Interaction between magmatism and deformation in the Mesozoic of the Lushan area,eastern China
The interaction between lithospheric deformation and magmatism is pivotal for understanding the structure and evolution of continents.However,the underlying mechanism remains elusive.This study presents an example of the coupling and feedback between magmatism and deformation during the Late Mesozoic in the Lushan area of eastern China.The Lushan area is composed of a Mesozoic Xingzi gneiss dome consisting of core,mantle,and cover.Numerous plutons and dikes are distributed in the core of the gneiss dome.The mantle is a 2-3 km thick contact metamorphic zone(sillimanite,staurolite,garnet,and mica)with migmatites in localities where strongly deformed zones(amphibolite facies)and ductile shear zones(greenschist facies)have developed from base to top.The cover of the gneiss dome is an unmetamorphosed and undeformed structural klippe named Hanyangfeng.We dated all types of plutons and dikes in the core.The results show that the emplacement ages of these intrusive rocks are essentially same within error(141±2 Ma),and no age difference is observed between the deformed and undeformed rocks.40Ar/39Ar dating of hornblende from the migmatite in the strongly deformed zone yielded a deformation age of 138.62±0.32 Ma.Interestingly,the timing of the deformation and metamorphism(approximately 140 Ma)of the Xingzi gneiss dome is similar to that of the magmatism.Another major geological feature of the Lushan area is highly fractionated magmatism in the core of the Xingzi gneiss dome.The rocks exhibit considerable variation,ranging from amphibolite gneiss,amphibolite,biotite hornblende diorite,quartz diorite,biotite monzogranite,alaskite,and pegmatite.Alaskite and pegmatite have significantly high Rb/Sr and low Zr/Hf values,indicating their highly fractionated nature.All intrusive rocks,irrespective of their silica content,display similar ages and a similar range of zircon Hf isotope ratios(εHf(t)=-10.5 to+2.2),suggesting a fractionation relationship between different rock types and a derivation of their primary basic magmas from an enriched lithospheric mantle.Zircon Hf isotope ratios from the intermediate-acidic plutons and dikes(εHf(t)=-10.5 to+0.8)are distinctly different from those of granitoids originating from the melting crust basement in South China(εHf(t)=-26.9 to-2.9),further strengthening the fractionation origin for various rock types from basic magma in the Lushan area.The synchronism of magmatism and deformation,close spatial association,and high fractionation evolution of magma in the Lushan area assisted in establishing a model in which magma-derived heat and large-scale deformation are genetically coupled and reciprocally influenced.Heats associated with deep-seated magma reservoirs and their derivatives may have caused regional metamorphism,forming a thermal aureole and progressive metamorphic zone,facilitating thrusting shearing,and forming a large-scale thrust nappe at the top of what is now Lushan Mountain(i.e.,the Hanyangfeng klippe).The extent of metamorphism and deformation requires a large hidden magmatic body at depth to account for the observed large thermal effects.The extremely diverse rock types of the Late Mesozoic magmatism,ranging from basaltic dikes to highly fractionated granites,require a long magma chamber duration,which is probably made possible by the thermal insulation of large-scale thrust nappe and ductile shear zones.The latter may have acted as a thermal discontinuity/barrier,slowing the cooling rate of the magma body.The more strongly the aureole is deformed,the longer the duration of the magma chamber,and vice versa.Thus,the Late Mesozoic geological evolution of the Lushan area is a type example of the interaction between magmatism and deformation,a mechanism that may be widely operative in both intraplate settings and orogenic belts.
LushanLate Mesozoicthrust nappegneiss domehighly differentiated magmatismdeep-seated magma and superficial structure
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