Petrogenesis And Geodynamic Processes Of The Indosinian Magmatism In The West Qinling Orogenic Belt, Central China

The West Qinling belt is the westward extension of the Qinling-Dabie-Sulu orogen in central China. It links the Kunlun and Qilian orogens to the west and the Songpan-Ganze belt to the south, making it an important tectonic syntaxis in China. The final amalgamation of China continents mainly took place during Indosinian stage, along the Mianlue and A’nimaque suture zones. The connection between these two sutures is still puzzling. One of the most profound phenomenon is that the Indosinian magmatism are widely distributed in the West Qinling and the adjacent East Qunlun and Qaidam terrans, Songpan-Ganze block, East Qinling and northwestern margin of the South Chian Block. These magmatic rocks may record the lithosphere evolution and geodynamic processes responding for the amalgamation of these micro-continental blocks in western China. The West Qinling is located on the central domain of these tectono-magma belts. An integral investigation of spatial and temporal distributions, rock types, and petrogenesis of Indosinian magmatic rocks in the West Qinling plays an important role in discussing the west Qinling Indosinian tectonic evolution, the distinct evolutions between east and west Qinling belts, and even the continent crust differentiation during the continents collisional.This thesis focuses on the Indosinian magmatism in the West Qinling. Combined with the previous relevant data, we carry out an integrated study of petrography, zircon U-Pb dating and Hf isotope, and whole-rock geochemical and Sr-Nd isotopic compositions for Indosinian magmatic rocks from the West Qinling. The West Qinling is characterized by the most widely distributed of High-K cal-alkalin I-type granitioids. Firstly, the Meiwu composite pluton, one of the largest batholiths in the middle part of the West Qinling, were choosed as the detailed study objects, to clarify the petrogenesis and emplacement mechanism for the composition diversity of I-type high-K calc-alkaline rocks; second, one type of rarely exposed rocks, garnet-bearing magmatic rocks and leucogranite, were discussed for their petrogenesis; then, the rock association and the spatial and temporal distribution of the Indosinian magmatism throughout the West Qinling were investigated for their petrogenesis and magma sources; finally, compared with the Indosinian magmatism in the adjacent areal, the geochronological framework for the Indosinian magmatism in the West Qinling were established and the geodynamic background for their formation were revealed. Our conclusions are summarized as follows:(1) The Meiwu pluton, a complex batholith, is composed of quartz diorite, granodiorite, and biotite granite, with abundant mafic enclaves and minor felsic enclaves in the granodiorites. The magma crystallization age is～240-245Ma. The different rock types exhibit distinct geochemical variation, indicating that the Meiwu pluton was constructed by multiple injections and repeated magma pulses assembly over a protracted period, rather than a single larger magma chamber. A united model for the construction and evolution of the Meiwu batholith is listed as bellow:(1) During the Early Indosinian, partial melting of the enriched lithospheric mantle generates the hydrous basaltic magmas. The high-T mafic sills were successively emplaced in the lower crust forming a Deep Crustal Hot Zones. The high-T hydrous basaltic magmas can also provide the necessary heat source and volatiles to induce melting of the lower crust;(2) The hydrous basalts that had experienced fractional crystallization of olivine and pyroxene formed the residual melt. The residual melt from hydrous basaltic magmas mixing with the melt derived from partial melting of crustal materials formed a hybrid magma; then the hybrid magma batches intruded to the upper crust and coalesced as a single magma body as the Meiwu quartz diorites. The outer part of the quartz diorites cooled rapidly preserving the primary isotopically diversity, whereas the inner part would have developed as a larger homogeneous magma chamber that underwent internal differentiation and crystallized slowly;(3) For random emplacement of the mafic sills, the single batches of melt can be generated by polybaric partial melting of the crustal materials over a range of depths through heat transfer from the cooling basalts. The melt segregation from different depths assimilated with minor the evolved mafic magma that then were successive intruded into the present lever of exposure, forming the Meiwu granodiorites;(4) The hybrid magmas analogue to the quartz diorites were injected into the host granitic magma and had experienced local mechanical exchange and diffusion exchange with the adjacent host rocks, producing various types of MME. The melts derived from the thickened mafic lower crusts that were subsequently intruded into the host granitic magma and disaggregated to the tonalitic enclaves;(5) The melts generated by increase degree of partial melting of a single source due to heat input from mantle magma were successive emplaced forming the Meiwu biotite granites.(2) The magma crystallization ages for Dewulu volcanic rocks, Sai’erqin garnet-bearing volcanic rocks and Xiahe garnet-bearing granite porphyry, and Xiahe leucogranite are～239-243Ma. Dewulu volcanic rocks are mainly composed of andesite and dacite, and Sai’erqin volcanic rocks range from andesite to rhyolite to volcaniclastic rocks. These rocks are enriched in Na2O (K2O/Na2O ratios mostly<1). The andesitic rocks range from metaluminous to weakly peraluminous with increasing SiO2contents, and the felsic rocks show strongly peraluminous (A/CNK>1.1). Geochemical and Sr-Nd-Hf isotopic compositions indicate that these rocks share a single metaluminous calc-alkaline andesitic magma source, which turned into strongly peraluminous magmas through AFC processes. The Xiahe leucogranite has high SiO2contents (～74%), high ISr values of～0.7103-0.7119, negative εNd(t)(～-12) andεNf(t)(～-11.3), with two-stages model ages of～ 1.8-2.2Ga, implying derivation from partial melting of reworked mature crust. The leucogranite contains very low HREE (YbN=0.1-0.2), with strongly fractionated REE patterns with (La/Yb)N ratios of400-486, indicating a large volume of residual garnet. Garnets from these rocks show distinct formation features. They are divided into three types:(i) crystallizing from the metaluminous to peraluminous magmas;(ii) inheriting from the melting source;(ⅲ) xenocryst.(3) To understand the temporal and spatial distribution of west Qinling Indosinian magmatism, thirty-two samples were selected for in situ LA-ICP-MS zircon U-Pb analyses. And four of them were reanalyzed by using the SIMS. We compile new results and nineteen published data. Our compilation shows two magmatism stages:(i) early Indosinian (-245-230Ma), concentrating on the central and west part of west Qinling belt;(ii) late Indosinian (-230-205Ma), widespread throughout the west Qinling belt.(4) Based on geochronology, petrology, and geochemistry data of west Qinling magmatic rocks, they are mainly composed of early Indosinian diorite (including mafic enclaves), low Sr/Y granitoids, and high Sr/Y granitoids (C-type adakite), and late Indosinian granitoids and mafic enclaves. Xiekeng pyroxene diorite is a typical example of early Indosinian diorite, including high-Al diorite and high-Mg diorite. Both of them were derived from partial melting of enriched lithospheric mantle that had been modified by slab-derived melt. The Xiekeng high-Al diorite was formed by the hydrous basaltic magmas that had experienced fractional crystallization of olivine and pyroxene and/or preferential accumulation of plagioclase, while the Xiekeng high-Mg diorite was formed by fractional crystallization of olivine and/or preferential accumulation of pyroxene. Early Indosinian low Sr/Y granitoids are high-K calc-alkaline series, and they display ISr=0.7063-0.7098, εNd(t)=-9.2--3.8, with T2DM=1.46-1.74Ga, indicating derivation from a early-middle proterozoic mafic K-rich lower crust. Their low (La/Yb)N (10.0-30.6) and Eu/Eu*(0.31-0.79) imply the residual mineral of amphibole and plagioclase in its source. The high Sr/Y granitoids show similar Sr-Nd isotopic compositions with those of the low Sr/Y granitoids, suggesting that they share a same source. Their high Ba, Sr contents and high Sr/Y and (La/Yb)N ratios imply the residual mineral face of garnet. Some samples exhibit slightly negative Eu anomalies, probably resulted from fractional crystallization of plagioclase during the emplacement. Most of the early Indosinian low Sr/Y and high Sr/Y granitoids show high Mg#, Cr, and Ni values. Besides, coeval diorites and mafic enclaves are widespread, suggesting a culmination of basalt underplating during early Indosinian. Late Indosinian granitoids commonly have low MgO, Cr, and Ni contents, and high K2O, Rb, and ISr values, indicating they were derived from more mature crust, with less juvenile material contribution.(5) Distinct geochemistry features of early and late Indosinian granitoids may imply different geodynamic setting. Combined with regional studies, we interpret this early Indosinian magmatism in the West Qinling to result from break-off of the subducted A’nimaque oceanic slab soon after collision. The slab break-off model can explain the linear distribution of the early Indosinian plutons and rapid uplift during the Middle Triassic in the West Qinling. Late Indosinian magmatic rocks are widely exposed in the Qinling orogen, the northwest part of the Yangtze craton, the Songpan-Ganzi belt, and the east Kunlun belt, suggesting all of these regions are in a post-collision setting. Widespread late Indosinian intrusions are most likely caused by delamination of thickened crust. The Jiangligou potassic granite shows similar geochemical features with those of A-type granite, and its crystallization age is210Ma. Coeval A-type granites and are also found in east Kunlun and Songpan-Ganzi. This A-type granitic magmatism may represent the end of central orogen processes, and the west Qinling belt turn into an extension setting.(6) Early Indosinian magmatism mainly concentrate on the central and west part of west Qinling belt, whereas the east Qinling was dominated by the late Indosinian magmatism, suggesting that West Qinling and East Qinling belts are in different geodynamic settings during early Indosinian. Whether the A’nimaque and Mianlue suture zones is connected or not is still in debate. The early Indosinian magmatism in the middle and west part of the West Qinling is related to the A’nimaque suture zone in spatial, and the east part (as well as east Qinling belt) magmatism related to the Mianlue suture zone in spatial. The distinct temporal evolutions of Indosinian magmatism between these two domains reveal that the A’nimaque Ocean and Mianlue Ocean were not connected before.