Crystallization and melt extraction of a garnet-bearing charnockite from South China: Constraints from petrography, geochemistry, mineral thermometry, and rhyolite-MELTS modeling

Abstract

It is common for granitic rocks in high-grade terranes to undergo amphibolite-granulite facies metamorphic overprint, thus recovering magmatic records from the metamorphic modification remains a major challenge. Here, we report an early Paleozoic, garnet-bearing Yunlu charnockite, which outcropped in the Yunkai terrane of the Cathaysia block from South China, and underwent amphibole-grade metamorphic overprint in the late Devonian. Field observation, micro-texture and mineral geochemistry combining with diffusion modeling constrain that the metamorphic overprint with extremely short-duration of ~0.2-0.5 Ma only influences a narrow rim of < 100 μm for most minerals. The magmatic information is able to be retrieved combining with rhyolite-MELTS modeling, mineral thermobarometry using mineral core compositions to quantitatively estimate magmatic pressure, temperature and melt H 2 O contents. Rhyolite-MELTS modeling results are evaluated by comparison with experimentally-determined phase relations for a peraluminous granite with ~69.83 wt.% SiO 2 at a pressure of ~500 MPa. The comparison results suggest that the modeling reproduces phase relationships of feldspars and quartz within 20-60 °C below 7.0 wt% melt H 2 O, but fails to work properly for all phases at melt H 2 O contents higher than 7.0 wt%. The modeling results using reconstructed primary magma composition of the Yunlu charnockite combining with the orthopyroxene-garnet-plagioclase-quartz thermobarometry and fluid inclusion analyses suggest that the magma was emplaced at pressure of ~600 MPa, temperature of >900 °C, and initial H 2 O content This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Geochemical variation in biotite from the Devonian South Mountain Batholith, Nova Scotia: Constraints on emplacement pressure, temperature, magma redox state and the development of a magmatic vapor phase (MVP)

“…The corona and symplectites observed are suggestive of subsolidus reaction textures such as those observed in metamorphosed and hydrothermally altered granitoids (cf. Rubbo and Bruno, 2003;Zhang et al, 2021) and do not imply that the minerals involved were coeval in the original magmatic rock.…”