The Early Paleozoic Volcanic Rocks And Charnockites In South China: Implications For The Magma Origin And Crystallization-Differentiation Processes In Intracontinental Orogeny

The early Paleozoic orogen in South China Block(SCB),which is considered to be one of the few examples of an intraplate orogen in the world,is characterized by an angular unconformity between preDevonian deformed rocks and Devonian strata as well as the absence of the Silurian strata in Cathaysia block and eastern Yangtze block.Meanwhile,this orogeny also produces voluminous early Paleozoic silicic igneous rocks which are pervasive and planar-shaped distributed in Cathaysia block and eastern Yangtze block.Previous works are mainly focused on the intrusive rocks,whereas the coeval volcanic rocks and charnockites are poorly constrained and studied,which limits our knowledgements about the early Paleozoic magmatism in SCB.Here,combining with previous data,the early Paleozoic volcanic rocks and garnet-bearing charnockites are choosen as examples.We thus conducted a detailed petrological,zircon U-Pb-Hf isotopic,whole-rock geochemistry,thermodynamic modeling and mineral thermobarometers study of early Paleozoic volcanic rocks and charnockites in SCB.In order to better understand the evolutional process and tectonic environment of magmatism as well as reveal the intrinsic connections between this early Paleozoic tectonothermal event and contemporary magmatism.Early Paleozoic volcanic rocks are mainly exposed in the southwest of the intracontinental orogen of the SCB at Mashan,Hekou and Chayuanshan.Zircon U–Pb age data show that these rocks formed at445–435 Ma,coeval with large-scale intrusive magmatism(450–430 Ma).These volcanic rocks could be divided into two groups,one group is Mashan and Hekou dacites and rhyolites,show high Si O2,low Mg O and low Fe2O3 contents.Whole-rock trace-element and isotopic compositions of the felsic volcanic rocks suggest that they were generated by partial melting of a Paleoproterozoic crustal component.Another group is Chayuanshan basalts,which are characterized by low Si O2,high Mg O,Cr and Ni contents,enrichment in LILE and depletion in HFSE.The low Nb/La ratios,high Th/Yb ratios and negative whole-rock ?Nd(t)values suggest that the basalts were derived from the metasomatized subcontinental lithospheric mantle(SCLM).The ‘‘subduction signature”(calc-alkaline affinity,enriched LILE and depleted HFSE)of the Chayuanshan basalts was supposed to be inherited from the source and it didn't reflect their generation in a subduction-related arc setting.Asthenospheric mantle upwelling and basaltic magma underplating may have been responsible for the partial melting of the crust and the metasomatized SCLM,which produced the Hekou and Mashan dacites and rhyolites and Chayuanshan basalts,respectively.For the intensive and extensive magmatism in SCB,we consider the following three factors are necessary: activation of the pre-existing suture zones,asthenospheric mantle upwelling and extensive partial melting of the crust.The Yunlu garnet-bearing charnockite is located at the central part of the Gaozhou complex,Yunkai terrane.It intruded the coeval gneiss migmatite and anatexis granite as a lenticular body in outcrops with an area of ~11 km2.Besides,the norite and quartz jotunite is distributed at the marginal of the charnockite pluton.Petrography,orthopyroxene-garnet-plagioclase-quartz thermobarometry,rhyolite-MELTS modeling and fluid inclusions analyses suggest that the magma was emplaced at a pressure of 600 MPa(~20 km),a temperature of > 900 °C,and an initial H2 O content of ~4.0 wt% with rare CO2 components.Two feldspar thermometry,rhyolite-MELTS modeling combined with previous experimental data suggest that the Yunlu magma is finally solidified at H2O-saturated(“wet”),~630 ?(“cold”)conditions.This recognition is different from metaluminous charnockite,which usually solidifies at H2O-unsaturated(“dry”)and > 800 ?(“hot”)conditions.The orthopyroxene-garnet,biotite-garnet,and biotite-orthopyroxene thermometers yield a consistent temperature range of 770-820± 60 °C,which is significantly higher than the H2O-saturated solidus temperature of ~630 ± 50 °C estimated from experimental results and two-feldspar thermometry.These results indicate that the early crystallized minerals(e.g.garnet,orthopyroxene and some euhedral biotite)of the Yunlu charnockite equilibrate at higher temperatures with crystallinities of ~30-45 %,rather than the H2O-saturated solidus conditions.We thus propose a hypothesis of melt extraction at 780-820 °C in a deep-seated,slowly cooling,partially crystalline magma reservoir.The melt extraction physically segregates the early crystallized minerals from residual interstitial melts and inhibits element diffusion equilibration between these minerals and interstitial melts,which result in the mafic minerals record the temperature of melt extraction(~780-820 °C).This hypothesis is also supported by the petrographic and geochemical evidence of the charnockite.Our study shows that melt extraction recorded in granites can be identified by combining micro-texture,mineral thermometry and rhyolite-MELTS modeling,which further provides quantitative insights into the fractionation process of silicic magmas.The Yunlu norite-quartz jotunite-charnockite association has identical emplacement age of ~430Ma.Petrography and mineral chemistry analyses show that the different rock types have same mineral assemblages and similar mineral compositions,e.g.garnet(almandine),orthopyroxene(hypersthene),biotite(Fe-rich biotite),plagioclase(andesine),k-feldspar,quartz and ilmenite.From norite to charnockite,they show linear variations on major-and trace-elements Haker diagrams.In addition,all rock types have similar and enriched whole Sr-Nd and zircon Hf isotopic compositions.Based on the above recognitions,we consider the Yunlu norite-quartz jotunite-charnockite association shares the same protolith,i.e.ancient(Paleoproterozoic)meta-sedimentary rocks,and the crystallization differentiation process in magma chamber controls the chemical variation of different rock types.The Yunlu noritequartz jotunite-charnockite association shows more mafic compositions compare to meta-sedimentary derived experimental melts,which suggest a cumulate origin.This observation could be also approved by petrographic characteristics,the ferromagnesian minerals and plagioclase + quartz exhibit clear cumulus textures in norite and charnockite,respectively.Therefore,we suggest that the Yunlu noritequartz jotunite-charnockite association is a series of basic-intermediate-silicic cumulates.The compositional variations are controlled by the mineral assemblages and proportions of crystal-melt separation under different crystallinities of Yunlu magma.Combined with mass balance calculation and thermodynamic modeling,the geochemical modeling results show that the norite is formed at ~15%crystallinity of the parental magma,which is an extremely low value.This crystallinity range is obviously lower than the popular viewpoint,as the crystal-liquid separation in silicic magma takes place most efficiently within a crystallinity of ~40-70 %.We consider that the hindering settling and compaction are main physical mechanisms that responsible for the crystal-liquid separation.The calculation results reveal both processes could be finished within ~0.1 Ma,which is comparable or less than the lifespan of the middle-upper crustal magma chambers with intermediate volumes.Lastly,this study shows that the norite could be formed by the crystal accumulation of crustal-derived silicic magma without the involvement of mantle-derived basic magma,which has important implications for the petrogenesis of norite.We consider that the early Paleozoic volcanic rocks and charnockites are formed in a post-orogenic extension environment in intra-continental orogen and the volcanism points to a ~445 Ma for the initial orogen collapse and post-orogenic extension.Asthenospheric mantle upwelling and basaltic magma underplating may have been responsible for the partial melting of the crust and the metasomatized SCLM,which produced the Chayuanshan basalts and Hekou and Mashan dacites and rhyolites,respectively.The high emplacement temperature(900 °C)and pressure(600 MPa)of the Yunlu charnockite require a high geothermal gradient(> 45 °C/km)in the Yunkai terrane,in which the melting temperature in the source(lower crust)exceeds 900 °C,also indicate a crustal anatexis result from the basaltic magma underplating.