| The East Kunlun orogen has gone through a very complex tectonic evolution history. It’s one of the particular areas in the Qinghai-Tibetan Plateau that contains substance records of both Proto-Tethys and Paleo-Tethys after the formation of the Precambrian basement. The East Kunlun enter the stage of intercontinental evolution since Mesozoic. There were lack of substance records in this period, but the limited deformation of sedimentary rocks(such as fold-thrust structures developed in the Babaoshan Fm molasse) indicated a strong Yanshan movement in the East Kunlun area. Other thermochronology studies(such as40Ar/39Ar) also show that there was an intense tectonic thermal event in Jurassic-Cretaceous period in the East Kunlun. The tectonic evolution research of the East Kunlun in Mesozoic-Cenozoic period is still very weak due to the lack of substance records. Some thermochronology methods with different closure temperatures provide an opportunity to solve this problem.Thermochronology results do not reflect the time of formation of the rocks, but the time of the thermal events the rocks experienced after it formed. With different closure temperature, especially the notion of Partial Annealing Zone(PAZ) in fission track (FT) method, we can reveal the modern history of the old geological body. It provides a viable solution for the tectonic evolution research in some area with less substance records during a geological period.Pyroxenite is a special rock which belongs to mafic rocks on chemical composition, and the ultramafic rocks on the mineral composition. It’s another important part of upper mantle beside peridotite. Pyroxenite could be formed and uplift to near-surface by tectonic emplacement or magma capture, even magma emplacement directly. Geological body formed by tectonic emplacement, such as the ophiolite belt and mafic igneous complex generally have the composition of pyroxenite; The basaltic magma often carry a variety of xenoliths, and many of the xenoliths are pyroxenite; But it is very rare that the pyroxenite magma formed in the mantle, rise directly and emplacement into the crust and formed a pyroxenite pluton. The pyroxenite in this study is such a special occurrence, which emplaced in the contact area between the Early Silurian granodiorite and Proterozoic basement metamorphic rocks. The magma of the pyroxenite was from the mantle source region, emplaced in the middle-upper crust, and then uplift and erode to the surface. Through a long vertical migration path and complex tectonic evolution history, the pyroxenite became a ideal sample for research. Based on detailed field survey and structural analysis, the thesis have done many aspects of research on the pyroxenite, such as petrology, geochemistry, zircon U-Pb geochronology and Lu-Hf isotope, fission track thermal chronology, fabric and microstructure, petrophysics. Combined with regional geological background and the previous research, we investigated the causes and its associated crust-mantle interaction of the pyroxenite, simulated the thermal history of pyroxenite after its emplacement, and then established a complete tectono-thermal history of the pyroxenite pluton eventually.The new findings and knowledge aehieved in this study are as follows:(1) The rock is mainly composed of clinopyroxene, orthopyroxene and amphibole, and minor plagioclase, quartz, biotite and iron opaque minerals. Amphibole and biotite were formed during retrograde metamorphism. The mineral composition of the rock is very similar with some mafic granulites and the difference between the two is that the orthopyroxene metamorphic or magmatic origin. If the orthopyroxene in the rock has a metamorphic origin, the rock is granulite, and if it is magmatic origin, the rock is pyroxenite. The discriminant analysis results suggested that the orthopyroxene are magmatogenic, thus the rock should be named pyroxenite rather than granulite.(2) The rock have high MgO, CaO and low A12O3and enriched in Rb and Th and depleted in Nb and Ti, high U, Th in the zircon, low176Hf/177Hf and negative εHf(t) values showing clear evidence for an enriched mantle source. Petrography, rare earth elements and other features of the rock showing that during the pyroxenite magma forming there had Si-rich melt added which came from the crust ingredients. In other words, there was crust-mantle interaction in the formation process of pyroxenite contrast with the results of previous studies.(3) Field occurrence of the pyroxenite pluton suggest that the pyroxenite pluton was formed after the mylonization of the surrounding rocks. In situ LA-ICPMS U-Pb dating, and trace element and MC-ICPMS Lu-Hf isotope analyses were undertaken on zircon grains from the pyroxenite sample. We have two major206Pb/238U age clusters,452-474Ma(A group) and255-270Ma(B group), which yield two weighted mean206Pb/238U ages of462.6±5.3Ma and261.2±3.0Ma respectively. Some of the zircon CL images have core-rim structures and the cores have oval shapes and cross-shaped zones and the rims with oscillatory zoning. That means the cores are metamorphic and the rims are igneous zircon. The segmented signal curves of some spots of LA-ICPMS zircon U-Pb dating also showing the multi-period growth of zircons in pyroxenite. Combined with field occurrence and petrography, we suggest that the age of261.2±3.0Ma is the best estimated age of emplacement time of the pyroxenite pluton. Age of462.6±5.3Ma, corresponding to the age of the granulite-facies metamorphism of Baishahe Group in the East Kunlun orogen, implying that the cores of zircons from the pyroxenite pluton were captured from the metamorphic rocks of Baishahe Group when the ultramafic pyroxenite magma underplated the overlying lower crust, and then, the magmatic zircon growth and wrapped on the metamorphic cores. The zircons have high U, Th and REE contents, low176Hf/177Hf and negative εHf (t) values of-12.1to-3.0showing clear evidence for an enriched mantle source and crust-mantle interaction during the magma formation and emplacement process. We propose a model for ultramafic magma underplatting and crust-mantle interaction in a subduction zone environment, in which subduction of an Paleo-Tethys(A’nyemaqen) oceanic slab at the Middle Permian led to fluid and Si-rich melt metasomatism, inducing partial melting of an enriched lithospheric mantle(peridotites) to form the ultramafic pyroxenite magma. The pyroxenite magma underplated the overlying lower crust, captured the metamorphic zircons of the granulite and exchange some trace elements, but didn’t result in the lower crust partial melting to form any felsic magma. The pyroxenite magma emplacement alone eventually. Crust-mantle interaction plays an important role in the entire process of the formation of pyroxenite, including the magma originated and emplacement. There are retrogression and alteration in the pyroxenite during the later uplift and exhumation process.(4) Although there is no preferred orientation on the pyroxenite outcrops and hand samples, the EBSD analysis results show that the mineral Clinopyroxene and orthopyroxene both have a strong crystallographic preferred orientation fabric(CPO), especially the orthopyroxene. On the fabric figure, there are preferred orientation(CPO) along the [001] axis and the (010) surface, and the [001] axis and (010) surface distribution were less complete ring. The transmission electron microscope (TEM) observation shows that, dislocation microstructures, such as free dislocation, dislocation bowing, dislocation arrays, dislocation wall (subgrain border) and a little dislocation net are widely developed in clinopyroxene and orthopyroxene of the pyroxenite samples.In contrast, the orthopyroxene have more free dislocations.Overall, there are both free dislocation and islocation tangles which under lower temperature conditions and dislocation bowing and incomplete dislocation loop which indicate the higher temperature plastic rheology conditions. These description implied the pyroxenite had experienced a uneven and very wide range of temperature conditions. The statistical results of the free dislocation density and the microstructures indicates these dislocations were formed by creep during the uplift process of the pyroxenite pluton.(5) The test results under normal temperature and pressure conditions show that the pyroxenite have apparent resistivity and velocity anisotropy. It’s consistent with the strong fabric of the orthopyroxene and clinopyroxene of the pyroxenite. The wave velocity estimated use the EBSD data show anisotropic characteristics too, this imply that the anisotropy of rocks controlled by the fabric of its main mineral composition. The presence of fabrics in mainly composed minerals of rocks are the origin of the significantly anisotropic characteristics of the physical properties in the rocks without any macro orientation structures.(6) Through the research of apatite fission track (AFT) thermochronology and thermal history modeling, we found that the tectono-thermal history of the pyroxenite since the emplacement can be divided into five-stages. The first stage from about260Ma to205Ma. The rock cooling to about40℃rapidly and the cooling rate was about2.9℃/Ma. We can estimate that the pyroxenite lifted about4.6km in this stage if it was assumed that the geothermal gradient was35℃/km, and the average uplift rate was84m/Ma. There was a temperature increase incident in the second stage from about205Ma to200Ma for a short time, the temperature of the pyroxenite from about40℃increased to about80℃, which may be due to the impact of nearby tectono-thermal events such as faulting, magma emplacement, etc. The third stage was about200Ma to130Ma, and the pyroxenite cooling slowly from80℃to50℃with an average cooling rate of0.4℃/Ma. The fourth stage from about130Ma to about5Ma, the pyroxenite cooling very slow, with the average cooling rate of0.06℃/Ma. In the fifth stage the pyroxenite cooling rapid since5Ma with the cooling rate of about8.6℃/Ma. The average uplift rate was about246m/Ma. It can be seen in the first and the fifth stage the pyroxenite were cooling and uplift very fast. The first stage corresponds to the uplift of the northern block of the East Kunlun related to the Paleo-Tethys subduction and continued extrusion in this area. The rapid cooling and uplift in the fifth stage was the result of rapid uplift and exhumation of the entire Qinghai-Tibetan Plateau since the Pliocene and local fault depression activities and differential uplifts.(7) The combination of zircon U-Pb geochronology method with higher closure temperature and zircon and apatite fission track thermochronology (FT) method with lower closure temperatures basic revealed the process of the emplacement, uplift, cooling the tectonic exhumation-thermal history and dynamics of the pyroxenite, which fully shows that the thermochronology methods, such as fission track, are effective means in study of the modern history of the old geological bodies. The combination of the two above can reflect the tectono-thermal history of a wider region in the space-time background. |
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