Font Size: a A A

A Study On Chronology, Petrogenesis And Region Tectonic Significance Of Early Cretaceous-Early Eocene Intrusive Rocks In Nyainqentanglha Area

Posted on:2011-01-29Degree:DoctorType:Dissertation
Country:ChinaCandidate:X F ChenFull Text:PDF
GTID:1100360332456157Subject:Institute of Geochemistry
Abstract/Summary:PDF Full Text Request
The Lhasa block is one of the most important tectonic magmatic belts of Tibet Plateau, has a very complex tectonic magmatic evolutionary history. Since Mesozoic, people understand less about the petrogenesis and magmatic processs of igneous rocks exposed in the central of Lhasa block, compared with that of igneous rocks exposed in the north and south of Lhasa block. Through detailed study on chronology, geochemistry, and petrogenesis of Early Cretaceous intrusive rocks exposed in Nyainqentanglha area, the central of Lhasa block. and comparing with the former study, the author put forward some ideas as follows:1. The zircon LA-MC-ICPMS U-Pb dating for Early Cretaceous granodiorite defines a crystallization age of 133.20±0.92Ma, two the inheritance ages of 2543Ma,2573Ma,respectively, indicating there may exist Archeozoic crust for Early Paleocene a crystallization age of 62.41±0.53Ma, for a Early Eocene a age of 49.92±0.24Ma.2. Early Cretaceous granodiorite are metaluminous to weak peraluminous, high-K calc-alkaline series metaluminous to weak peraluminous, high-K calc-alkaline series, have average Mg index of 40, (87Sr/86Sr)i of 0.71947-0.72503,εNd(t)of-12.6--12.7, tDM of about 1927Ma, and the highest magma temperature to reach 830℃calculating by concentration of zirconium in whole-rock and zircon, which indicate the granodioritic magma are mainely originated from partial melting of ancient crust being closely related to underplating of mantle-derived magma. Quartz diorite are poor in Si,alkalinity, rich in Mg,Fe,Ca, metaluminous, calc-alkaline to shsoshonite series, have Nb/Ta ratio of 20, Sr/Y ratio of 9, which indicate quartz -ioritic magma originated from partial melting of mantle. Both granodiorite and quartz diorite show the trace element distribution characteristic as well as calc-alkaline rocks in island arc or active continental margin. Granodiorite and quartz diorite may be derived form the same magmatic processes. Quartz diorite originate from crystallization differentiation of mantle source magma assimilating and contaminating less crust material; while granodiorite originate from crystallization differentiation of massous crust source magma mixing less mantle source magma. Connecting with the predecessors research results, it can be supposed that the petrogenesis may be related to the subducting towards north of Neo-Tethyan.3. Early Paleocene intrusive rocks consist of quartz diorite and granodiorite. Both of them are high-K calc-alkaline series, metaluminous granorite. Rare-earth elements analysis for Early Paleocene intrusive rocks showed that heavy and light rare-earth elements high fractionation((La/Yb)N of 17-36), indicating the magmas originating in deep crust, and leaving garnet as residue. There are remarkble distinct between Adakite and TTG series and Early Paleocene intrusive rocks. According to basalt partial melting experiment, presume that Early Paleocene intrusive rocks originate from metabasalt partial melting under high pressure, relating to mantle magmas metasomatised in subduction zone, and source region rich in REE.4. Early Eocene intermediate-mafic intrusive complexes consist of gabbro-diorite, biotitediorite, and quartz diorite. They show magmas evolving towards subalkaline oleiiticbasalt series. Gabbro-diorite show the major and trace characteristic of mantle source magamas, so they may originate from partial melting of mantle rocks. Sr and Nd isotope and trace element geochemistry study indicate that intermediate-mafic intrusive complexes originate from crust melt mixing mantle melt, and parent magmas most likely is from asthenosphere mantle. Sr isotope mixing simulation show the percent of crust material in magmas is 8%. Regional correlation study and geodynamics analysis indicate the petrogenesis of Early Paleocene intermediate-mafic intrusive complexes is related to the rolling back and then breaking off of Neo-tethys oceans crust, asthenosphere mantle materials acending along fault cutting lithospheric mantle.5. Space-time comparative study on between Nyainqentanglha Early Cretaceous intrusive rocks and Mezoic arch-type igneous rocks exposed in other place in Lhasa block show that at Early Cretaceous Neo-tethys oceans crust subducted towards north of Lhasa block, while Bangonghu-Nujiang oceans crust subducted towards the south,and bothe of their leading edge sinking into asthenosphere mantle under Shiquanhe-Yongzhu-Namucuo-Jiali ocean.6. Study on the trend, emplacement strutrue and geochemistry characteristic of The intrusive rocks older than Early Eocene intrusive rocks show that Nyainqentanglha vast batholith emplaced along the fault of NE trend exsiting ahead of Early Eocene7. It is expected to find some Ag-Pb-Zn hydrothermal vein deposits in Nyainqentanglha area.
Keywords/Search Tags:Nyainqentanglha, zircon U-Pb dating, Early Cretaceous, Early Paleocene, Early Eocene, petrogenesis, partial melting under high pressure, oleiiticbasalt series, contamination, vast batholith, Ag-Pb-Zn hydrothermal vein deposit
PDF Full Text Request
Related items