Petrogenesis Of Archean Shengousi Complexes In The Anshan Area

Anshan area is one of the very few palces where preserved the ancient Archeancrust and existed3.8to2.5Ga different times and types Archean rocks, whichcontinuous record the evolutionary history of the Archean crust. Anshan area is a classicplace for studying early crustal material composition and its evolution. The Shengousicomplexes in the Anshan area is an important outcrop where exist Eoarchean rocks, andas a microcosm of Archean crustal evolution. Accurately determining composition,geochronological framework and petrogenesis of the Shengousi complexes on theoutcrop scale which will contribute to understanding the tectonic-magmatic-thermalevolution processes of various Archean stages in the Anshan area.Shengousi complexes can be divided into four rock units base on petrology andfield occurrence relationships, namely trondhjemitic gneiss, trondhjemitic migmatites,fine-grained trondhjemite and granitic pegmatite respectively. Besides, trondhjemiticmigmatites includ banded biotite plagioclase gneiss, gray gneiss, light-coloredtrondhjemite and biotite schist. Trondhjemitic gneiss unit was cut and wrapped by otherrocks, indicating it is the first phase rock unit. The gradational contact，wrapped witheach other and syntectonic concordant occurrence fabrics among the different rocks oftrondhjemitic migmatites unit show characteristics of the migmatite. Fine-grainedtrondhjemite unit is characterized by fine-grained texture and massive structure whichconsistent with the Lishan trondhjemite. Granitic pegmatite is the last stage rock unitwhich cut and wrapped other rocks.We performed SIMS zircon U-Pb dating on representative samples of theShengousi complexes. The ages of trondhjemitic gneiss between3.77to3.81Ga， andcontain³.60and³.45Ga discordant ages. The ages of the different rocks of thetrondhjemitic migmatite have similar ages, ranging from3.35to3.31Ga, some samplecontain3.56³.61Ga inherited zircon. Based on geochronology datas，the Shensousicomplexes can be divided into three stage ages,3.81³.77,3.33³.31and³.13Garespectively. In addition，some ages of3.64³.60,³.55and³.45Ga should be thelead loss ages of3.8Ga zircon, recording the thermal metamorphism.Trondhjemitic gneiss has high contents of SiO₂, Al₂O₃and Na₂O and low K₂O,TFe₂O₃, MgO, CaO and TiO₂contents and Na₂O/K₂O>1, which indicate that thegneiss’s protolith typically is a trondhjemite. The trondhjemitic gneiss are characterized by right-slop chondrite-normalized rare earth element （REE） patterns and showedobvious light rare earth elements （LREEs） enrichment and high rare earth elements（HREEs） depletion. In addtion, most of the samples possess （La/Yb）Nvalue range from7to21, and the LREEs without strong fractionation, and with negative Eu anomalies（0.55to0.80）. Furthermore, the gneiss dpletion in Sr element, therefore, it is indicatethat the garnet is not to be the residual in the source region, but the amphibole andplagioclase are the main residual phase. The gnesis have a charateristics of negativeTa-Nb-Ti toughs show that contain Ti minerals （such as rutile） are also residual in thesource region. These rocks are low Sr, Sr/Y, Mg#, Cr and Ni contents show withoutinteraction with the mantle wedge in the formation process, which are significantdifferent from the Cenozoic adakites which formed by subducting of hot oceanic crust.Zircon saturation thermometer calculation results reveal that the trondhjemitic magmaformed at about800℃. Based on comparative study with the Archean tholeiitichigh-temperature and high-pressure experiment results， the trondhjemitic magmaformed in8¹0Kbar,800℃and contained3to4%H₂O, and the rocks melting degreeis about15%. Therefore, we argue the derived-mechanism of3.8Ga trondhjemite in theAnshan area is that partial melting of the hydrous basaltic rocks （amphibolites） in thebottom of the continental crust （or oceanic crust） that affected by Mantle magmaupwelling.The feature of the petrologic assemblage, geology and petrology of thePaleoarchean trondhjemitic migmatites show that they were formed by crustal anatexis.The light-colored trondhjemite, gray trondhjemitic gneiss and biotite schist correspondrespectively to the leucosome, residuum and melanosome, they have linear variationcharacteristics in the plot of the major elements. Besides, On the primitive mantle-normalized spider diagram, the trace elements show that enrichment of large ionlithophile elements （LILEs） and LREEs, depletion of Eu, Nb, Ta, P and Ti in thetrondhjemite and trondhjemitic gneiss, while enrichment of P, Ti and HREE, depletionof Eu, Th, Sr in the biotite schist. These three type rocks have varying degrees of thenegative Eu anomalies, suggest that they have inherited protolith nature. Thegeochemical covariant corresponds to the rule of element migration during the crustalanatexis, suggesting that the trondhjemitic migmatites experienced partial melting.Zircon saturation thermometer calculation results reveal that the anatexis occurred at ⁷30oC, producing the amounts of melt （¹5vol.%）. The comparison between themineral assemblage and the experimental petrology display that trondhjemiticmigmatites were formed by partial melting of the early stage TTG at2⁴kbar under thewater-saturated conditions. Biotite was not involved in the reaction of the partialmelting, and they were the residuals due to the loss of the melt, not involving theretrograde process.The evolution of crust in the Anshan area during Eoarchean-Paleoarcheanunderwent three stages:（1） the upwelling mantle magma cause partial melting of thehydrous basaltic rocks （amphibolites） in the bottom of the continental crust （or oceaniccrust） to form the3.8Ga trondhjemitic-tonalitic rocks；（2）3.8Ga trondhjemiteexpericend the3.66³.64、3.55and3.45Ga heating metamorphic events;（3） the earlierTTG rocks remelted to form the Paleoarchean trondhjemitic migmatites during3.33³.31Ga.