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The Eastern Carpathians are thrust to the east and north over their Eastern European foreland, tectonically covering it over an area several hundred kilometers across. Information about the nature of the underthrust part of the Carpathian foreland can be obtained from the rock fragments preserved in the sedimentary successions of the Carpathian fold and thrust belt, specifically in the Outer Dacides and the Moldavides. Fragments of felsic rocks occurring within the sedimentary units of the Upper Cretaceous successions of the Moldavides have long been attributed to the Cuman Cordillera—an intrabasinal ridge in the Eastern Outer Carpathians. This work is the first complex geochemical and geochronological study on the exotic igneous clasts of the Cuman Cordillera. Igneous clasts from the southern part of the Moldavides (Variegated clay nappe/formation) are investigated here. They include mainly granites and rhyolites. Phaneritic rocks are composed of cumulus plagioclase, albite, amphibole and biotite, and intercumulus quartz and potassium feldspar, with apatite, magnetite, sphene, and zircon as main accessories, while the porphyritic rocks have a mineral assemblage similar to that mentioned above, displayed in a porphyritic texture with a usually crystallized groundmass. SHRIMP U-Pb zircon dating indicated the 583–597 Ma age interval for magma crystallization. Based on calcareous nannofossils, the depositional age of the investigated igneous clasts is Cenomanian to Maastrichtian, implying that the Cuman Cordillera was an emerged piece of land, herein an active source of sediments in the flysch basin for at least 40 Ma, from the Early Cretaceous (Aptian) to the Late Cretaceous (Maastrichtian). The intrusive and subvolcanic rocks show similar trends for trace and major elements, evincing their comagmatic nature. The enrichment in LILE and LREE relative to HFSE and HREE, as well as the element anomalies (e.g., negative Nb, Ta, and Eu and positive Rb, Ba, K, and Pb) suggest a convergent continental plate margin tectonic setting. Mineral chemistry suggests magma crystallization in relatively oxic conditions (magnetite series), during ascent within a depth of 15 km to 5 km. The igneous rocks attributed to the Cuman ridge display compositional and geochronological features similar to Brno and Thaya batholiths in the Brunovistulian terrane, which could be a piece of the Carpathian foreland not covered by the Tertiary thrusts. Our data confirm the non-Carpathian origin of the igneous clasts, revealing a Neoproterozoic history of the Carpathian foreland units, which include a Cadomian/Pan-African continental arc, exposed mainly during the Late Cretaceous as an intrabasinal island of the Alpine Tethys, traditionally known as the Cuman Cordillera. Full article

The alkaline complex in the southwest region of Luxi Terrane of the North China Craton is spatially correlated with the newly discovered Longbaoshan REE deposit. Its petrogenesis, however, remains ambiguous. In this study, we present an integrated petrology, whole-rock geochemistry, sphene U-Pb and rare earth element data from the Longbaoshan alkaline complex to investigate the petrogenesis, magma source and tectonic evolution. The Longbaoshan alkaline complex consists of mafic to intermediate rocks of hornblende diorite and alkaline hornblende syenite porphyry, biotite monzonite porphyry and aegirine diorite porphyrite. The hornblende diorites show a composition of low SiO₂, high MgO, Fe₂O₃ and moderate Na₂O, CaO and are metaluminous and medium-to-high-K calc-alkaline. The hornblende syenite porphyries, biotite monzonites and argirine diorite porphyrites display a relatively higher content of SiO₂, Na₂O, K₂O and Al₂O₃ and lower contents of MgO, Fe₂O₃ and CaO and are metaluminous, peralkaline, high-K calcic-alkaline and shoshonite. The sphene U-Pb data shows that the parent magma of the hornblende diorite was emplaced at ca. 120 Ma. All these samples show a common depletion in Th, Nb-Ta and Zr-Hf and enrichment in large ion lithophile elements (e.g., Pb, Ba, Sr) and Light Rare Earth Elements. The magma may have experienced fractionation of pyroxene, amphibole, sphene, apatite and zircon during its evolution. The variable La content, La/Sm, Rb/Sr and (Ta/Th) _N ratios indicate that the parent magma may produce by partial melting of a mantle source that was interacted with sediment-derived melts in a subduction setting. Therefore, we propose that the parent magma of the Longbaoshan alkaline complex was derived from a lithospheric mantle which was metasomatized by sediment-derived melt in a prior subduction process. The enriched magma was emplaced through an extension process and experienced subsequent fractionation and assimilation with the continental crust during the rollback of the Paleo Pacific Ocean plate. Full article

The Luxi Terrane (eastern China) exposes widespread Early Cretaceous alkaline rocks, whereas their petrogenesis remains controversial, including fractional crystallization, partial melting and crustal contamination regime. Here, we present petrology, geochemistry, sphene U-Pb geochronology and trace element data from the syenogranite, quartz syenite and quartz monzonite of the Guandimiao alkaline complex rocks to investigate their petrogenesis. Geochemical data suggest that these alkaline rocks show alkalic and peralkaline characters, and high Ga/Al ratios, SiO₂, light rare-earth element (LREE), Zr and Nb, and low MgO, CaO, Eu contents, corresponding to A-type granites. Sphene trace elements in syenogranite and quartz monzonite show obvious fractionation between LREE and heavy rare-earth element (HREE) and high Th/U ratios, indicating a magmatic origin. They yield U-Pb lower intercept ages of 128 ± 2.3 Ma and 127 ± 1.3 Ma, representing the crystallization ages of these alkaline rocks. The negative correlations between CaO, Fe₂O₃ (Total), MgO, P₂O₅, TiO₂, MnO and the pronounced depletion in Nb, Ta and Ti suggest that the alkaline rocks were formed by fractional crystallization. Additionally, the positive correlation between La/Hf and La, Th and Th/V, Ce/Yb and K₂O, and Tb/Yb and Yb suggest that the alkaline melts are generated by partial melting. Such high Rb/Nb, (Th/Nb)_N and Nb/Th ratios indicate crustal contamination during the magma emplacement. We, therefore, propose the magma source of the alkaline rocks in the Guandimiao complex originated by partial melting of lithospheric mantle, which experienced fractional crystallization and crustal contamination processes during its emplacement. Such complex alkaline rocks were probably formed in an extensional back-arc setting induced by the retreat of the subducting Izanagi plate. Full article