Chronology And Geochemistry Of Magmatic Rocks In Namling-Yangbajing At Lhasa Terrane, Tibet

There are many Mesozoic and Cenozoic magmatic rocks in the south andcentral Lhasa area, their petrogenesis and geological setting is still ongoing debateup to now. This dissertation reports the data of chronology, bulk-rock major element,trace element and Hf isotopes of the magmatic rocks in the Namling region of SouthLhasa Terrane and Yangbajing region of Central Lhasa Terrane, Tibet, aiming toprovide more answers for the solving of these questions.Namling area are mainly diabase stock and feldspar granite dikes. Yangbajingarea are mainly granite, rhyolite, and metamorphic rocks. The result of zircon U-Pbdating indicate: the age of Nanmling intrusive rocks is93Ma and the age ofYangbajing granite and rhyolite samples are65Ma and61Ma respectively, the ageof one metamorphic rock sample correspond with the peak age of inherited zirconsof peraluminous granitoids in central Lhasa block. Namling granite trace elementsfrom zircon similar to the trace elements in igneous Zircons; Diabase has highercontent of trace elements of Zircon. Yangbajing granite trace elements from zirconsimilar to earlier reported granitoid zircon trace elements, has more negative Euanomaly compare to Namling; Biotite-quartz schist has lower content of traceelements; rhyolite trace elements in zircon similar to granite.Intrusive rocks in central Lhasa area are high-K calc-alkaline to shoshoniticrocks, in south Lhasa area are medium-to high-K calc-alkaline rocks. All thesamples in Namling are enriched in LREE and depleted in HREE most samplesshow well-developed to weak negative Eu anomalies; Rb, Th, U are enriched and Ba,Nb, Ta, Ti, Sr are depleted. Magmatite trace elements characteristics in Yangbajingshow marked difference. Two-mica moyites (YB1101YB1102) showwell-developed positive Eu anomalies. Biotite moyites (YB1105YB1106) showweak negative Eu anomalies. Two-mica granites (YB1103YB1YB2) showwell-developed negative Eu anomalies. All samples in Namling indicate positive eHf(t) values and relative young Hf model ages. Samples in Yangbajing indicatenegative eHf(t) values and relative old Hf modal ages. The petrogenesis of the magmatic rocks in Namling is juvenile crust materialsbut in Yangbajing are juvenile crust or mantle materials mixing with the old crust.The Namling magmatic rocks are formed in a setting of northward subducton ofTethyan sea floor and following estrusion as well as east to west extension whichcaused the rising asthenosphere material. The Yangbajing magmatic rocks areformed in a setting of India-Eurasia continental collision, in combination withgeochemical features further support the initial collision time is65Ma or earlier.