Anatexis Of Metapelites And Genesis Of S-type Granites In Helanshan

The Helanshan complex is located in the western part of the Khondalite Belt,North China Craton?NCC?,and is mainly composed of khondalites and S-type granites,which are important to the tectonic evolution of the Paleoproterozoic Khondalite Belt.Although many important results on the politic granulites and S-type granites in the Helanshan complex have been achieved,there still some questions,such as the metamorphic evolution and tectonic siginificance of medium-pressure pelitic granulites in this region;the relationship between the genesis of S type granites and orogenic process,which need to be further clarified.In this paper,we have carried to a combined study of petrography,electron probe analysis,phase equilibrium modelling,zircon U–Pb dating,whole-rock major and trace element analyses,and whole-rock Nd isotopes on the medium-pressure pelitic granulites and S-type granites,in order to reveal the metamorphic evolution characteristics and metamorphic time of medium-pressure pelitic granulites,and the geochemical characteristics and genesis of S-type granites in Helanshan complex,and this implications on the tectonic evolution of the Khondalites belt.The main results are summarized as follow:?1?Based on petrographic observations,the metamorphic evolution stages of this kind of pelitic granulites can be divided into four stages?M₁–M₄?,of which the M₁–M₃ stages are suprasolidus and the M₄ was formed by the cooling across the solidus.The peak metamorphic stage?M₁?is represented by the rim of garnet porphyroblasts along with inclusions of sillimanite+quartz±biotite,and rutile relicts+plagioclase+K-feldspar in the matrix;the M₂is defined by the ilmenite that has partially replaced rutile;and the M₃ is characterized by the replacement of garnet by cordierite.Phase equilibrium modelling defines P–T conditions of9.6–10.2 kbar at 825–830°C for the M₁ stage,6.2–10 kbar at 805–835°C for the M₂ stage,5.5–6.6 kbar at 795–835°C for the M₃ stage,and<6.2 kbar at<820°C for the M₄ stage.A clockwise P–T evolution is defined for the MP pelitic granulites in the Helanshan complex,involving isothermal decompression followed by nearly isobaric cooling.SHRIMP zircon U–Pb dating yielded a metamorphic age of 1931±21 Ma,constraining the time of the nearly IBC process.?2?The 1.95 Ga S-type granites of the Helanshan is a typical peraluminous granites?A/CNK=1.15–1.25?.The S-type granites have relatively high variable content of SiO₂,ranging from 64.49 to 74.07 wt%,and relatively high TiO₂?0.14–0.91 wt.%?,Al₂O₃?13.0–15.60 wt.%?,K₂O?4.99–5.93 wt.%?,Fe₂O₃^T?1.10–1.47 wt.%?and MgO?0.33–0.38 wt.%?contents,but low Na₂O/K₂O ratios?0.37–0.81?.The rocks are enriched in LREE and relatively flat HREE,and with the negative to positive Eu anomaly.Whole-rock Nd isotope analysis shows that the pelitic granulites and 1.95 Ga S-type granites in Helanshan have similar values of?_Nd?t?,–1.62^–0.85 and–0.98⁰.11 respectively,indicating that the 1.95Ga S-type granites was formed by the anatexis of regional metapelites,without the addition of mantle material.?3?These data of granites from this study and literature in Helanshan complex suggest that the S-type granites varied widely of compositions and higher Fe+Mg than experimental glasses.Geochemical modelling of major elements shows that the change of composition is related to the capture of ferromagnesian minerals?such as garnet,ilmenite and biotite?from the restite of the source region into the melt.?4?Previos studies have pointed out that the formation of the 1.93–1.90 Ga S-type granites are closely associated with the addition of mantle material,and it's isotopic value of?_Nd?t?is higher than that of the 1.95 Ga S-type granites in Helanshan.Thus,we think that the change of the value of?_Nd?t?of S-type granites can be used to indicated different orogenic stages in the whole evolution process of orogenic belts.That is to say,the 1.95 Ga S-type granites were formed by anatexis of metapelites in the process of syn-collision,while the formation of 1.93–1.90 Ga S-type granites accompanied by the addition of mantle materials indicated that the orogenic belts entered the post-collision extension stage and accompanied by local upwelling of mantle material.?5?The continent-continent collision between the Yinshan and Ordos Terranes at ca.1.95 Ga,metapelites underwent granulite facies anatexis and formed high-medium pressure pelitic granulites and 1.95 Ga syn-collision S-type granites during this process;and subsequent slab breakoff led to extention of the orogen and rapid uplift accompanied by local upwelling of mantle material,which resulted in decompression melting and/or heating melting of metapelites again?due to mantle magmatic underplating and intrusion?and the formation of1.93–1.92 Ga ultrahigh temperature granulites and 1.93–1.90 Ga post-collision S-type granites.