphiboles and other minerals of varied origins were incorporated into the ascending magmas, forming the Sanggeda intrusion.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/app14188435/s1, Table S1: Major element compositions (wt.%) of amphiboles and physical–chemical conditions from the Sanggeda gabbros, South Tibet. Table S2: Major element compositions (wt.%) of clinopyroxene from the Sanggeda gabbros, South Tibet. Table S3: Trace element compositions (ppm) of amphiboles from the Sanggeda gabbros, South Tibet. Table S4: Trace element compositions (ppm) of clinopyroxene from the Sanggeda gabbros, South Tibet. Table S5: Major element (wt.%) and trace element (ppm) compositions of the Sanggeda gabbros, South Tibet.

Author Contributions

Conceptualization, X.J. and X.L.; data curation, X.J. and X.L.; formal analysis, X.J.; funding acquisition, Z.L.; investigation, X.J. and X.L.; methodology, X.J. and X.L.; project administration, Z.L.; supervision, Z.L.; validation, X.J. and X.L.; visualization, X.J. and X.L.; writing—original draft, X.J.; writing—review and editing, X.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research is funded by the China Geological Survey project (121201102000150069-01).

Institutional Review Board Statement

Not appliable.

Informed Consent Statement

Not appliable.

Data Availability Statement

Data are contained within the article or in the Supplementary Materials.

Acknowledgments

We thank Wenchao Jing and Zongchang Wu for assistance with sample collection, Xuejun Li and Jinhua Hao for help with the EPMA and LA–ICPMS analyses, Jie Li for his assistance in the data computation, Xiaowei Li for English language improvement, and Guochen Dong for valuable suggestions in modifying this manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Applsci 14 08435 g001
Figure 1. (a) Simplified tectonic map of the Tibetan Plateau (modified from [34]), showing the location of the Lhasa terrane and the study area. (b) Detailed geological map of the Zedong ophiolite at the eastern of the IYZSZ (modified from [35]). (c) Geological cross-section AA’ shows the contact relationships between the Sanggeda gabbros and the Middle Cretaceous volcanic rocks. Abbreviations are as followings: JSSZ = Jinshajiang Suture Zone, BNSZ = Bangong–Nujiang Suture Zone, SNMZ = Shiquan River–Nam Tso Mélange Zone, LMF = Luobadui–Milashan Fault, IYZSZ = Indus–Yarlung Zangbo Suture Zone, SL = southern Lhasa subterrane, CL = central Lhasa subterrane, and NL = northern Lhasa subterrane.
Figure 1. (a) Simplified tectonic map of the Tibetan Plateau (modified from [34]), showing the location of the Lhasa terrane and the study area. (b) Detailed geological map of the Zedong ophiolite at the eastern of the IYZSZ (modified from [35]). (c) Geological cross-section AA’ shows the contact relationships between the Sanggeda gabbros and the Middle Cretaceous volcanic rocks. Abbreviations are as followings: JSSZ = Jinshajiang Suture Zone, BNSZ = Bangong–Nujiang Suture Zone, SNMZ = Shiquan River–Nam Tso Mélange Zone, LMF = Luobadui–Milashan Fault, IYZSZ = Indus–Yarlung Zangbo Suture Zone, SL = southern Lhasa subterrane, CL = central Lhasa subterrane, and NL = northern Lhasa subterrane.
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Figure 2. Field photographs of the Sanggeda rocks. (a) The intrusion is in fault contact with the andesite. (b) Gabbro intruded into the ophiolite, causing significant serpentinization and schistosity in the latter. (c) The contact boundary between fine-grained hornblende gabbro and porphyritic hornblende gabbro. (d) Hornblendite cumulate enclave within the fine-grained hornblende gabbro. (e) Fine-grained hornblende gabbro. (f) Porphyritic hornblende gabbro. (g) Detailed view of the contact boundary in (b). (h) Gabbro vein intruding into the ultramafic rock after the felsic rock. Dashed lines indicate the lithologic boundary. The diameter of the coin is 20.5 mm.
Figure 2. Field photographs of the Sanggeda rocks. (a) The intrusion is in fault contact with the andesite. (b) Gabbro intruded into the ophiolite, causing significant serpentinization and schistosity in the latter. (c) The contact boundary between fine-grained hornblende gabbro and porphyritic hornblende gabbro. (d) Hornblendite cumulate enclave within the fine-grained hornblende gabbro. (e) Fine-grained hornblende gabbro. (f) Porphyritic hornblende gabbro. (g) Detailed view of the contact boundary in (b). (h) Gabbro vein intruding into the ultramafic rock after the felsic rock. Dashed lines indicate the lithologic boundary. The diameter of the coin is 20.5 mm.
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Figure 3. Photomicrographs of the Sanggeda mafic intrusion and various types of amphiboles. (a) Fine-grained hornblende gabbro, showing amphibole of Type 1. (b) Porphyritic hornblende gabbro, showing amphibole of Type 2, Type 3P, and Type 4P. (c) Hornblendite, showing amphibole of Type 3C and Type 4C. (d) Clinopyroxene in cumulate. (e) Amphibole Type 3P. (f) Amphibole Type 4P. Under plane-polarized transmitted light. Amp = amphibole, Cpx = clinopyroxene. The suffixes P and C denote crystals present as phenocrysts and cumulates, respectively.
Figure 3. Photomicrographs of the Sanggeda mafic intrusion and various types of amphiboles. (a) Fine-grained hornblende gabbro, showing amphibole of Type 1. (b) Porphyritic hornblende gabbro, showing amphibole of Type 2, Type 3P, and Type 4P. (c) Hornblendite, showing amphibole of Type 3C and Type 4C. (d) Clinopyroxene in cumulate. (e) Amphibole Type 3P. (f) Amphibole Type 4P. Under plane-polarized transmitted light. Amp = amphibole, Cpx = clinopyroxene. The suffixes P and C denote crystals present as phenocrysts and cumulates, respectively.
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Figure 4. Mineral compositions of representative amphiboles and clinopyroxenes from the Sanggeda intrusion. (a,b) Classification of amphiboles according to the nomenclature of [45]. (c) Data for clinopyroxene from hornblendite cumulate enclaves plotted on the enstatite–ferrosilite–diopside–hedenbergite quadrilateral of [50]. Except for Type 1, hollow and solid symbols represent crystals dispersed in the matrix and cumulate enclaves, respectively.
Figure 4. Mineral compositions of representative amphiboles and clinopyroxenes from the Sanggeda intrusion. (a,b) Classification of amphiboles according to the nomenclature of [45]. (c) Data for clinopyroxene from hornblendite cumulate enclaves plotted on the enstatite–ferrosilite–diopside–hedenbergite quadrilateral of [50]. Except for Type 1, hollow and solid symbols represent crystals dispersed in the matrix and cumulate enclaves, respectively.
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Figure 5. Major element compositional profiles across amphibole crystals of various types: (a) Type 2; (b) Type 3C; (c) Type 4P; (d) Type 4C. The red arrows in the figure indicate the starting and ending positions of the profiles, whether rim to core or rim–core–rim.
Figure 5. Major element compositional profiles across amphibole crystals of various types: (a) Type 2; (b) Type 3C; (c) Type 4P; (d) Type 4C. The red arrows in the figure indicate the starting and ending positions of the profiles, whether rim to core or rim–core–rim.
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Figure 6. Amphibole major element composition in bivariate diagrams versus Mg#: (a) SiO2–Mg#; (b) TiO2–Mg#; (c) Al2O3–Mg#; (d) CaO–Mg#; (e) Na2O–Mg#; (f) K2O–Mg#.
Figure 6. Amphibole major element composition in bivariate diagrams versus Mg#: (a) SiO2–Mg#; (b) TiO2–Mg#; (c) Al2O3–Mg#; (d) CaO–Mg#; (e) Na2O–Mg#; (f) K2O–Mg#.
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Figure 7. Chondrite-normalized REE patterns and primitive-mantle-normalized trace element patterns for amphiboles and clinopyroxene. Chondrite and primitive mantle values are from [51]. The compositions of the core, mantle, and rim are presented as grey, red, and blue lines, respectively.
Figure 7. Chondrite-normalized REE patterns and primitive-mantle-normalized trace element patterns for amphiboles and clinopyroxene. Chondrite and primitive mantle values are from [51]. The compositions of the core, mantle, and rim are presented as grey, red, and blue lines, respectively.
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Figure 8. (a) Chondrite-normalized REE and (b) primitive-mantle-normalized multi-element patterns for the Sanggeda intrusion. Whole-rock compositions from [42], lower continental crust compositions from [3], and chondrite, primitive mantle, oceanic island basalt (OIB), and normal-type mid-oceanic ridge basalt (N-Type MORB) values from [51].
Figure 8. (a) Chondrite-normalized REE and (b) primitive-mantle-normalized multi-element patterns for the Sanggeda intrusion. Whole-rock compositions from [42], lower continental crust compositions from [3], and chondrite, primitive mantle, oceanic island basalt (OIB), and normal-type mid-oceanic ridge basalt (N-Type MORB) values from [51].
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Figure 9. Amphibole-melt equilibrium diagrams. The curves represent the range where amphibole and melt compositions are in equilibrium, based on iron-magnesium exchange. Whole-rock compositions from [42] were used to calculate the Mg# content of the melt. The inset shows the range of Mg# of the amphiboles.
Figure 9. Amphibole-melt equilibrium diagrams. The curves represent the range where amphibole and melt compositions are in equilibrium, based on iron-magnesium exchange. Whole-rock compositions from [42] were used to calculate the Mg# content of the melt. The inset shows the range of Mg# of the amphiboles.
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Figure 10. Diagrams for selected amphiboles determined by the amphibole thermobarometer [55,60]: (a) P–T; (b) logfO2–T; (c) T–H2Omelt. Bt = biotite, Pl = plagioclase, Opx = orthopyroxene, Cpx = clinopyroxene, Mgn = magnetite, Ilm = ilmenite, Ol = olivine, Mg-Hbl = magnesiohornblende, Tsc-Prg = tschermakitic pargasite.
Figure 10. Diagrams for selected amphiboles determined by the amphibole thermobarometer [55,60]: (a) P–T; (b) logfO2–T; (c) T–H2Omelt. Bt = biotite, Pl = plagioclase, Opx = orthopyroxene, Cpx = clinopyroxene, Mgn = magnetite, Ilm = ilmenite, Ol = olivine, Mg-Hbl = magnesiohornblende, Tsc-Prg = tschermakitic pargasite.
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Figure 11. Trans-crustal magma plumbing system: (a) subduction of the oceanic lithosphere and structure of multiple magma chambers; (b) individual magma storage and its chamber; (c) schematic diagram showing the crystal texture characteristics and composition differences of four amphibole types (not to scale).
Figure 11. Trans-crustal magma plumbing system: (a) subduction of the oceanic lithosphere and structure of multiple magma chambers; (b) individual magma storage and its chamber; (c) schematic diagram showing the crystal texture characteristics and composition differences of four amphibole types (not to scale).
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Table 1. Types of amphibole crystal populations.
Table 1. Types of amphibole crystal populations.
TypeRockMineral CharacteristicsHabitRepresentative Sample Number and Photomicrographs
Type 1Fine-grained hornblende gabbroAbout 0.1 mm in size, light green to light brown-yellow with obvious pleochroism, homogeneous composition without growth zoningEuhedralD17-B1-02, Figure 3a
Type 2Porphyritic hornblende gabbro0.5–3 mm, light green to light brown with obvious pleochroism, interference colors range from second-order blue to blue-green, hexagon, with normal cleavage, partially with embayed outlines, homogeneous but a few with thin growth zoningEuhedral/subhedralD17-B1-01,
Figure 3b,e
Type 3PPorphyritic hornblende gabbro0.5–3 mm, light green to light brown-yellow with obvious pleochroism, interference colors range from second-order blue to blue-green, hexagon, with normal cleavage, homogeneous but a few with thin growth zoningEuhedral/subhedralD17-B1-03,
Figure 3b,e,f
CCumulate0.5–3 mm, light green to light brown-yellow with obvious pleochroism, interference colors range from second-order blue to blue-green, hexagon, short cylindrical, with normal cleavage, homogeneous without growth zoningEuhedral/subhedralD17-B2-01,
D17-B2-02,
Figure 3c,d
Type 4PPorphyritic hornblende gabbro0.5–3 mm, green to brown-yellow with obvious pleochroism, interference colors range from second-order blue to blue-green, octagon, short cylindrical, remaining clinopyroxene cleavages, with or without thin growth zoning at the edgesEuhedral/subhedralD17-B1-04,
D17-B1-05,
Figure 3b,f
CCumulateShort cylindrical, remaining clinopyroxene cleavages, green to brown-yellow with obvious pleochroism, interference colors range from second-order blue to blue-green, without thin growth zoningEuhedral/subhedralD17-B2-03,
Figure 3c,d
P and C denote crystals present as phenocrysts and cumulates, respectively.
Table 2. Crystallization conditions of amphibole crystal population.
Table 2. Crystallization conditions of amphibole crystal population.
TypeSampleT (°C)P (MPa)Continental Depth (km)∆NNOlogfO2H2Omelt (wt.%)
MinMaxAvgMinMaxAvgMinMaxAvgMinMaxAvgMinMaxAvgMinMaxAvg
Type 1D17–B1–02847881868931761273.56.74.81.01.31.1−11.5−11.2−11.45.15.65.3
Type 2D17–B1–0191910189781395143415.319.412.90.31.00.7−10.8−9.3−9.84.45.94.9
Type 3PD17–B1–039121035101129076763211.029.023.90.20.80.5−10.8−8.8−9.5 5.57.26.5
Type 3CD17–B2–019841051103045780466017.330.424.90.50.90.6−9.7−8.7−9.05.66.96.3
D17–B2–029591034101029380462211.130.423.50.61.00.8−9.9−8.8−9.14.96.85.8
Type 4PD17–B1–04954106210351237806334.629.523.9 0.40.90.5−10.3−8.5−9.0 5.37.36.7
D17–B1–059621025100028975157710.928.421.8 0.11.30.4−10.1−8.9−9.7 5.17.46.5
Type 4CD17–B2–039651038101847087262617.833.023.70.40.80.6−10.0−8.9−9.25.26.65.8
Temperature, fO2, and H2Omelt values calculated using the formulations of Ridolfi [55]. T = –151.487Si* + 2041, Si* = Si + [4]Al/15 − 2[4]Ti − [6]Al/2 − [6]Ti/1.8 + Fe3+/9 + Fe2+/3.3 + Mg/26 + BCa/5 + BNa/1.3 − ANa/15 + A[ ]/2.3). ΔNNO = 1.644Mg* − 4.01, Mg* = Mg + Si/47 − [6]Al/9 − 1.3[6]Ti + Fe3+/3.7 + Fe2+/5.2 − BCa/20 − ANa/2.8 + A[ ]/9.5. H2Omelt = 5.215[6]Al* + 12.28, [6]Al* = [6]Al + [4]Al/13.9 − (Si + [6]Ti)/5 − CFe2+/3 − Mg/1.7 + (BCa + A[ ])/1.2 + ANa/2.7 − 1.56K − Fe#/1.6. Pressure is calculated using an extended calibration of the Larocque and Canil [17] barometer, as published by Krawczynski. P(MPa) = 1675AlVI − 48. Pressure (MPa) was converted to continental depth (km) based on the equation P = ρgh, where ρ = 2700 kg/m3 and g = 9.8 m/s2.
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Jiang, X.; Luo, Z.; Liu, X. The Petrogenesis and Geological Implications of the Sanggeda Gabbros, Southern Tibet: Insights from the Amphibole Crystal Population. Appl. Sci. 2024, 14, 8435. https://doi.org/10.3390/app14188435

AMA Style

Jiang X, Luo Z, Liu X. The Petrogenesis and Geological Implications of the Sanggeda Gabbros, Southern Tibet: Insights from the Amphibole Crystal Population. Applied Sciences. 2024; 14(18):8435. https://doi.org/10.3390/app14188435

Chicago/Turabian Style

Jiang, Xiumin, Zhaohua Luo, and Xiao Liu. 2024. "The Petrogenesis and Geological Implications of the Sanggeda Gabbros, Southern Tibet: Insights from the Amphibole Crystal Population" Applied Sciences 14, no. 18: 8435. https://doi.org/10.3390/app14188435

APA Style

Jiang, X., Luo, Z., & Liu, X. (2024). The Petrogenesis and Geological Implications of the Sanggeda Gabbros, Southern Tibet: Insights from the Amphibole Crystal Population. Applied Sciences, 14(18), 8435. https://doi.org/10.3390/app14188435

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