Petrogenesis Of Early Mesozoic Peraluminous Charnockite In South China:From Source Partial Melting To Shallow Magma Chamber Crystallization

Exposed granites of continental crust have micro-to crust-scale compositional diversities,reflecting the diversities of magma sources,magmatic processes and the involved intensive parameters.It thus remains challenging towards a unified model to understand granite formation and its role in the differentiation of continental crust.An important perspective is to decipher the detailed magma processes combining mineralogical and petrological characterizations with experimental and modeling findings.The early Mesozoic Qinzhou Bay Granitic Complex(QBGC)from South China,containing abundant rock types,i.e.orthopyroxene-bearing granite(the Jiuzhou pluton)and subvolcanic rock and their coexisting restitic granulite enclaves,provides an example to understand the processes and conditions of granitic magma formation and crystallization.The orthopyroxene crystals in the granite and subvolcanic rock are present either as euhedral-subhedral grains or as anhedral grains that have been partially reacted to be biotite.These crystals are different from those crystals in the granulite enclaves in both textures(crystal shape and size)and compositions(mostly Al2O3 content),thus should be of magmatic origin.The orthopyroxene-bearing peraluminous granite thus can be named as peraluminous igneous charnockite,which rarely outcrops and differs from the more widely reported metaluminous charnockite in mineral assemblages,whole-rock compositions and magma fluid compositions.In this dissertation,detailed mineralogical and petrographical characterizations combining thermodynamic modeling are used to estimate the P-T-XH2O conditions of magma formation and crystallization,to understand the phase relationship and preservation of orthopyroxene in granitic magma,to decipher the detailed magma crystallization processes involved in the formation of peraluminous charnockite,and to shed new lights on the petrogenesis of the widely outcropped granites in continental crust.The restitic granulite enclaves hosted by the peraluminous charnockite show two-stage metamorphic mineral assemblages.Peak assemblage is produced by biotite-dehydration melting reaction at 905±15 ? and 0.68±0.03 GPa,and retrograde assemblage is formed as a result of isothermal decompression to pressure of 0.10-0.26 GPa with the ascent and emplacement of magma.Mineral compositions of the charnockite record the cooling of magma,i.e.XMg values(XMg= Mg/(Fe+Mg))of the orthopyroxene decrease from 0.52 to 0.35,corresponding to magma crystallization conditions varying from 880±30 ? and 2.8±0.5 wt%melt H2O content to 750 ?±30 ? and 5.6±0.5 wt%melt H2O content.The charnockite completely solidifies at lower temperature(?650±30 ?)and higher melt H20 content(?5.8 wt%,water-saturated at 0.2 GPa),i.e.at relatively "cold" and "wet" conditions.Such crystallization conditions are different from the relatively "hot" and "dry" conditions(i.e.>800 ? and<3 wt%melt H2O content)of the metaluminous chanrockite.The charnockite at stratigraphically low levels of the Jiuzhou pluton gradually changes to relatively evolved orthopyroxene-free granite at stratigraphically high levels.The charnockite and part of orthopyroxene-free granite in the Jiuzhou pluton have more mafic whole-rock compositions than metasediment-sourced experimental melt.The upward-deflected plagioclase crystal size distributions(CSD)for large grain size(>3.8 mm)in the charnockite,and step-zoned plagioclase and alkali feldspar crystals in the charnockite and in the orthopyroxene-free granite suggest open-system fractionation by melt extraction,crystal partial dissolution,and subsequent crystallization from trapped minimum melt.Crystal cluster and chain fabrics and viscous deformation are more abundant in the charnockite than in the overlying orthopyroxene-free granite,suggesting that gravitational,compaction-driven fractionation increased towards the bottom of the pluton.Field observation,petrographic characterization and thermodynamic modeling suggest that gravitation-driven compaction reduces trapped melt fraction of the Jiuzhou crystal mush with thickness of ?100 meters from>30 wt%at the upper levels to?10 wt%at the lower levels.Significant melt extraction restricted back-reaction with high-temperature phases during progressive crystallization,which preserved orthopyroxene during the solidification of granitic magma.The Jiuzhou pluton from the charnockite to the upper orthopyroxene-free granite shows vertical,pluton-scale compositional zoning,which advocates incremental growth of cumulate as a result of incremental fractionation.Incremental fractionation may occur in many zoned granite plutons worldwide,causing their whole-rock compositions to deviate from their primary melt compositions.The majority of exposed peraluminous granites in South China is orthopyroxene-free,which have comparable K2O/Na2O and Rb/Sr ratios and zircon saturation temperature with the orthopyroxene-bearing granite in the QBGC,indicating production by fluid-absent melting.Melting experiments suggest that fluid-absent melting produces high temperature and moderately H2O-poor melts,i.e.temperature>800 ? and melt H20 content<4-5 wt%.Such melts have the potential to crystallize orthopyroxene,indicating that orthopyroxene may be a common phase at the early stage of magma crystallization.The reason why outcrop of peraluminous charnockite is rare may be that the primary magma of exposed granite is high-silica(mostly larger than?70 wt%),which cannot preserve orthopyroxene when melt extraction occurs at crystallinities of>40 wt%;for the low-silica magma,if no melt extraction occurs during magma crystallization,orthopyroxene will be completely resorbed by the low-temperature,H2O-and alkali-rich intersitial melt.