Magma Evolution And Deep Geological Processes Of Early Permian Tarim Large Igneous Province

Huge quantities of Early Permian igneous rocks were found in Tarim Basin, Xinjiang Province,NW China.The overall coverage of the igneous rocks was up to 200,000km².These igneous rocks have always been the key interest to geologists.In this thesis,systematic researches have been carried out as to Early Permian igneous rocks,focusing on the magma evolution and deep geological processes of the large igneous province.Up to now,several points have been addressed as follow:1.Based upon the nomination of Early Permian Tarim LIP by our research group, more works have been down to support the conclusion.Early Permian Tarim LIP consists of ultrabasic-basic-felsic rock types.The ultrabasic rocks are ultramafic rocks, explosive breccia and pyroxenite.Basalt and diabase are the main components of basic series,while felsic series outcropping as syenite and syenite porphyry.Basalts were widely erupted across the west,northwest,southwest part of the basin,with estimated area of more than 200,000km².2.According to Zircon SHRIMP U-Pb dating,The lowest unit of Keping basalts erupted at about 290Ma(289.5±2.0Ma),while the top unit reveals an age of 288.9±3.4Ma,which means the basalt eruption probably ended at 289Ma.SHRIMP U-Pb age of syenite porphyry is 278.4±2.2Ma,which is of the same age as syenite (277～278Ma).So the temporal series for Tarim LIP are:basalt eruption came first, ultramafic intrution,then diabase followed,syenite porphyry and syenite intruded at last.3.Early Permian Tarim LIP consists of a range of igneous rocks,with SiO₂ content ranging from 31%to 67%.All the igneous rocks are rich in large ion lithosphile elements(LILE) as well as high field-strength elements(HFSE),depleted in HREE.The geochemical features are to some extent similar to OIB.Wajilitag explosive breccia and diabase,Xiaohaizi ultramafic rocks and diabase,syenite, syenite porphyry are belonging to alkali series,while Keping basalt and diabase are at the boundary between alkali and sub-alkali series.Based on the apparent difference ofεNd and Ta/Hf value,two types of magma source were brought in to explain their petrogenesis:one is Keping basalts(εNd＜0) derived from lithospheric mantle;another is intrusive rocks and Tabei basalts(εNd＞0) related to depleted mantle.4.Magma evolution of Tarim LIP mainly happened in the way of crystal fractionation,including the fractionation of olivine,clinopyroxene and plagioclase. There are seldom captures found in Tarim igneous rocks,which means no significant crust contamination happened to Early Permian Tarim igneous rocks.Early Permian Tarim igneous rocks are mainly from two magma sources,ultramafic rocks and pyroxenite were respectively from～16%and～5%melting of garnet lherzolite; Keping basalts were probably from 5%melting of spinel-garnet lherzolite.All of the other igneous rocks are from these two magma sources.5.Early Permian Tarim large igneous province was caused by the action of mantle plume.A lot of evidences show that there was a plume under Tarim block during early Permian.Uprising of the plume resulted in uplift of the crust.Such huge quantities of flood basalts were erupted during such a short period(1～2Ma).A lot of dyke swarms occurred in Tarim basin.At the final stage of plume-caused magmatism, felsic magma was evolved in and intruded into the crust,ultra high MgO content magmatic rocks are related to hot mantle plume.Trace elements ratios(e.g.Ta/Hf＞0.3) and isotopic data also support the existence of mantle plume.6.According to the characteristics of Early Permian igneous rocks in Tarim basin,we put forward a petrogenic model for the Tarim large igneous province.At the early stage,hot plume caused the low-level partial melting of lithosphere mantle;later, sudden uplift of plume lead to melting of itself due to its adiabatic pressure release. This is different from Bahana model and Deccan model,either by partial melting of lithosphere or mantle plume.7.The general evolution process of Tarim LIP should be like this:First,the uprising mantle plume met with the lithosphere mantle,causing the low degree partial melting of lithosphere mantle,forming the magma for Keping basalts.After eruption of basaltic magma,mantle plume got more room to move up,leading to pressure release and followed by plume melting.The magma can feed ultrarnafic intrusions and explosive breccia.Also,the evolved magma can be the parental magma for Tabei basalts and diobases,and even syenite porphyry and syenite.Pyroxenite can be the accumulates from the fractional crystallization of the primitive magma.