Thermomagmatic evolution of Mesoproterozoic crust in the Blue Ridge of SW Virginia and NW North Carolina: Evidence from U-Pb geochronology and zircon geothermometry
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New geologic mapping, petrology, and U-Pb geochronology indicate that Mesoproterozoic crust near Mount Rogers consists of felsic to mafic meta-igneous rocks emplaced over 260 m.y. The oldest rocks are compositionally diverse and migmatitic, whereas younger granitoids are porphyritic to porphyroclastic. Cathodoluminescence imaging indicates that zircon from four representative units preserves textural evidence of multiple episodes of growth, including domains of igneous, metamorphic, and inherited origin. Sensitive high-resolution ion microprobe (SHRIMP) trace-element analyses indicate that metamorphic zircon is characterized by lower Th/U, higher Yb/Gd, and lower overall rare earth element (REE) concentrations than igneous zircon. SHRIMP U-Pb isotopic analyses of zircon...Keywords:
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Petrogenesis
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Geochronology
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Underplating
Adakite
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Quartz monzonite
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Numerous intrusive bodies of mafic–ultramafic to felsic compositions are exposed in association with volcanic rocks in the Late Permian Emeishan large igneous province (ELIP), southwestern China. Most of the granitic rocks in the ELIP were derived by differentiation of basaltic magmas with a mantle connection, and crustal magmas have rarely been studied. Here we investigate a suite of mafic dykes and I-type granites that yield zircon U-Pb emplacement ages of 259.9 ± 1.2 Ma and 259.3 ± 1.3 Ma, respectively. The εHf(t) values of zircon from the DZ mafic dyke are –0.3 to 9.4, and their corresponding TDM1 values are in the range of 919–523 Ma. The εHf(t) values of zircon from the DSC I-type granite are between –1 and 3, with TDM1 values showing a range of 938–782 Ma. We also present zircon O isotope data on crust-derived felsic intrusions from the ELIP for the first time. The δ18O values of zircon from the DSC I-type granite ranges from 4.87‰ to 7.5‰. The field, petrologic, geochemical and isotopic data from our study lead to the following salient findings. (i) The geochronological study of mafic and felsic intrusive rocks in the ELIP shows that the ages of mafic and felsic magmatism are similar. (ii) The DZ mafic dyke and high-Ti basalts have the same source, i.e., the Emeishan mantle plume. The mafic dyke formed from magmas sourced at the transitional depth between from garnet-lherzolite and spinel-lherzolite, with low degree partial melting (<10%). (iii) The Hf-O isotope data suggest that the DSC I-type granite was formed by partial melting of Neoproterozoic juvenile crust and was contaminated by minor volumes of chemically weathered ancient crustal material. (iv) The heat source leading to the formation of the crust-derived felsic rocks in of the ELIP is considered to be mafic–ultramafic magmas generated by a mantle plume, which partially melted the overlying crust, generating the felsic magma.
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The Hercynian, post-collisional Karkonosze pluton contains several lithologies: equigranular and porphyritic granites, hybrid quartz diorites and granodiorites, microgranular magmatic enclaves, and composite and lamprophyre dykes. Field relationships, mineralogy and major- and trace-element geochemistry show that: (1) the equigranular granite is differentiated and evolved by small degrees of fractional crystallization and that it is free of contamination by mafic magma; (2) all other components are affected by mixing. The end-members of the mixing process were a porphyritic granite and a mafic lamprophyre. The degree of mixing varied widely depending on both place and time. All of the processes involved are assessed quantitatively with the following conclusions. Most of the pluton was affected by mixing, implying that huge volumes (>75 km3) of mafic magma were available. This mafic magma probably supplied the additional heat necessary to initiate crustal melting; part of this heat could have also been released as latent heat of crystallization. Only a very small part of the Karkonosze granite escaped interaction with mafic magma, specifically the equigranular granite and a subordinate part of the porphyritic granite. Minerals from these facies are compositionally homogeneous and/or normally zoned, which, together with geochemical modelling, indicates that they evolved by small degrees of fractional crystallization (<20%). Accessory minerals played an important role during magmatic differentiation and, thus, the fractional crystallization history is better recorded by trace rather than by major elements. The interactions between mafic and felsic magmas reflect their viscosity contrast. With increasing viscosity contrast, the magmatic relationships change from homogeneous, hybrid quartz diorites–granodiorites, to rounded magmatic enclaves, to composite dykes and finally to dykes with chilled margins. These relationships indicate that injection of mafic magma into the granite took place over the whole crystallization history. Consequently, a long-lived mafic source coexisted together with the granite magma. Mafic magmas were derived either directly from the mantle or via one or more crustal storage reservoirs. Compatible element abundances (e.g. Ni) show that the mafic magmas that interacted with the granite were progressively poorer in Ni in the order hybrid quartz diorites—granodiorites—enclaves—composite dykes. This indicates that the felsic and mafic magmas evolved independently, which, in the case of the Karkonosze granite, favours a deep-seated magma chamber rather than a continuous flux from mantle. Two magma sources (mantle and crust) coexisted, and melted almost contemporaneously; the two reservoirs evolved independently by fractional crystallization. However, mafic magma was continuously being intruded into the crystallizing granite, with more or less complete mixing. Several lines of evidence (e.g. magmatic flux structures, incorporation of granite feldspars into mafic magma, feldspar zoning with fluctuating trace element patterns reflecting rapid changes in magma composition) indicate that, during its emplacement and crystallization, the granite body was affected by strong internal movements. These would favour more complete and efficient mixing. The systematic spatial–temporal association of lamprophyres with crustal magmas is interpreted as indicating that their mantle source is a fertile peridotite, possibly enriched (metasomatized) by earlier subduction processes.
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Fractional crystallization (geology)
Igneous differentiation
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