The Genesis And Formation Mechanism Of Lvliangshan Composite Granite Body And Enclosed Gneiss And Garnet Amphibolite Lenses

The North Qaidam orogenic belt, lies in the north margin of Tibet plateau between the Qilian block and Qaidam block, is a typical continental subduction ultrahigh pressure (UHP) metamorphic belt. In this paper, we performed integrated studies of petrography, geochemistry and geochronology on the Lvliangshan granitic intrusion and enclosed granitic gneiss and garnet amphibolite/amphibolite lenses. We determined the genesis and formation history of them and further discussed the relationship between the enclosed granitic gneiss block and garnet amphibolite/amphibolite in the granitic intrusion with the UHP eclogites and their country granitic gneisses in the North Qaidam UHP belt. The main achievements of the thesis include the following aspects:1. Systematic field and petrology studies show that the Lvliangshan granite body is a composite granite body composed of porphyritic granite, biotite granite, synogranite, monzogranite and two mica granite. The gneissic schistosity of the granitic gneiss and garnet amphibolite/amphibolite enclosed in the composite granite body is parallel to that of the regional gneisses, indicating that the enclosed granitic gneiss (garnet amphibolite/amphibolite) is in situ and the intrusion rate of the magma is slow.2. Systematic geochemistry studies show that the porphyritic granite and biotite granite have characteristics of C type adakitic rocks and are formed by partial melting of thickened mafic lower crust during continental subduction and eclogites during exhumation, respectively. The synogranite has features of A type granite and is formed by low pressure and high temperature partial melting of felsic rocks during post-collisional extension stage. The monzogranite associated has characteristics of I type granite and is partial melting product of intermediate-basic rocks in the continental crust. The two mica granite has characteristics of strongly peraluminous S type granite and is formed by partial melting of metapelitic rocks during derooting and collapsing at the post-orogenic stage.3. The magmatism of the Lvliangshan composite granite body were divided into three periods according to detailed in situ zircon U-Pb dating:The first of which occurred at 436-430Ma and is represented by the porphyritic granite; The second period magmatism occurred at 400-390Ma and is represented by biotite granite, synogranite and monzogranite; The last period occurred at 365-360Ma and is represented by the two mica granite, the major rock of the composite granite body.We also obtained the protolith age of 910±10Ma and metamorphic age of 407±5Ma for the granitic gneiss enclosed in the composite granite body, the protolith age of 870-855Ma, peak metamorphic age of 430±2Ma and retrograde age of 401±5Ma for the garnet amphibolite as well as metamorphic age of 398±4Ma for amphibolite enclosed in the granitic gneiss. The retrograde metamorphic age (401±5Ma) of the garnet amphibolite is consistent with the metamorphic ages of the granitic gneiss (407±5Ma) and the amphibolite (398±4Ma) as well as the second stage granite in Lvliangshan (400-390Ma) and the time of regional magmatic activity, indicating that the exhumation of the North Qaidam UHP terranes is accompanied with large scale granitic magmatism.4. Combined the results of petrology, geochronology and geochemistry in this study and the published data about the North Qaidam UHP belt, the genesis and formation history of the Lvliangshan composite granite body are briefly inferred as following:(1) At 436-430Ma, the continental crust subducted to the mantle depth and formed coesite-bearing UHP metamorphic rocks and meanwhile the thickened mafic lower crust partial melted and formed the porphyritic granite with C type adakite characters; (2) At 400-390Ma, the deeply subducted eclogite terranes exhumated extensively. As a result, the UHP rocks underwent retrogression and the eclogite terranes partial melted due to decompression and dehydration and formed biotite granite with C type adakite features, synogranite with A type granite characters formed in extensition setting and monzogranite with I type granite characteristics; (3) At 365-360Ma, the orogenic belt unrooted and collapsed and formed abundant strongly peraluminous two mica granite.5. According to the comprehensive study on occurrence, petrology, geochemistry and geochronology, the granitic gneiss and garnet amphibolite/amphibolite enclosed in different place of the composite granite body have similar occurrence, protolith affinity, protolith age and peak metamorphic age with the granitic gneisses and continental-type (retrograde) eclogites in the North Qaidam UHP belt, indicating that they are captured nearly in situ by the slowly intruded granitic magma.