The Study Of Chehe Rock In Tianshui Area,Western Qinlin

The study area is located in the west of the north Qinling orogenic belt that is important combination of the north qinling and qilian orogenic belt.The Chehe rocks, invaded in Early Paleozoic Caochuanpu rock in Tianshui area, is mainly composed of porphyaceous granite,which has a small amount of mafic microgranular enclaves, is the most important symbol of crust-mantle mixing. while the predecessors are lack of deep study about the crust-mantle magma mixing mechanism and the cause of mafic microgranular enclaves. Based on detailed field geological survey, petrography, petrology, major elements, trace elements and U-Pb zircon SHRIMP dating, this paper has done detailed exploration and discussion about the formation age, cause, tectonic environment and its geological significance. According to the research, we get the main points as follows:1.The zircon U-Pb Shrimp dating of the Chehe host granite shows that the zircon ages of the host rock is 210±1.7Ma(MSWD=1.74) respectively. Thus the formation age of the host rock is late Triassic, it belongs to Early Mesozoic granitoid of indo-china period. Which is not the same time of simple complex massif for Caochuanpu granite, it is a composite batholith formed by polyphasic intrusion.2. The minerals of the Chehe rocks include hornblende and sphene without rich aluminium mineral like garnet and muscovite. On the main elements, higher contene of SiO2(72.57%~74.13%), K2O/Na2O(1.35~1.47), higher K2O+Na2O(8.02%-8.6%),Al2O3( 13.57%~13.91%), A/CNK(0.99-1.07), the Chehe host rock is sub-aluminum-Weak aluminum granit with 10000Ga/Al(2.05-3.97). On the trace elements, the host rocks is similar to A type granite with poor Ba、Ti、P and Sr, whereas the host rocks are higher Sr and δEu than the typical A-type granite rock with weakly negative Eu anomaly and positive anomaly individually. Therefore the Chehe granite shows both A type and I type, but differs from both.3. The Chehe rock has a small amount of mafic microgranular enclaves, the inclusions have varial shape. Within the inclusions, there developed acicular apatite and quartz phenocryst included by microgranular enclaves, the trace elements Nb-Ta, Zr-Hf shows tank shaped distribution. All previous provide important evidence of magma mixture.4. Research on higher Na clinopyroxene inclusion that developed in mafic microgranular enclaves shows that the mafic microgranular enclaves has formed through two stages. The first stage is generating early products by crystal differentiation of pyroxene and other minerals: the mixture of high temperature basic magma and lower temperature acidic magma caused rapid crystallization differentiation of high temperature basic magma, in this stage,some basic mineral assemblage like monoclinic pyroxene starts to crystallize.The second stage is crystallizing mafic microgranular enclaves: after the magma experienced crystallization differentiation, it invaded upward with residual partial basic and acidic magma.At the same time, the monoclinic pyroxene starte to crystal. After then, the rest of basic magma more rapid coolling and crystallization in one location,which resulted that part of the fine clinopyroxenee are encapsulated by amphibole and other minerals packed.5. Researchs on higher Na clinopyroxene show that the depth of magma mixture is over 69 kilometers at least, this thickness represents the minimum thickness of crust in the study area of the late Triassic period. From this the Study area has experienced collision orogenic activity during late Triassic, the thickness of the continental crust is equivalent to the Himalayan region of the earth’s crust thickness of nowadays.6. From Comprehensive analysis of the regional geological background and the tectonic environment diagram, we can get the Chehe granite of early Mesozoic should be formed under the post-collisional- after collision stage of extension.