The Metamorphism Of Sumdo High/Ultra High Pressure In Tibet And Its Tectonic Significance

The HP/UHP metamorphic rocks are considered to be important evidence to constraint the regional tectonic evolution process and to invert of the evolution history of ancient ocean.The Sumdo HP/UHP metamorphic zone,located in the central part of the Lhasa plate,is an important newly identified HP/UHP metamorphic zone on the Qinghai-Tibet Plateau in recent years.And it is considered to be an important progress in the field of high-pressure metamorphism studies on the Qinghai-Tibet Plateau.Since the discovery of eclogites in Sumdo area,new eclogite outcrops have been discovered in Jilang,Xindado,Bailang and other areas,which not only expand the distribution range of Sumdo HP/UHP metamorphic belt,but also make some progress in terms of chronology,metamorphic temperature,pressure conditions and metamorphic phase equilibrium simulation.As the eclogite is affected by the obvious later reformation and the complex tectonic activities in the region,the degree of research on the Sumdo HP/UHP metamorphic zone is still relatively low.Regarding the original rock properties,formation mechanism and regional impact of the HP metamorphic rock Issues such as indications of structural evolution have not yet been resolved.Zircon U-Pb isotopic dating studies of eclogite samples in the Sumdo HP/UHP metamorphic zone show that the peak metamorphic ages of the eclogite in the Sumdo area are 266Ma—265Ma,and the zircon weighted mean ²⁰⁶P/²³⁸U ages are 266.4±9.3Ma,265.1±5.3Ma and 266.5±2.5Ma respectively;the peak metamorphic age of Longyasong eclogite yield 268Ma-261Ma,and the zircon ²⁰⁶P/²³⁸U weighted mean age are 268.3±7.3Ma,260.5±6Ma and 261±5.1Ma,respectively;the zircon²⁰⁶P/²³⁸U weighted mean age of Jilang eclogite is 254±12Ma,so the peak metamorphic age of eclogite is Middle Permian.Through the 40Ar-39Ar isotope dating of muscovite in the direct wall rocks of the eclogites in Sumdo and Longya Sumdo area,the effective plateau ages obtained are 234.9±2.7Ma and 223.3±2.3Ma,respectively,indicating that the exhumation age is Late Triassic.Based on the study of petrography and mineralogy of eclogite,combined with electron probe composition test results,the metamorphic mineral assemblage of eclogites in Sumdo and Longya Sumdo areas was determined;the peak metamorphic conditions of eclogites were estimated.Finally,the peak metamorphic P-T conditions of the Sumdo eclogite are 606-695?and 23.5-27.5k Pa;the P-T conditions of the Longya Sumdo eclogite peak metamorphism are 593-679?and 21.6—25.8k Pa.Petrochemical studies have shown that the Middle and Late Permian eclogites in the Sumdo HP/UHP metamorphic zone have the characteristics of low silicon,rich magnesium and aluminum,similar to the geochemical characteristics of basic volcanic rocks;The Ti O₂ content is highly variable,and the trends of rare earth element partitioning curves has obvious changes in groupings,with protolith properties of N-MORB and E-MORB;the Middle Triassic eclogite is obviously enriched in light rare earth elements and depleted in heavy rare earth elements,and has typical OIB geochemical characteristics.Combining regional evidences such as Middle-Late Permian OIB-type ophiolite and Late Carboniferous OIB type volcanic rocks,this paper believes that the eclogites with different geochemical properties on the Sumdo HP/UHP metamorphic belt may be the product of long-term interaction between the mantle column and mid-ocean ridges.Eclogite-type rutile deposits have attracted increasing attention due to their ease of mining,high availability of components and low environmental pollution.Based on the petrography and mineralogy of rutile in eclogites in the Sumdo HP/UHP zone,this paper analyzes the Ti O₂ content of these rocks by geochemical methods and it is found that there may be titanium-rich mafic protoliths in the Sumdo HP/UHP zone,which is the material basis for the formation of eclogite-type rutile deposits;the results of the temperature and pressure conditions of eclogite indicate that there is high/ultra-high pressure metamorphism in the region,which is conducive to the preservation of rutile in large quantities;In addition,the study of the peak metamorphic age of the eclogites and the ⁴⁰Ar-³⁹Ar dating of their direct contact with the surrounding rocks indicate that the eclogites in the Sumdo HP/UHP metamorphic zone have experienced a rapid exhumation rate,which is beneficial to the preservation of rutile in the eclogite.Therefore,this paper concludes that the eclogite-type rutile deposits in the Sumdo HP/UHP metamorphic zone possess the basic conditions for mineralization and have a good prospecting prospect for mineralisation.Based on the research results of the Sumdo HP/UHP metamorphic zone,combined with the regional ophiolite,magmatic rock and stratigraphic data,this paper explores the formation mechanism of the Sumdo HP/UHP metamorphic rock and establishes a preliminary evolutionary model of the Sumdo HP/UHP metamorphic rock.During the Late Carboniferous-Early Permian,the Sumdo Paleo-Tethys ocean began to take shape,and the widely exposed Upper Carboniferous-Lower Permian Sumdo Formation in the region represented the sedimentary record during the initial opening of the Sumdo Paleo-Tethys Ocean Basin;During the Middle to Late Permian(274Ma-254Ma),intra-oceanic subduction occurred in the Sumdo Paleo-Tethys Ocean,during which Late Permian eclogite were formed.During the Middle Triassic(238Ma-230Ma),part of the upper regional oceanic island base(OIB-type oceanic crust)was dragged downwards as the Sumdo Tethys oceanic crust continued to subduct to the surface;During the Late Triassic(238 Ma-230 Ma),the Bailang eclogites were folded back to the surface along the subduction channels as the subducting plate broke off and generated large differential stresses.During the Early Jurassic(199Ma-163Ma),a volcano-magma arc was formed on the southern margin of the Central Lhasa block,with large-scale intrusions of granitoids and other rocks in the Upper Carboniferous-Lower Permian Sumdo Formation at the end of the Late Triassic;this was accompanied the southward accretion of the southern margin of the Central Lhasa block and the ophiolite fragments,forming the Late Triassic-Early Jurassic arc magmatism in the region.