Petrogenesis Of Late Phanerozoic Ore-forming Porphyries In The Western Tianshan, Xinjiang, NW China: Implications For Metallogenetic Geology Background

The western Tianshan Orogenic Belt is located in southwest part of the Central Asian Orogenic Belt （CAOB）, which is one of the largest accretionary collages and the site of Phanerozoic crustal growth on Earth. As a north part of the Yili palte, the western Tianshan is sandwiched between the Junggar Basin in the north and the Tarim Basin in the south, i.e., orogenic belt between Junggar plate and Yili plate. The widespread of granitoids with diverse sources is confirmed in the area, similar to the whole CAOB. Previous studies are concentrated in the geochronology of these granitoids, whereas the distribution, rock association and petrogenesis of these granitiods are still under debate. Thus, it is important to study these granitiods, particularly Carboniferous-Permian granitoids, because Carboniferous to Permian is a critical period for final amalgamation of the CAOB. They are also accompanied by extensive copper （molybdenum, zinc） polymetallic mineralization. This paper reports several ore-forming porphyry intrusions from four deposits in a bid to discuss the petrogenesis of these granitiods and some important implications for metallogeny, with the aim of providing constraints on tectonic evolution and crustal growth mechanism and guides for exploration programs in the area and, by inference, elsewhere in the Chinese Tianshan and the CAOB.Based on previous results, this paper reports zircon U-Pb geochronology, mineral chemistry, major and trace elements, and Sr-Nd-Hf isotopic data for the ore-forming porphyries in the western Tianshan. The following achievements and conclusions havebeen made.1. In the Lamasu deposit, the ore-forming porphyries are tonalitic and granodioritic rocks. The ore-forming porphyry rocks in the Lailisigao’er deposit are porphyritic monzogranite and porphyritic granodiorite stocks, while those in the 3571 deposit are porphyritic granodiorite stocks. LA-ICPMS zircon U-Pb dating on ore-forming porphyries from the Lailisigao’er and 3571 deposits yields ages of 346.1±1.2 Ma and 354.0±0.68 Ma, respectively. The trace element compositions of these porphyries from these three deposits are similar to those formed in a continental arc setting and are characterized by enrichment of large ion lithophile elements （LILE） and depletion of high field strength elements （HFSE） and heavy rare earth element （HREE） coupled with slightly negative Eu-anomalies. These rocks also show high (⁸⁷Sr/⁸⁶Sr)t （0.70722⁰.71028） and lowε_Nd（t） values （-3.71⁺0.17）, coupled with depletion of Ba relative to Th and elevated Th/Ce, Nb/Y and Th/Yb ratios, suggesting that the porphyry magma originated from a partial melting of subducted sediments, mixed with minor melts produced by partial melting of mantle wedge components and involvement also of lower continental crust during emplacement.2. In the Kekesai deposit, the ore-forming porphyries consist of monzogranite and granodiorite. LA-ICPMS U-Pb zircon analyses suggest that the monzogranitic rocks were formed at 305.5±1.1 Ma, and include two groups of inherited zircons （358-488 Ma and 1208-1391 Ma）, whereas the granodioritic rocks were formed at 288.7±1.5 Ma. The monzogranites belong to strongly peraluminous, high-K calc-alkaline to transitional series, suggesting a supracrustal origin characteristic of S-type granites. TheεHf（t） values range from -1.08 to +10.12,ε_Nd（t） values vary from -1.04 to -1.12, and initial ⁸⁷Sr/⁸⁶Sr (I_Sr) are 0.7057 to 0.7059, indicating the magmas derivation from reworking of evolved juvenile crust and their geochemical affinity to I-type granitoid. Combining the geochronology, elemental and isotope geochemistry with regional geological settings, we suggest that the monzogranitic magmas were generated by partial melting of thickened continental crust, which was composed predominantly of sediments eroded from late Ordovician to early Carboniferous continental arc volcanic rocks. In contrast, the granodiorites have adakitic compositional characteristics, such as high Al₂O₃, Sr, Ba and low Y contents, coupled with fractionated rare earth element patterns （（La/Yb）n=12.7¹6.8）, and negligible to positive Eu anomalies （Eu/Eu?=1.00¹.20）. The granodioritic rocks exhibit metaluminous to slightly peraluminous medium to high-K calc-alkaline characteristics. They are characterized by relatively low TiO₂, P₂O₅, Th and high MgO, Mg# contents and strong depletion of high field strength elements and heavy rare earth element. They exhibit juvenile isotopic signatures, with I_Sr ranging from 0.70545 to 0.70552,ε_Nd（t） from +1.02 to +1.03 andεHf（t） from +5.3 to +12.3. These features suggest that the granodiorites probably resulted from melting of thickened mafic lower crust, with subsequent melt-mantle interaction.3. Based on our studies and previous results, the ore-forming porphyries in the Tianshan formed in two different periods. The first group formed in between Middle Devonian and early Carboniferous in a continental arc environment related to subducted oceanic crust. The second group formed in the Permian in a late-collision stage related to a significant change in regional tectonic framework from collision-related compression to extension. Different types of deposits could be linked to distinct sources, that is, the formation of molybdenum deposits could be attributed to greater crustal contamination involved in magma generation.4. Middle Devonian to early Carbonifeous ore-forming porphyries might be related to the southward subduction of the Junggar ocean in a continental arc setting, whereas late Carboniferous to Permian ore-forming porphyries were probably derived from partial melting of thickened continental crust and that interacted with mantle materials due to underplating of mantle-derived magma in the post-collisional setting.5. In the western Tianshan, mechanisms of late Phanerozoic continental growth are included lateral accretion of successive accretion of arc complexes and subduction accretion in subduction zones prior to early Carboniferous and vertical addition of underplates in crust-mantle interface. In addition, extensive post-collisional magmatism with multiple magma source reservoirs in the western Tianshan, could plausibly be explained by a tectonic model of slab break-off and lithospheric plate-scale strike-slip shearing regimes. They are not only the Phanerozoic juvenile crust materials, but also are probably an important symbol of the underplating of mantle-derived basaltic magmas and the vertical growth of continental crust in west Tianshan area during the post-collision stage of Late Paleozoic.