Petrogrnesis Of Granites And Their Relation To Tectonic Evolution Of Orogen In The East Part Of Qinling Orogenic Belt

Qinling orogenic belt experienced a multi-phase, multi-stage, long time and complex tectonic evolution, accompanied by the corresponding activities of granitic magma. The granitoid studies on petrogenesis including source rock research for various-stage granites involved the formation of the Qinling orogenic belt, in particular the geodynamic processes and evolution of each stage magmatic activity. Nine granite bodies in east part of the North Qinling area were selected to be studied in the thesis. These plutons were distributed on both sides of the Shangdan suture. Using the methods of systematic geochemical, zircon geochronology and Hf isotope geochemistry, we divided stages of magmatic activity, and futher clarified the material source and petrogenesis of granites and geodynamic significance.Based on previous work, in this thesis we obtained ages of nine granites by the method of LA-ICP-MS Zircon U-Pb. Combined with regional geological setting the granitoid magmatism can be divided into three periods based on zircon U-Pb ages of granites, which include four stages:the first stage is early Ordovician(O1), Banshanping granite(quartz diorite 486±6.8Ma), equivalent to Caledonian; the second stage is middle Ordovician-early Silurian(O2-S1), Sikeshu granite(porphyritic biotite granite 464.1±4.6Ma, quartz diorite 468.5±4.1Ma), Zhonggangmiao reservoir granite(porphyritic granite 460.3±3.9Ma), southwest of Huichizi granite(red granite 450.2±2.6Ma), the northwest(monzogranite 445.6±6.1Ma), the east(granite 432.1±3.8Ma), Piaochi granite(gray granite 436.2±6.7Ma), equivalent to Caledonian; the third stage is middle Triassic-late Triassic(T2-T3), Dongjiankou granite(granodiorite 242.3±2.5Ma), Zhashui granite(porphyritic granite 230.6±2.7Ma), Shahewan granite(rapakivi granite 228.4±1.4Ma), equivalent to Indosiniari; the fourth stage is late Jurassic, Mangling granite(porphyritic granite 152.6±1.4Ma), equivalent to Yanshanian.Geochemical studies show that each stage granite has a different genetic type of rock and tectonic setting. The first stage of Caledonian Banshanping withⅠ-type granite characteristecs formed in the subduction of oceanic crust background. The second phase granites include both S-type(for example Piaochi granite) and S-I type granite, which formed in the background of continental crust subduction. Both the emergence of two genetic types of rock at the same stage implies that these granites were not a single resource and there may be contain some mantle components. Both the Indosinian and Yanshanian granite are I-type rock.By the study of Hf isotopes, we got some results about material source of various stages of granite. The first stage of the Caledonian Banshanping granite is the product of Neoproterozoic mantle material(I-type, zirconεHf(t)>0); In the second stage, the soruce rock of S-type Piaochi granites is sedimentary of Mesoproterozoic crust(zirconεHf(t)<0). Other rocks were derived from the product of crust-mantle mixing(I-type, the heterogeneous range of zirconεHf(t))-In the Indosinian granites Dongjiangkou granite came from partial melting of Mesoproterozoic ancient crust(I-type, zirconεHf(t)<0). Zhashui and Shahewan granites from partial melting of the proliferation of Neoproterozoic crust(mantle) with a small amount of Mesoproterozoic igneous crust material(crust) (Ⅰ-type, positive and negative zirconεHf(t) values). In Yanshanian the source rock of Mangling granite was igneous material of Proterozoic ancient crust(I-type, ancient crust(I-type, zirconεHf(t)<0).Based on above studies, we discussed the geodynamic significance of different stages of granitic magmatism, and obtained three conclusions as follows:(1) In the early Paleozoic, closure pattern of the Qinling Ocean may be irregular"S-shape"or "scissors-like shape". The earliest closing time of oceaneic crust is about middle Ordovician-early Silurian, and began in the eastern part of Qinling. (2) In the early Mesozoic, the subduction time of Mian-Lue oceanic crust should be earlier than 245Ma-242Ma and from east to west in Qinling area. And the ocean closed around 242-228Ma and entered the continental collision stage. (3) In Yanshanian, several of granites mainly distributed along the main faults or secondary faults in Qinling orogen, which may be that the south-east of China including Qinling area was affected by the plate subduction from Western Pacific in late Jurassic time.