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1.
Using Fourier transform infrared spectroscopy (FTIR), we measured water contents of quartz and feldspar for four thin sections of felsic mylonite and two thin sections of banded granitic gneiss col- lected from a ductile shear zone of middle crust along the Red Rivers-Ailaoshan active fault. The ab- sorbance spectra and peak position suggest that water in quartz and feldspar of granitic gneiss and felsic mylonite occurs mainly as hydroxyl in crystal defect, but also contains inclusion water and grain boundary water. The water contents of minerals were calculated based on the absorbance spectra. Water content of feldspar in granitic gneiss is 0.05 wt%-0.15 wt%, and that of quartz 0.03 wt%-0.09 wt%. Water content of feldspar ribbon and quartz ribbon in felsic mylonite is 0.095 wt%-0.32 wt%, and those of fine-grained feldspar and quartz are 0.004 wt%-0.052 wt%. These data show that the water content of weakly deformed feldspar and quartz ribbons is much higher than that of strongly deformed fine-grained feldspar and quartz. This suggests that strong shear deformation leads to breakage of the structures of constitutional water, inclusion and grain boundary water in feldspar and quartz, and most of water in minerals of mylonite is released to the upper layer in the crust.  相似文献   

2.
Mineral deformation and rock flow mechanism in the lithosphere are related to the rheological behavior and weakening mechanism of the continent. Natural deformation behaviors of feldspars are not well understood due to the complexity of their mineral compositions, crystal structures, as well as changing deformation conditions. The refined microstructure,fabric and composition of major minerals in the deformed granitic rocks within the Gaoligong shear zone(GLGSZ), southwestern Yunnan, China, were studied. With increasing mylonitization, two fabric types of end-members have been distinguished(type-I banded granitic mylonite and type-II banded ultramylonite). The two types of deformed granitic rocks have the same mineral assemblage, but different mineral modes. The type-I banded granitic mylonite has a greater proportion of K-feldspar(mostly present as porphyroclasts)plagioclasequartz±biotite, however, the type-II banded ultramylonite has a greater proportion of fine-grained plagioclaseK-feldsparquartz±biotite. The crystallographic preferred orientation(CPO) patterns of quartz combined with two-feldspar geothermometer, confirm that the quartz grains in the type-I and type-II granitic rock have undergone high-temperature dislocation creep deformation. The K-feldspar grains in the matrix of type-II banded ultramylonite show a dominant(100) [010] slip system with dislocation creep recrystallization, while the fine-grained plagioclase grains present a weak CPO pattern with superplastic flow. The K-feldspar porphyroclasts show grain-size reduction associated with mineral composition and fabric transformation. The myrmekite formation with the fine-grained neocrystallization of plagioclase and quartz significally replaced the K-feldspar porphyroclasts. Finally, the fine-grained neocrystallization plagioclases were formed further into the high strain localized ultramylonites with superplastic flow.  相似文献   

3.
Monazite is an important accessory mineral in the pelitic granulites(Weihai area, Sulu orogen), and is also a powerful monitor for understanding the metamorphic evolution of the granulites. The pelitic granulites incongruously occur as lenses in granitic gneisses. The lithologies of the lenses gradually change from core to margin: The undeformed coarse-grained granulite,the foliated fine-grained granulite, the garnet-biotite-gneiss, and the migmatized granulite. The lens cores mostly preserve a peak granulite-facies metamorphic mineral assemblage of garnet + plagioclase(antiperthite) + quartz + sillimanite + biotite with accessory minerals of rutile, zircon, and monazite. The lens margins display a fluid-induced retrogression. In order to decipher the metamorphic processes of the pelitic granulites, a combined study of BSE imaging, U-Pb dating, and trace element composition for the monazites from the metapelitic lens were conducted. Monazites from the undeformed coarse-grained granulite only record a Paleoproterozoic age(1832±7 Ma, n=40). Monazites from the other lithologies yield the inherited Paleoproterozoic age and Triassic overgrowth age. For example, monazites from the migmatite yield intercept ages of 1818±10 and 211±22 Ma(n=56) with Triassic concordant age of 223.8±2.9 Ma. The Paleoproterozoic monazites are characterized by remarkable depletion in HREE and Y with obviously negative Eu anomalies, indicating their formation equilibrated with garnet and feldspar under granulite-facies conditions. During Triassic fluid modification, the monazite bright rims assimilated Th and Si but released U, HREE, Y, and P. This process resulted in that the Triassic overgrowth monazites have higher HREE and Y contents, and lower Th and U contents with relatively low Th/U ratios. Thus, the monazites in the pelitic granulites recorded a Paleoproterozoic metamorphic event and Triassic fluid modification. The Weihai pelitic granulites might have a tectonic affinity with the North China Craton. Therefore, the Paleoproterozoic pelitic granulites were mechanically drawn into the orogen during the Triassic continental collision, and subsequently were remoulded by the fluids during its exhumation.  相似文献   

4.
<正>Granites are a common type of felsic intrusive rock that are composed of quartz,feldspar and micas as rock-forming minerals.They are the final products of the high-temperature,magmatic,predominantly endogenic,chemical differentiation of the earth(Clarke,1996).As the most abundant plutonic rocks in the continental crust,granites may form in various tectonic settings such as marginal arc,collisional orogen and intraplate rifting(Barbarin,1999).Granites are widespread in fossil orogens of China,through Archean to Cenozoic in time.In keeping with the general  相似文献   

5.
It is generally believed that trondhjemitic rock, an important component of TTG rocks, is the anatectic product of mafic rocks. However, in many TTG gneiss terranes, for instance, the granulite facies terrane in Eastern Hebei, trondhjemites occur as small dikes, intrusions or leucosomes in tonalitic gneisses, suggesting their origin of in-situ partial melting. Based on the petrological analysis of a tonalitic gneiss sample from Eastern Hebei, in combination with zircon U-Pb dating, we investigated the petrogenesis of trondhjemite through simulating anatectic reactions and the major and trace element characteristics of the product melt at different pressures(0.7, 1.0 and 2.0 GPa). The results indicate that hornblende dehydration melting in a tonalitic gneiss at 0.9–1.1 GPa and 800–850°C, corresponding to the high-T granulite facies, with melting degrees of 5–10wt.% and a residual assemblage containing 5–10wt.% garnet, can produce felsic melts with a great similarity, for instance of high La/Yb ratios and low Yb contents to the trondhjemitic rocks from Eastern Hebei. However, the modelled melts exhibit relatively higher K2 O, and lower CaO and Mg~# than those in the trondhjemitic dikes and leucosomes from Eastern Hebei, suggesting that the leucosomes may not only contain some residual minerals but also be subjected to the effect of crystal fractionation. The zircon U-Pb dating for the tonalitic and trondhjemitic rocks in the Eastern Hebei yields a protolith age of 2518±12 Ma and a metamorphic age of 2505±19 Ma for the tonalitic gneiss. The latter age is consistent with a crystallization age of 2506±6 Ma for the trondhjemitic rock, confirming a close petrogenetic relation between them.  相似文献   

6.
The Shapinggou porphyry molybdenum(Mo) deposit, located in Jinzhai County, Anhui Province, China, is the largest in the Qinling-Dabie Mo Metallogenic Belt. The intrusive rocks in the Shapinggou Mo ore district formed in the Yanshanian can be divided into two stages based on zircon U-Pb dating and geochemical features. This study focuses on the late stage intrusions(quartz syenite and granite porphyry), which are closely genetically related to molybdenum mineralization. Petrographic observations identified two quartz polymorphs in the quartz syenite and granite porphyry, which were derived from the same magmatic sources and similar evolutionary processes. The quartzes were identified as a xenomorphic β-quartz within quartz syenite, while the quartz phenocrysts within the granite porphyry were pseudomorphous b-quartz, characterized by a hexagonal bipyramid crystallography. The pseudomorphous b-quartz phenocrysts within the granite porphyry were altered from b-quartz through phase transformation. These crystals retained b-quartz pseudomorph. Combined with titanium-inzircon thermometry, quartz phase diagrams, and granitic Q-Ab-Or-H_2O phase diagrams, it is suggested that the quartz syenite and granite porphyry were formed under similar magmatic origins, including similar depths and magmatic crystallization temperatures. However, the β-quartz within quartz syenite indicated that the crystallization pressure was greater than 0.7 GPa, while the original b-quartz within the granite porphyry was formed under pressures between 0.4 and 0.7 GPa. The groundmass of the granite porphyry which formed after the phenocryst indicated a crystallizing pressure below 0.05 GPa. This indicates that the granite porphyry was formed under repetitive and rapid decompression. The decompression was significant as it caused the exsolution of the ore-forming fluids, and boiling and material precipitation during the magmatic-fluid process. The volumetric difference during the phase transformation from b-quartz to β-quartz caused extensive fracturing on the granite porphyry body and the wall rocks. As the main ore-transmitting and ore-depositing structures, these fractures benefit the hydrothermal alteration and stockwork-disseminated mineralization of the porphyry deposit. It is considered that the pseudomorphous β-quartz phenocrysts of the porphyritic body are metallogenic indicators within the porphyry deposits. The pseudomorphous β-quartzes therefore provide evidence for the formation of the porphyry deposit within a decompression tectonic setting.  相似文献   

7.
Polycrystalline quartz ribbons in high-grade metamorphic rocks from the Daqingshan region, are typi- cal microfabrics of, and provide information for, deep crust deformation and metamorphism. The quartz ribbons have straight boundaries and extend stably along gneissosity. They truncate other mineral grains in the rocks and may contain inclusions of such minerals that are lens-shaped and oriented. They frequently end into branching termination. Analysis fluid inclusions in polycrystalline quartz rib- bons reveal that the complex types of fluid inclusions are inhomogeneously distributed. They are ob- viously different from inclusions captured at granulite facies, in both fluid compositions and T-P esti- mations. Based on microfabric and fluid inclusion analysis, the polycrystalline quartz ribbons are suggested to be formed by SO2-rich fluids filling micro-fractures that are parallel to early gneissosity. The SO2 composition is derived from the deformed host rocks. The fluid phase has significant effects on the rheological characteristics, fracturing of rocks, and formation of quartz ribbons.  相似文献   

8.
There are several Pan-African granitoid plutons widely distributed in Shillong Plateau,NE India.Nongpoh(506.7 ± 7 Ma) and Mylliem(480-430 Ma) plutons were chosen for the petrological and geochemical study to constrain their petrogenesis.Nongpoh pluton consists of coarse-grained porphyritic quartz-monzogranite(NQM)and Mylliem pluton consists of medium to coarse-grained porphyritic granite(MG).The constituent minerals are K-feldspar,plagioclase,quartz,and biotite in both granitoids with accessory minerals of hornblende,zircon,sphene,and ilmenite.Both NQM and MG are metaluminous to weakly peraluminous(A/CNK=0.82-1.06),exhibit varied ranges of SiO_2(NQM:58.4-64.9 wt%;MG:66.9-69.9 wt%),and display a clear compositional gap in the Harker variation diagram.NQM contains higher abundances of CaO,MgO and Fe2O_3~t and similar K_2O and total alkali contents compared to MG.They have distinctive geochemical features typical of highly fractionated I-type granitoids such as higher abundances of K_2O,Al_2O_3,MgO,CaO,Al_2O_3+ CaO 15 wt% and A/CNK 1.1,low P_2O_5 content,enrichment in LILE,depletion in HFSE and HREE and highly fractionated REE patterns with moderate Eu anomalies,implying magma generation in a post-collisional extension setting and thinning induced asthenosphere upwelling,accompanied by the partial melting of the overlying enriched lithospheric mantle.The multi-element diagram of both NQM and MG shows pronounced negative anomalies at Ba,Nb,Sr,P,Zr,and Ti which implies a major role of crystal fractionation in their petrogenesis.High concentrations of Th,U,and Pb in the granitoid types point to additional involvement of crustal components in their generation.However,MGs with more pronounced positive spikes at U,Th,and Pb compared to NQM in multi-element diagram suggests the involvement of more felsic crustal material.The observed geochemical features of the granitoid types thus suggest that they are genetically unrelated to each other and their parental magmas were modified during magmatic differentiation processes.We suggest that the NQM and MG were formed as a result of fractional crystallization of compositionally diverse hybrid magmas produced due to mingling and mixing of an enriched lithospheric mantle-derived melts with lower crust-and middle crust-derived melts respectively during a period of extension late in the cycle of PanAfrican orogeny.  相似文献   

9.
Neoproterozoic igneous and metamorphic complexes occur as tectonic domes in the Longmen Mountains of the western margin of the Yangtze Block, and are important in reconstructing the Rodinian supercontinent and constraining the timing and mechanism of tectonic denudational processes. The Pengguan dome consists of granitic intrusions and metamorphic rocks of the Huangshuihe Group and is tectonically overlain by ductilly deformed Sinian to Paleozoic strata. The plutonic intrusions consist of granites with abundant amphibolite enclaves. New LA-ICP-MS zircon U-Pb dating yielded an emplacement age of 809±3 Ma and a protolith age of 844±6 Ma for the granite. The granitic rocks have geochemical signatures typical of A-type granites, indicating their formation under an extensional environment, by melting of newly formed tonalite-trondhjemite-granodiorite (TTG) rocks. A detachment fault, characterized by variable ductile shear deformation of S-C fabric and ESE-ward kinematics, separates the Pengguan dome from the Sinian-Paleozoic cover. 40Ar/39Ar dating of muscovite from the mylonite in the detachment fault of the dome demonstrates that ductile deformation occurred at ~160 Ma. This study indicates the existence of a Neoproterozoic magmatic arc-basin system, which was denudated by a Jurassic middle crustal ductile channel flow along the Longmenshan thrust belt.  相似文献   

10.
Clay mineralogy, texture size and statistical analyses were carried out on surface sediments from the continental shelf of Chennai, Bay of Bengal, India. The purpose of this study is to characterize the clay mineral distribution and its relation to the hydrodynamics off Chennai to identify the sources and transport pathways of the marine sediments. Characterization of clay minerals in coastal sediments by Fourier Transform Infrared （FTIR） spectroscopy has provided the association of quartz, feldspar, kaolinite, chlorite, illite and iron oxides （magnetite and hematite） derived from river catchments and coastal erosion. Kaolinite, chlorite, illite, iron oxides, and organic matter are the dominant minerals in Cooum, and Adayar region. High quartz and feldspar zones were identified in Marina, which are being confined the sand zone and paralleling the coast. The strong relationships among the wave energy density, sand, quartz and carbonate revealed that wave induced littoral drift system play a dominant role in transportation and deposition of sediments in the Chennai coast. The sediment texture and minerals data are in agreement well with the previous results of hydrodynamics and littoral drift models in this region. Multivariate statistical analyses （correlation, cluster and factor analyses） were carried out and obtained results suggested that clay minerals and organic matter are trapped in silt and clay particles, whereas quartz, feldspar and carbonate are associated with sand particles. Results of sediment sources and transport processes from this study will be useful to predict the fate of the pollutants released from land or the potential change in sediment delivery to coastal areas.  相似文献   

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