Geochemical Study Of Early Paleozoic Igneous Rocks From The Qilian Orogen

Mafic-felsic igneous rocks in subduction zones usually record the reworking and recycling of subducted crustal materials,and are thus the important targets for studying crust-mantle interaction and tectonic evolution of subduction zones.The Qilian orogen was builted by the convergence between the Qaidam block,the Qilian block,and the Alax block,with the tectonic development from oceanic subduction to continental collision in the early Paleozoic.It is an ideal area to study material recycling and crust-mantle interaction in subduction zones.A number of early Paleozoic arc igneous rocks with different lithologies are exposed in the Qilian block,which provide good research objects for identifying reworking and recycling of different subducted crustal materials and reconstructing the tectonic evolution history of the orogen.This dissertation provides a comprehensive study of mineral-rock geochemistry for early Paleozoic mafic-felsic igneous rocks from the southeastern part of the Qilian block.The results not only reveal their heterogeneous source and complex formation process,but also provide insights into crust-mantle interaction in the ancient oceanic subduction channel.It has great significance for understanding the reworking and recycling of subducted crustal components and tectonic evolution of this orogen.Subduction zone is a critical place for crust-mantle interaction,and the addition of subducted crustal material into the mantle is an important mechanism of crustmantle material exchange.However,the understanding of crust-mantle interaction and later magmatic processes is still insufficient.Here we undertake an integrated study of whole-rock major-trace elements and Sr-Nd-Hf isotopes,zircon U-Pb ages and Hf-O isotopes as well as in-situ mineral major-trace elements and Sr isotopes for the mafic intrusives from the Jinyuan and Jishishan areas in the Qilian orogen,northwestern China.The Jinyuan and Jishishan mafic igneous rocks show the consistent early Paleozoic zircon U-Pb ages of ca.448 to 450 Ma,corresponding to the subduction of the South Qilian ocean between the Qaidam block and the Qilian block,indicating that the Jinyuan and Jishishan early Paleozoic mafic rocks are syn-subduction igneous rocks in the oceanic subduction zone.They display arc-like trace element distribution patterns with enrichment in large ion lithophile elements(LILE)and light rare earth elements(LREE)but depletion in high field strength elements(HFSE),relatively depleted Sr-Nd-Hf isotope compositions with(87Sr/86Sr)i ratios of 0.7028 to 0.7050,εNd(t)values of 1.0 to 4.2 and εHf(t)values of 4.8 to 10.4,high and variable zirconδ18O values of 5.2 to 7.8‰.In comparison,the Jinyuan mafic igneous rocks have less depleted Sr-Nd-Hf isotope compositions and higher zircon δ18O values than those of the Jishishan mafic rocks.These elemental and isotopic features indicate that they were derived from a heterogeneous mantle source modified by different crustal materials.Model calculations confirm that the heterogeneous mantle would be generated by reaction of mantle wedge peridotite with different proportions of the subducted Proto-Tethyan oceanic crust-derived aqueous solutions and sedimentderived hydrous melts.Partial melting of the heterogeneous mantle would generate multiple batches of mafic melts with different geochemical compositions,resulting in magma recharge and mixing processes prior to emplacement.These processes were recorded not only by the whole-rock geochemical compositions,but also by the complex textural and compositional zonings of clinopyroxene and amphibole.Thus,in addition to the heterogeneous mantle resulted from crust-mantle interaction,the magma process such as magma recharge and mixing also can be recorded by the synsubduction mafic arc igneous rocks.In addition,an integrated study of zircon U-Pb ages and Lu-Hf isotopes,wholerock major-trace elements and Sr-Nd-Hf isotopes as well as amphibole and plagioclase major-trace elements is also presented for early Paleozoic mafic intrusives from the Heishishan area in the Qilian orogen,northwestern China.The results indicate that the mantle source of these mafic igneous rocks was modified by the subducted oceanic crustal materials,their compositional variation was mainly resulted from magma differentiation and recharge processes during emplacement.Zircon U-Pb dating yields consistent ages of 449±2 Ma and 449±3 Ma,respectively,for the Heishishan hornblende gabbro and diorite,indicating that the Heishishan mafic rocks were formed at the subduction stage of the South Qilian ocean.These mafic intrusives show arc-like trace element distribution patterns with enrichment in LILE and LREE but depletion in HFSE,and weakly enriched to depleted Sr-Nd-Hf isotope compositions with whole-rock(87Sr/86Sr)i ratios of 0.7040 to 0.7063,εNd(t)values of2.3 to-0.3 and εHf(t)values of 1.6 to 4.9,and zircon εHf(t)values of 0.2 to 5.9.These geochemical features indicate their origination from a metasomatic mantle source with involvement of the subducted Proto-Tethyan oceanic crustal materials.Some samples exhibit high Th/Nb and(La/Yb)N ratios,suggesting that the metasomatic agents are composed of seafloor sediment-derived hydrous melts and basaltic oceanic crust-derived aqueous solutions.Amphibole and plagioclase in the Heishishan hornblende gabbro and diorite show complex textural and compositional zonings,recording four stages of mineral crystallization during magma evolution.Combined the whole-rock and mineral geochemical characteristics,we suggest that multiple batches of mafic melts would be produced by partial melting of the metasomatic mantle source,they experienced variable degrees of magma differentiation and recharge prior to emplacement,resulting in geochemical diversity of these mafic igneous rocks.Therefore,in addition to the nature of crust-mantle interaction at mantle depths,the magma evolution and recharge at crustal depths have also played an important role on the petrogenesis and diversity of arc-type mafic igneous rocks above the oceanic subduction zone.During the evolution of orogen,granitoids can be produced by partial melting of different crustal rocks.In order to further understand the tectonic evolution of the Qilian orogen,an integrated study of zircon U-Pb ages and trace elements,whole-rock major-trace elements and Sr-Nd-Hf isotopes is presented for early Paleozoic granites from the Bamishan and Heishishan areas in the Qilian orogen,northwestern China.The Bamishan and Heishishan granites show different zircon U-Pb ages of ca.473472 Ma and 456-455 Ma,respectively,corresponding to the subduction of the South Qilian ocean between the Qaidam block and the Qilian block.They all display adakitic characteristics and arc-like trace element distribution patterns with enrichment in LILE and LREE but depletion in HFSE.The Bamishan granites have depleted Sr-Nd isotope compositions with(87Sr/86Sr)i ratios of 0.7044 to 0.7046,εNd(t)values of 2.6 to 3.7,Th and U contents of 2.5 to 7.0 ppm and 0.5 to 1.3 ppm,respectively,and K2O/Na2O ratios of 0.35-1.06.However,the Heishishan granites have weakly enriched Sr-Nd isotope compositions with(87Sr/86Sr)i ratios of 0.7049 to 0.7056,εNd(t)values of-1.4 to 0.3,higher Th and U contents of 10.1 to 22.3 ppm and 1.8 to 3.7 ppm,respectively,and higher K2O/Na2O ratios of 0.94-1.19.These geochemical features indicate that the Bamishan granites were derived from the partial melting of subducted basaltic oceanic crust with small amount of oceanic sediment,representing continental crust growth,whereas the Heishishan granites originated from partial melting of thickened juvenile mafic arc crust,representing reworking of continental crust.In addition,zircon(Dy/Yb)N ratio,Nb,Th and U contents of the Bamishan and Heishishan granites could also distinguish their different magma sources.In summary,the Qilian orogen experienced a series of processes during the subduction of the Proto-Tethyan South Qilian ocean,such as dehydration/partial melting of subducting oceanic crust,melt/fluid-mantle wedge peridotite interaction,arc magmatism,growth and rewoking of continental arc crust.