Early-Middle Indosinian Weiya Quartz Diorite, Eastern Segment of the Middle Tianshan Mountains, NW China: Implications for Intra-Continent Subduction and Partial Melting of Juvenile Lower Crust
摘要
AN id=EnChDivSummary name="EnDivSummary">Voluminous cognition has resulted from intensive studies of the geology and Hercynian tectonic envents in the eastern Tianshan Mountains.To date,howeven,it has not been unknown whether the Indosinian tectonic evolution was a continuing episode of Hercynian orogeny or it was significantly controlled by other tectonic regimes.Hence integrated petrographic,geochemical and isotopic studies of the Indosinian magmatism are critical to understand the tectonic evolution of the eastern Tianshan Mountains in Triassic.The Weiya quartz diorite,occurring in the eastern segment of the Middle Tianshan Mountains,was formed in early-middle Indosinian.The rock is mainly composed of quartz(5%~10 %),plagioclase(An 35-40,55%~60 %),K-feldspar(1%~10 %),biotite(10%~15 %) and hornblende(5%~10 %).Geochemical study indicates that the Weiya quartz diorite is characterized by varing SiO2 contents(ranging from 62.40 % to 65.25 %),higher Al2O3 conent(16.06~16.90 %),and lower TiO2(0.70%~0.88 %),with high ratios of Na2O/K2O(1.26~1.75) and aluminium saturation indices(ASI) of 0.91~0.93.These features suggest that it can be defined as high K calc-alkaline rocks.The rock is enriched in large ion lithopile elements(LILE) such as Cs,Rb,Ba,K and Pb and high field strength elements(HSFE) such as U,Th and Zr,depleted in HFSE such as Nb.All samples display similar chondrite-normalized REE patterns with enrichment in LREE,depletion in HREE,and significant fractionation(LaCN/YbCN=12.07~17.29) between LREE and HREE.Sr,Nd and O isotopic analyses show that the Weiya quartz diorite has an initial Sr and Nd values of 0.70672～0.70879和0.512413～0.512441,respectively,O isotope values of 8.64 ‰ to 8.73 ‰,εNd(t) values of 1.64~2.16,and an Nd model ages(TDM) of 0.79~0.80 Ga.All these features strongly indicate that the Weiya quartz diorite is derived from neither direct fractionation of mantle-derived magmas nor deep hybrid sources between mantle-derived magmas and crustal felsic melts,but a product of partial melting of middle-high K metabasaltic rocks in juvenile lower crust under the conditions of temperature of 1000~1100℃ and pressure of 1600~1800 MPa.Authors believe that driving force and heating source responsible for diagenesis of the Weiya quarzt diorite may be originally related to subduction occurring in this region,which was likely triggerd by northward subduction of the Paleo-Tethys oceanic plate during late Permian to Triassic periods.Our research displays that eastern Tianshan Mountains could not be the extension of the Hercynian orogeny,but has been influenced significantly by the Paleo-Tethys tectonic regime.
