Geochemistry of lower crustal xenoliths from Neogene Hannuoba basalt, North China craton: implications for petrogenesis and lower crustal composition
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Fractional crystallization (geology)
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Laboratory measurements of compressional and shear wave velocity to confining pressures of 600 MPa for a suite of representative samples collected from the Pikwitonei granulite belt and God's Lake domain, an Archean crustal cross section in the northwestern Superior Province, provide the basis of comparison of these terranes with the seismic characteristics of Archean lower crust. We found that felsic rocks in the Pikwitonei granulite belt and God's Lake domain, which make up the bulk of these terranes, have a similar average compressional wave velocity of 6.5 km/s at 600 MPa, indicating that felsic rocks show little velocity change across the amphibolite–granulite facies transition. Compressional wave velocities for mafic rocks from each terrane are between 7.1 and 7.3 km/s. Apparent Poisson's ratio ranges from 0.24 to 0.26 and 0.26 to 0.28 for felsic and mafic rocks, respectively. These velocity data compare favorably with data for similar lithologies from the Kapuskasing uplift. Using the relative abundances of the constituent lithologies, the weighted average compressional wave velocities of the God's Lake domain and Pikwitonei granulite belt at 600 MPa are 6.56 and 6.63 km/s, respectively. These values, coupled with velocity distribution functions based on the population statistics and relative abundance for each lithology, show that there is no correspondence between the seismic characteristics of the Pikwitonei granulite belt and typical Archean and Proterozoic lower crust. The average properties of the Pikwitonei granulite belt and God's Lake domain, however, correspond well with typical Archean and Proterozoic middle crust. This suggests that either the Pikwitonei granulite belt represents an extreme felsic end member of Archean lower crust or that the deepest levels of the Superior Province crust are not exposed in the Pikwitonei granulite belt. Similar distribution function diagrams for acoustic impedance show that the Pikwitonei granulite belt is characterized by high acoustic impedance contrasts, but the high-impedance component is low in abundance. If the strong reflections observed under the Pikwitonei granulite belt in recent Lithoprobe surveys are not due to other causes, such as favorably oriented bodies of metamorphosed banded iron formation, diabase, or rock units not exposed in this region but present at depth, then they are caused by surprisingly small volumes of mafic metavolcanic rocks.
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Abstract High‐pressure granulites are generally characterized by the absence of orthopyroxene. However, orthopyroxene is reported in a few high‐pressure, felsic–metapelitic granulites, such as the Huangtuling felsic high‐pressure granulite in the North Dabie metamorphic core complex in east‐central China, which rarely preserves the high‐pressure granulite facies assemblage of garnet + orthopyroxene + biotite + plagioclase + K‐feldspar + quartz. To investigate the effects of bulk‐rock composition on the stability of orthopyroxene‐bearing, high‐pressure granulite facies assemblages in the NCKFMASHTO (Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –Fe 2 O 3 ) system, a series of P – T – X pseudosections based on the melt‐reintegrated composition of the Huangtuling felsic high‐pressure granulite were constructed. Calculations demonstrate that the orthopyroxene‐bearing, high‐pressure granulite facies assemblages are restricted to low X Al [Al 2 O 3 /(Na 2 O + CaO + K 2 O + FeO + MgO + Al 2 O 3 ) < 0.35, mole proportion] or high X Mg [MgO/(MgO + FeO) > 0.85] felsic–metapelitic rock types. This study also reveals that the X Al values in the residual felsic–metapelitic, high‐pressure granulites could be significantly reduced by a high proportion of melt loss. We suggest that orthopyroxene‐bearing, high‐pressure granulites occur in residual overthickened crustal basement under continental subduction–collision zones and arc–continent collision belts.
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According to the thermal structure of the lithosphere, the continental crust is located within the range of the MBL (mechanical boundary layer) and its heat transfer is controlled by thermal conductivity of the rocks in the crust. Therefore, based on the theory of thermal conductivity and the same thermophysical parameters as those obtained in previous studies, we calculated the thermal state of the crust underplated by basaltic magma. The result suggests that a felsic magma layer, thinner than 250 m, would be generated at 850℃ by the emplacement of a basalt magma sill (1200℃, 500 m thick) into continental crust during a very short period (≤ 2700 a). Geological and geochemical characteristics and isotopic chronological data of the bimodal volcanic rock association in the Changpu Formation of the Yutian Group in the Longnan-Xunwu area of Southern Jianxi suggest that the rhyolite of this association might be derived directly from partial melting of the upper crust caused by emplacement of quartz tholeiite magma. The Mesozoic continental crust in SE China (≤50 km in thickness) was located in the MBL and the amount and formation age of the felsic magma generated by the underplating of basaltic magma would be constrained by heat conductivity of the rocks in the crust. Granite and rhyolitic rocks dominate (90%) the Mesozoic igneous rock in SE China, while basaltic rocks are minor with their formation ages different from those of felsic rocks. Therefore, the formation of the extensive felsic igneous rocks in SE China may have not been caused by the underplating of basalt.
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A first-order approximation of the lithological make-up of an orogen's middle and lower crust can provide insights into its structure, as well as the tectono-metamorphic and geodynamic processes taking place there. In this study, we investigate the possible lithological and chemical composition of Taiwan's middle and lower crust by matching in situ physical properties measured by the TAIGER tomography data with isotropic wavespeeds, density, and major element composition for a variety of upper amphibolite and granulite facies rocks modelled at ambient pressure and temperature using the AbersHacker Macro. The modelling suggests that Taiwan's middle crust is possibly comprised of some combination of biotite-poor metapelite, garnet-poor felsic granulite, mafic granulite, amphibolite, and marble. The lower crust is likely comprised of mafic granulite, garnet-rich felsic granulite, biotite-free metapelite, and eclogite. Furthermore, the modelling shows that the modal abundance of garnet and/or sillimanite has a significant effect on physical properties, elevating seismic wavespeeds and density of felsic rocks to those of mafic rocks. The modelled wt% major oxide composition suggests that Taiwan's middle and lower crust have a more mafic chemical composition than that of global compilations of the continental crust. Nevertheless, this reflects the choices made when assigning rock types for the lithological mix used to calculate the wt% oxides, since increasing the percentage of garnet-rich metapelite and felsic granulite would result in a more felsic bulk composition.
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