A Geochemical Study Of Late Paleozoic Mafic Igneous Rocks From Western Tianshan

Mafic igneous rocks at convergent plate boundaries generally record the recycling of crustal components at convergent plate boundaries.These mafic rocks are the ideal targets to investigate the different types of curst-mantle interaction in subduction zones,the tectonic evolution of oceanic subduction zones,as well as the nature of mantle sources.The Western Tianshan,located at the southwestern margin of the Central Asian Orogenic Belt in western China,was eventually amalgamated by the closure of the two major branches of the Paleo-Asian Ocean(PAO)along with the accretion of multiple arcs,accretionary complexes,seamounts,and oceanic plateaus.The Yili-Central Tianshan block was affected by the southward subduction of the North Tianshan ocean and northward subduction of the South Tianshan Ocean in the Late Paleozoic.This makes the western Tianshan an ideal target to trace the different types of crust-mantle interaction.Late Paleozoic mafic igneous rocks with different ages and compositions were emplaced on both the north and south sides of the Yili-Central Tianshan Block,and they can be used to identify the recycled crustal components during the oceanic subduction.This dissertation provides a comprehensive study of petrology and geochemistry for Late Paleozoic mafic igneous rocks from the western Tianshan.The results provide insights into the recycling of crustal components into the mantle sources of these mafic rocks.This bears the involvement of different properties of the materials derived from subducting slab and related crust-mantle interaction,the tectonic evolution of oceanic subduction zones,and the different types of geochemical transfer processes in oceanic subduction zones.For a long time,it has been widely believed that metasomatism of the overlying mantle wedge peridotite by slab-derived fluids plays a key role in the generation of arc igneous.rocks above subduction zones.However,the melange model argues that components from the subducting plate become physically mixed at the slab-mantle interface where partial melting of the peridotite subsequently occurs after being hybridized by the melange material.Then it diapirically rises into hotter portions of the wedge resulting in the formation of island arc magmatic rocks.Thus,the identification of the way that subducting crustal materials involved into the mantle and understanding the origin of an island arc are topics of great importance.Here,we carried out an integrated geochemical study on two types of Late Paleozoic mafic igneous rocks from northwestern Tianshan in western China to distinguish slab aqueous solutions from sediment melts in generating arc magmatism rather than the melange model.Zircon/Titanite U-Pb dating yields ages of 320-317 Ma for gabbro diorites and 307Ma Ma for diabase dikes,respectively.These mafic rocks all exhibit typical arc-like trace element distribution patterns,but show a series of differences in stable Li and radiogenic isotope compositions and the other geochemical variables.Generally,the gabbro diorites are characterized by the enrichment in fluid-mobile elements such as Rb,Ba,K,Pb and Sr,high δ7Li and δ18O values and depleted in Sr-Nd-Hf isotope compositions.By contrast,the diabase dikes are enriched in melt-mobile elements such as LILE,LREE and Th,relatively low δ7Li and slightly enriched Sr-Nd-Hf isotope compositions.There are also systematic differences and remarkable correlations between the Li isotope compositions and slab fluid indicators such as whole-rock Rb/Nd,Sr/Nd,Nb/U,Th/La,Th/Nd and Th/Yb ratios for the two types of mafic igneous rocks.To sum up,these geochemical differences can be explained by the metasomatism of mantle wedge peridotites by two distinct properties of liquid phases derived from subducting North Tianshan oceanic slab,in which the subducting slab(including the basaltic oceanic crust and pelagic sediments)-derived aqueous solutions metasomatism is required for the generation of gabbro diorites,while the pelagic sediments-derived hydrous melts metasomatism is required for the generation of diabase dikes.This qualitative identification is further confirmed by quantitative model calculations of trace elements and Sr-Nd-Li isotopes in the two types of mafic igneous rocks.Therefore,the studied mafic igneous rocks provide the geochemical evidence for chemical metasomatism of the mantle wedge peridotite by incorporating two properties of metasomatic agents in oceanic subduction channel.The subducting oceanic crust is generally composed of basaltic igneous crust and seafloor sediment,which may give rise to different compositions of liquid phases for causing metasomatism of the mantle wedge.Although the liquid phases can be aqueous solutions or hydrous melts,or both,the origin,properties and proportions in the mantle sources of mafic magmatic remain to be determined.This chapter presents on a combined study of whole-rock Li isotopes and zircon O isotopes in addition to whole-rock major-trace elements and Sr-Nd-Hf isotopes in Late Paleozoic mafic igneous rocks from the southern zone of western Tianshan in western China.The results are used to effectively identify and distinguish aqueous solutions derived from basaltic oceanic crust and sediments.Zircon U-Pb dating yields consistent ages of 313 ± 3 Ma to 305 ± 1 Ma for magma crystallization.The mafic igneous rocks exhibit arc-like trace element distribution patterns and depleted whole-rock Nd-Hf isotopes but slightly high(87Sr/86Sr)i ratios of 0.7039 to 0.7056.They also show positive zircon εHf(t)values and slightly higher zircon δ18O values of 5.2-7.6‰.In addition,these mafic igneous rocks display low(La/Yb)N,Th/Yb,Th/Nb and Th/Nd ratios but high Ba/Th,Ba/La,Pb/Ce and Li/Y ratios,consistent with their origination from mantle sources that contain subducting oceanic crust-derived aqueous solutions.There are covariations in whole-rock Sr isotopes with Th/La and Rb/Nb ratios,indicating a contribution from terrigenous sediment-derived fluids to their mantle source in addition to basaltic igneous crust-derived fluids.Based on the slightly higher zircon 18O values but variably lower whole-rock δ7Li values of-0.8 to 3.5‰ for the target rocks than those of mantle values,respectively,both altered oceanic basalt-and terrigenous sediment-derived fluids are identified in the mantle source of these mafic igneous rocks.Model calculations for trace elements and Sr-Nd-Li isotopes further confirm that the geochemical compositions of these mafic igneous rocks can be explained by chemical reaction of the depleted MORB mantle wedge peridotite with the mixed fluids to generate the ultramafic metasomatites at subarc depths.Therefore,the chemical metasomatism of the mantle wedge is a key mechanism for incorporating crustal components into the mantle source of arc-like mafic igneous rocks above oceanic subduction zones.Although high-Mg andesites(HMA)have attracted increasing attention due to their unique geochemical composition and important geological significance,there is no consensus on their petrogenesis.The present study indicates that the subducting terrigenous sediment-derived hydrous melts exerts a primary control on the composition of Paleozoic HMA in the northern zone of western Tianshan in western China.These HMA are composed of basaltic andesite and andesite.They generally exhibit arc-type trace element distribution patterns and weakly enriched Sr-Nd-Hf isotope compositions.Some show remarkably higher Ba/La,Ba/Th,Rb/Nb and U/Th ratios than normal mid-ocean ridge basalts(MORB).Furthermore,most of them are characterized by higher Th/Nb,Th/Yb and Th/Nd ratios but lower Nb/U ratios relative to normal MORB,similar to those of terrigenous sediments.Whole-rock Nd isotopes covary with Rb/Nb and Th/Yb ratios,indicating contributions from both oceanic basalt-derived aqueous solutions and terrigenous sediment-derived hydrous melts.Together with their high zircon δ18O values,it appears that the mantle source of the target HMA contains the terrigenous sediment-derived hydrous melts in addition to the subducting altered oceanic basalt-derived aqueous solutions.In the studied HMA,the andesite generally exhibits higher contents of large ion lithophile elements(LILE)and light rare earth elements(LREE)and more enrichments in Sr-Nd-Hf isotopes than the basaltic andesite.These differences indicate that the mantle source of andesite would contain more subducting sediment-derived hydrous melts than that of basaltic andesite.As a consequence,relatively Si-rich and Si-poor pyroxenite sources were respectively generated as the mantle sources of the andesite and basalt ic andesite.This qualitative interpretation is verified quantitatively by modelling the geochemical transfer from subducting oceanic crust into the mantle wedge.Model calculations indicate that the addition of～3%oceanic basalt-derived aqueous solutions and 4-12%terrigenous sediment-derived hydrous melts into the mantle wedge peridotite can account for the geochemical compositions of the target HMA.Therefore,the HMA in northwestern Tianshan provide the geochemical evidence for the crust-mantle interaction during the oceanic subduction in the Paleozoic.As such,the subducting terrigenous sediment-derived hydrous melts play a dominant role in generating the composition of the lithochemically fertile,geochemically enriched mantle sources and thus in the origin of HMA above oceanic subduction zones.