Reflectance Spectral Features and Significant Minerals in Kaishantun Ophiolite Suite, Jilin Province, NE China

This study used spectrometry to determine the spectral absorption of five types of mafic-ultramafic rocks from the Kaishantun ophiolite suite in Northeast China. Absorption peak wavelengths were determined for peridotite, diabase, basalt, pyroxenite, and gabbro. Glaucophane, actinolite, zoisite, and epidote absorption peaks were also measured, and these were used to distinguish such minerals from other associated minerals in ophiolite suite samples. Combined with their chemical compositions, the blueschist facies (glaucophane + epidote + chlorite) and greenschist facies (actinolite + epidote + chlorite) mineral assemblage was distinct based on its spectral signature. Based on the regional tectonic setting, the Kaishantun ophiolite suite probably experienced the blueschist facies metamorphic peak during subduction and greenschist facies retrograde metamorphism during later slab rollback.


Introduction
Ophiolites are segments of oceanic crust that have been residually accreted in convergent boundaries [1][2][3].The principal focus of recent studies related to ophiolites has been on the spatial and temporal patterns of felsic to mafic-ultramafic rock suites.These patterns are generated through processes of magmatic differentiation and separate melting episodes in specific tectonic settings [1][2][3][4].The basic assumption is that the type and characteristics of a given ophiolite will be linked to its geological environment.
Mineral assemblages that include olivine, pyroxene, hornblende, etc. are helpful for ophiolite type discrimination.Unfortunately, due to severe alteration, the mineralogical compositions of many ophiolite samples are difficult to recognize in the field.The traditional techniques that have been used to resolve this problem, such as microscopic identification analysis and dissolution methods, typically require samples to be removed from an outcrop and separated into its constituent minerals, which makes the process inefficient and difficult.This limits the ability to perform ophiolite mineral component discrimination.
Multispectral remote sensing has already been effectively utilized to identify different lithologies and altered minerals [4][5][6][7].This technique may be a feasible substitution for the recognition of olivine and its associated minerals, such as pyroxene, chlorite, and epidote, in their solid state.Electromagnetic radiation emitted by the sun interacts with materials and can be detected by remote sensors, which can determine its spectral pattern.This "material fingerprint" can be used to distinguish similar minerals and to determine their compositions [7].

Samples and Methods
We collected rock samples from the Caixiudong part of the Kaishantun area for our analysis.Photographs and sample locations of representative specimens are displayed in Figure 3 and Table 1, respectively.

Samples and Methods
We collected rock samples from the Caixiudong part of the Kaishantun area for our analysis.Photographs and sample locations of representative specimens are displayed in Figure 3 and Table 1, respectively.Samples microstructures and mineral textural relationships were observed with a Leica DMLP Optical Microscope (Leica DMLP GmbH, Wetzlar, Germany) and an FEI Quata-200 Genesis spectrometer (Hillsboro, OR, USA).An EDAX32 scanning electron microscope (SEM) (JEOL, Tokyo, Japan) was used to test the element composition of minerals at the Institute of Geology, Chinese Academy of Geological Sciences.This experiment measured chemical compositions, and discriminated between mineral phases by using energy dispersive spectrometer (EDS) (JEOL, Tokyo, Japan).SEM images recorded in backscatter electron mode and were taken in low-vacuum mode with a focus distance of 18 mm, at 20 keV and 0.5 Torr.A microanalysis system, JEOL-5610LV EDSJEOL, Tokyo, Japan, was used to measure the diameter and the phase compositions of a given selection.
The spectra of mineral samples from the Kaishantun ophiolite suite were measured with a TerraSpec spectrometer (Analytical Spectral Devices, Inc., version 6.4, Malvern PANalytical, Boulder, CO, USA) with NKLST-RSIIA image analysis and remote sensing information, located at China RS Geoinformatics Co, Ltd. (Beijng, China) Table 2 contains the technical specifications of the experimental apparatus.
The experimental procedure was based on Labspec 4 in the ViewSpecPro User Manual.The spectrometer was connected to the laptop and a programmed schedule was set up.The RS3 ASD Inc. software package (Malvern PANalytical, Boulder, CO, USA) was used to record and process the data.
The reflectance spectral measurements of rock samples were taken in the laboratory by using an ASD spectrometer with a 350-2500 nm wavelength range.The resolutions are 1.4 nm at 350-1000 nm; and 2 nm at 1000-2500 nm.Spectral analysis was measured at 10 random spots on the rock samples and then to get averaged representative spectrum of each samples.We measured the samples without  Samples microstructures and mineral textural relationships were observed with a Leica DMLP Optical Microscope (Leica DMLP GmbH, Wetzlar, Germany) and an FEI Quata-200 Genesis spectrometer (Hillsboro, OR, USA).An EDAX32 scanning electron microscope (SEM, JEOL, Tokyo, Japan) was used to test the element composition of minerals at the Institute of Geology, Chinese Academy of Geological Sciences.This experiment measured chemical compositions, and discriminated between mineral phases by using energy dispersive spectrometer (EDS, JEOL, Tokyo, Japan).SEM images recorded in backscatter electron mode and were taken in low-vacuum mode with a focus distance of 18 mm, at 20 keV and 0.5 Torr.A microanalysis system, JEOL-5610LV EDSJEOL, Tokyo, Japan, was used to measure the diameter and the phase compositions of a given selection.
The spectra of mineral samples from the Kaishantun ophiolite suite were measured with a TerraSpec spectrometer (Analytical Spectral Devices, Inc., version 6.4, Malvern PANalytical, Boulder, CO, USA) with NKLST-RSIIA image analysis and remote sensing information, located at China RS Geoinformatics Co, Ltd. (Beijng, China) Table 2 contains the technical specifications of the experimental apparatus.
The experimental procedure was based on Labspec 4 in the ViewSpecPro User Manual.The spectrometer was connected to the laptop and a programmed schedule was set up.The RS3 ASD Inc. software package (Malvern PANalytical, Boulder, CO, USA) was used to record and process the data.
The reflectance spectral measurements of rock samples were taken in the laboratory by using an ASD spectrometer with a 350-2500 nm wavelength range.The resolutions are 1.4 nm at 350-1000 nm; and 2 nm at 1000-2500 nm.Spectral analysis was measured at 10 random spots on the rock samples Minerals 2018, 8, 100 5 of 19 and then to get averaged representative spectrum of each samples.We measured the samples without a sunlight source using an accessory light and revised the experimental results by testing a white reference, which the RS3 ASD Inc. software automatically calibrated to a reflection coefficient of 1.To avoid the polluting effects of ambient light, the detector was carefully screened after this calibration.We measured six points on each sample to discern the distinctive spectral features that are used to distinguish different minerals.

Item Parameter
Spectral Range 350 to 2500 nm Spectral Resolutions 3 nm @ 700 nm 6 nm @ 1400 nm 6 nm @ 2100 nm Sampling Intervals 1.4 nm between 350 and 1000 nm 2 nm between 1000 and 2500 nm Signal to Noise Values 9500 DN @ 700 nm 5000 DN @ 1400 nm 800 DN @ 2100 nm The minerals' spectral data were stored in TEXT files by the ViewSpecPro and RS3 software.These TEXT data were converted to Microsoft Excel files and plotted with the CorelDRAW Graphics Suite 2017 (Corel Corporation, Ottawa, ON, Canada).The raw data are included in the Supplementary Materials.Table 3 contains the list of experimental minerals and their spectral file names.

Petrology
Eight representative samples from the Kaishantun ophiolite suite were examined in this study for their petrological characteristics.The sample locations are shown in Figure 2, and their salient details are listed in Table 1.The samples exhibited signs of intense tectonic shear deformation and alteration, with corresponding minerals that were mostly subjected to low-grade metamorphism.The samples included peridotite, diabase, basal, pyroxenite, and gabbro.A brief description of the petrographic features of the different rock types is given below, and representative photomicrographs are shown in Figure 4.Under the microscope, samples display metasomatic pseudomorph texture, comprising serpentine (60-70%), chlorite (25-35%), and actinolite (10-15%), and the magnetite is also associated with mineral pyroxene (Figure 4a-d).The plagioclase and other felsic minerals are deficiency in the samples with almost no clinopyroxene.The serpentine, which replaced olivine, displays pale yellow color, and is slightly pleochroic, with subhedral and foliated or net-vein texture and flake size of about 0.01 to 0.5 mm diameter.The mineral contains fine grained spinel inclusions of spinel at the core domains.It is usually surrounded by foliated chlorite, with flake size of about 0.02 to 0.2 mm diameter and abnormal interference color.The actinolite partly replaced pyroxene, and formed pseudomorph structure, has allotriomorphic fine grain (<0.1 mm).Magnetite occurs along the grain boundary of actinolite and as the accessory mineral of serpentine-an alteration product.

Intensely Altered Diabase
Sample 170615-6 is an altered diabase in the Kaishantun ophiolite suite.The hand specimen rock is dark colored, medium to coarse grained, and blocky structure.
The actynolin crystals, which replaced clinopyroxene, are subhedral to anhedral, yellowish to greenish, columnar and granular-columnar in shape, ranging in size from 0.2 to 1 mm and showing non-preferred orientation.The zoisite grains are mostly subhedral, and set in granular or columnar texture with size less than 0.1 mm.The mineral shows obvious pleochroism from pale yellow to light green.The mineral partly assembled subhedral tabular texture and showing non-preferred orientation, as an alteration product replaced plagioclase.The glaucophane displays subhedral to anhedral, granular in shape, and ranging in fine grained diameter from 0.01 to 0.1 mm.

Intensely Altered Basalt
Samples 170615-7-2 is a basic rock in the Kaishantun ophiolite suite.The hand specimen rock is dark grey colored, and blocky structure with part alteration.
Under the microscope, sample displays palimpsest texture, comprising chlorite (40-55%), zoisite (25-30%), and minor pumpellyite (5-10%) (Figure 4f).The rocks are altered.The plagioclase and other Under the microscope, samples display metasomatic pseudomorph texture, comprising serpentine (60-70%), chlorite (25-35%), and actinolite (10-15%), and the magnetite is also associated with mineral pyroxene (Figure 4a-d).The plagioclase and other felsic minerals are deficiency in the samples with almost no clinopyroxene.The serpentine, which replaced olivine, displays pale yellow color, and is slightly pleochroic, with subhedral and foliated or net-vein texture and flake size of about 0.01 to 0.5 mm diameter.The mineral contains fine grained spinel inclusions of spinel at the core domains.It is usually surrounded by foliated chlorite, with flake size of about 0.02 to 0.2 mm diameter and abnormal interference color.The actinolite partly replaced pyroxene, and formed pseudomorph structure, has allotriomorphic fine grain (<0.1 mm).Magnetite occurs along the grain boundary of actinolite and as the accessory mineral of serpentine-an alteration product.

Intensely Altered Diabase
Sample 170615-6 is an altered diabase in the Kaishantun ophiolite suite.The hand specimen rock is dark colored, medium to coarse grained, and blocky structure.
The actynolin crystals, which replaced clinopyroxene, are subhedral to anhedral, yellowish to greenish, columnar and granular-columnar in shape, ranging in size from 0.2 to 1 mm and showing non-preferred orientation.The zoisite grains are mostly subhedral, and set in granular or columnar texture with size less than 0.1 mm.The mineral shows obvious pleochroism from pale yellow to light green.The mineral partly assembled subhedral tabular texture and showing non-preferred orientation, as an alteration product replaced plagioclase.The glaucophane displays subhedral to anhedral, granular in shape, and ranging in fine grained diameter from 0.01 to 0.1 mm.

Intensely Altered Basalt
Samples 170615-7-2 is a basic rock in the Kaishantun ophiolite suite.The hand specimen rock is dark grey colored, and blocky structure with part alteration.
Under the microscope, sample displays palimpsest texture, comprising chlorite (40-55%), zoisite (25-30%), and minor pumpellyite (5-10%) (Figure 4f).The rocks are altered.The plagioclase and other felsic minerals are deficiency in the sample with almost no clinopyroxene.The chlorite displays pale or dark green color, and has low interference color, with subhedral and anhedral texture and granular or columnar in shape-grain size less than 0.2 mm diameter.The zoisite is fine porphyritic-columnar in shape (1-2 mm), subhedral, and mostly present as blastoporphyritic texture.The pumpellyite is subhedral to anhedral, mainly schistose structure with dark green, flake diameter ranging 0.02-0.2mm.Minerals occurs along the boundary of chlorite with abnormal interference color.

Intensely Altered Pyroxenite
Pyroxenite sample 170615-10 is dark colored and compact massive, with epidote (40-50%), glaucophane (15-25%) and chlorite (5-10%) (Figure 4g).The rocks are altered.The plagioclase and other felsic minerals are deficient in the sample with almost no clinopyroxene.Except partly medium grained (0.5-1 mm), the epidote displays grain size usually less than 0.1 mm, subhedral columnar or granular, and shows light yellow color with little pleochroism, as an alteration product replaced plagioclase.The glaucophane displays subhedral to anhedral, granular in shape, and ranging in fine grained diameter from 0.1 to 0.3 mm.The glaucophane occurs as tabular relics with blue or purple under plane-polarized light.The chlorite mainly occurs along grain boundaries and cleavage traces of the epidote.

Intensely Altered Gabbro
Samples 170615-11 were collected from near sample 170615-10.The hand specimen rock is dark grey colored, fine to medium grained, and has foliated structure.
The plagioclase is sericitization in medium degree with colorless subhedral tabular in shape.Plagioclase displays polygonal distortion and deformation with wavy extinction.Pyroxene shows pyroxene-rich bands, associating with plagioclase.The pyroxenes are well oriented with brown to dark green colored and subhedral granular in these bands.Actinolite grain, which might regard as an alteration pseudomorph of pyroxene, usually occurs interlocked with the plagioclase as a relict crystal.The actinolite is slightly oriented with pale yellow colored, and surround the pyroxene as a dark colored rim.

Chemical Compositions
The mineral testing position and chemical composition characteristics of the Kaishantun ophiolite suite are shown in Figure 5 and Table 4.Ten spots in each sample were measured by an energy dispersive spectrometer (EDS).Compared with standard chemical compositions in "An introduction to the rock-forming minerals" [23], the EDS data show the chemical composition of samples.Formula recalculation was used AX Win 2007 [24].Mineral within sample 17615-1 displays Na 2 O contents ranges from 0.04 to 0.13 wt % (average value 0.09%).MgO contents ranges from 35.31 to 43.02 wt % (average value 37.71%).Al 2 O 3 contents ranges from 1.04 to 2.92 wt % (average value 1.72%).SiO 2 contents ranges from 32.48 to 42.07 wt % (average value 38.27%).MnO contents ranges from 0.02 to 0.82 wt % (average value 0.45 %).FeO T and Cr 2 O 3 contents range from 4.09 to 6.87 wt % (average value 5.21%) and from 0.13 to 0.97 wt % (average value 0.54%), respectively.The chemical compositions are close to common serpentine with an approximate mineral formula that can be inferred as Mg 3 Si 2 O 5 (OH) X [23].value 0.27%).The chemical compositions are close to common zoisite with an approximate mineral formula that can be inferred as Ca2Al3[SiO4][Si2O7]O (OH)X [23].

Reflectance Characteristics
The results of the white reference tests, which can detect any meaningful variation in the standard spectra, are shown in Figure 6.There was no significant change during the experiment.Figure 7 presents the spectra of eight rock samples from the Kaishantun ophiolite suite.Six spectrum measurements from different parts of each sample showed no variation of absorption features during the measurements.Mineral types had distinctive absorption features and similar shapes, and the spectra of different parts of each rock sample were not identical (Figure 7).     5 includes all spectral absorption features from these tests.

Discussion
As the dominant species used for type differentiation of ophiolite suites, it is important to be able to distinguish typical minerals such as actinolite, epidote, zoisite, and glaucophane from the overall mineral assemblage in rocks.However, it is difficult to distinguish them from associated minerals and also from other femic-ultramafic minerals in the ophiolite suite, as they possess identical tectonic dynamic setting and various degrees of alteration.The traditional techniques, such as microscopic identification and the dissolution method with manual sorting, are inefficient, uneconomical, and time-consuming.Fortunately, multispectral remote sensing based on mineral spectra is a promising tool for identification of different lithologies and altered minerals.
Under normal solar spectrum (300-2500 µm), several minerals have distinct absorption features because of electronic transition, vibrational overtones, charge transfer and conduction.Actinolite has a distinct Fe, Mg-OH combination absorption feature within the 2300-2350 nm range and a less intense absorption feature at 1400 nm depending on electronic transition processes (Fe 2+ ) (Figure 8a).Epidote shows distinct 2200 nm absorption and a less intense 2235 nm absorption feature due to the Al-OH bond and Fe-Mg content (Figure 8a).Similarly, glaucophane exhibits intense absorption feature at 1945 nm and 1400 nm and a weak absorption feature within the 2315-2386 nm range, derived from its Fe-Mg content (Figure 8a).Chlorite displays pronounced absorption at 2310-2330 nm caused by the Fe-Mg content and a shallow absorption feature at 1950 nm (Figure 8b).Zoisite has a strong absorption at 720 nm and a less intense absorption feature at 1800 nm and within the 2310-2375 nm range due to Fe, Mg-OH and Al-OH bond (Figure 8b).Moreover, serpentine shows a strong Fe, Mg-OH absorption feature at 2350 nm, a less intense absorption feature at 1410 nm, and a weak absorption feature at 470 nm depending on electronic transition processes (Fe 3+ )(Figure 8b).The reflectance of peridotite samples (Sample 17615-1, -2, -3, and -4) from Kaishantun ophiolite suite showed strong absorption features at 1400, and 2350.Their shapes compared with standard single mineral spectra presented in the USGS spectral library [25].The spectral curves were similar to the spectra of serpentines (intense absorption feature) and chlorites and minor signatures of epidotes.Sample 17615-1 had higher reflection values, emphasizing that it has more leucocratic minerals, such as plagioclase, but its standard absorption peaks and shapes suggest that there is no difference in its primary mineral components.To confirm the availability of these absorption features, chemical compositions of the minerals in the ophiolite samples were presented.Samples 17615-1 and 17615-2 had minerals with serpentine features, while 17615-3 contains epidote and Sample 17615-4 has minerals closer to actinolite.The results were consistent with the microscopic observation.
In addition, according to the spectral absorption features and shapes reported in this study, Sample 17615-6 shows a distinct absorption feature at 1960 nm, a less intense absorption feature within the 2135-2385 nm range, and a minor absorption feature at 1400 nm and 420 nm, which are attributed to the existence of glaucophanes, actinolites and chlorites.Sample 17615-7-2 exhibits pronounced absorption at 1920 nm and 2350 nm and spectral curves were similar to the spectra of epidotes and minor actinolites.Sample 17615-10 showed an intense absorption features at 1920 nm, a less strong absorption feature within the 2260-2350 nm range and a minor absorption feature at 1400 nm, 1160 nm and 450 nm, which are due to the presence of glaucophanes, zoisites and actinolites.Sample 17615-11 has a distinct absorption feature within the 2330-2385 nm range, a less strong absorption feature at 1920 nm, and a weak absorption feature at 1410 nm, 1050 nm and 410 nm, the spectral curves were similar to the spectra of actinolites (intense absorption feature) and minor signatures of zoisites.These absorption features are in accordance with chemical compositions of the minerals.In chemical composition, Samples 17615-6 and 17615-7-2 had minerals with glaucophane and zoisite features, respectively.Samples 17615-10 and 17615-4 has chlorite and actinolite minerals.The results are correspond to microscopic observation.
Therefore, we can conclude that both the blueschist facies (glaucophane + epidote + chlorite) and the greenschist facies (actinolite + epidote + chlorite) exist in the Kaishantun ophiolite suite.Wu (2003) reported that chloritoid + carpholite + phengite and actinolite + zoisite + barroisite mineral assemblages were found as a blueschist facie in the northern part of Kaishantun ophiolite suite, but they did not discuss the implications of the presence of these minerals [26].Blueschist forms in highpressure, low-temperature environments, and regions associated with subduction of oceanic material reflectance of peridotite samples (sample 17615-1, -2, -3, and -4) from Kaishantun ophiolite suite showed strong absorption features at 1400, and 2350.Their shapes compared with standard single mineral spectra presented in the USGS spectral library [25].The spectral curves were similar to the spectra of serpentines (intense absorption feature) and chlorites and minor signatures of epidotes.Sample 17615-1 had higher reflection values, emphasizing that it has more leucocratic minerals, such as plagioclase, but its standard absorption peaks and shapes suggest that there is no difference in its primary mineral components.To confirm the availability of these absorption features, chemical compositions of the minerals in the ophiolite samples were presented.Samples 17615-1 and 17615-2 had minerals with serpentine features, while 17615-3 contains epidote and sample 17615-4 has minerals closer to actinolite.The results were consistent with the microscopic observation.
In addition, according to the spectral absorption features and shapes reported in this study, sample 17615-6 shows a distinct absorption feature at 1960 nm, a less intense absorption feature within the 2135-2385 nm range, and a minor absorption feature at 1400 nm and 420 nm, which are attributed to the existence of glaucophanes, actinolites and chlorites.Sample 17615-7-2 exhibits pronounced absorption at 1920 nm and 2350 nm and spectral curves were similar to the spectra of epidotes and minor actinolites.Sample 17615-10 showed an intense absorption features at 1920 nm, a less strong absorption feature within the 2260-2350 nm range and a minor absorption feature at 1400 nm, 1160 nm and 450 nm, which are due to the presence of glaucophanes, zoisites and actinolites.Sample 17615-11 has a distinct absorption feature within the 2330-2385 nm range, a less strong absorption feature at 1920 nm, and a weak absorption feature at 1410 nm, 1050 nm and 410 nm, the spectral curves were similar to the spectra of actinolites (intense absorption feature) and minor signatures of zoisites.These absorption features are in accordance with chemical compositions of the minerals.In chemical composition, samples 17615-6 and 17615-7-2 had minerals with glaucophane and zoisite features, respectively.Samples 17615-10 and 17615-4 has chlorite and actinolite minerals.The results are correspond to microscopic observation.
Therefore, we can conclude that both the blueschist facies (glaucophane + epidote + chlorite) and the greenschist facies (actinolite + epidote + chlorite) exist in the Kaishantun ophiolite suite.Wu (2003) reported that chloritoid + carpholite + phengite and actinolite + zoisite + barroisite mineral assemblages were found as a blueschist facie in the northern part of Kaishantun ophiolite suite, but they did not discuss the implications of the presence of these minerals [26].Blueschist forms in high-pressure, low-temperature environments, and regions associated with subduction of oceanic material beneath either oceanic or continental crust are characterized by blueschist and greenschist facies [27].Although some blueschists could also be produced in the collision zone, discrimination about the formation of their environment needs to consider the regional tectonic setting [27].Kaishantun ophiolite suite is located at eastern part of Xar Moron-Changchun-Yanji Fault due to the subduction of the Paleo-Asian Ocean in the Late Permian [13,28,29].Tang (2007) reported the SHRIMP age 286 Ma of granite pebbles in Kaishantun ophiolite suite [30].They are, therefore, typical signs of subduction tectonics [26].It is deduced that Kaishantun ophiolite suite mainly experienced the blueschist facies metamorphic peak during subduction and greenschist facies retrograde metamorphism during later slab rollback.

Conclusions
We measured the spectra of main type samples from the Kaishantun ophiolite suite in Northeast China.Peridotite showed absorption peaks at In conjunction with its chemical composition, the blueschist facies (glaucophane + epidote + chlorite) and the greenschist facies (actinolite + epidote + chlorite) mineral assemblage can be recognized based on their spectral signatures.4 Considered about the regional tectonic setting, the Kaishantun ophiolite suite probably experienced the blueschist facies metamorphic peak during subduction and greenschist facies retrograde metamorphism during later slab rollback.

Figure 6 .
Figure 6.Spectral of white reference before and after the experiment.Figure 6. Spectral of white reference before and after the experiment.

Figure 6 .
Figure 6.Spectral of white reference before and after the experiment.Figure 6. Spectral of white reference before and after the experiment.

Figure 6 .
Figure 6.Spectral of white reference before and after the experiment.

Table 2 .
Technical specifications of the ASD Inc. TerraSpec ® spectrometer.

Table 3 .
File name list of reference and sample spectra.
Note: Data file in the Supplementary Material.

Table 5 .
Summary of Mineral Absorption Peaks from This Study, with Reference Samples from the United States Geological Survey Spectral Librar.