| As the southern part of Altaids or “Central Asian Orogenic Belt”,the Beishan Orogenic Collage(BOC)was formed by continuous docking of multiple microcontinents,island arcs,ophiolitic belts and accretionary complexes,which contributed to Phanerozoic continental growth in Central Asia.The Gongpoquan arc,located in the middle BOC,is separated from the Huaniushan arc to the south by HongliuheXichangjing ophiolite mélange and Mingshui-Hanshan terrane to the north by Shibanjing-Xiaohuangshan ophiolite mélange.Large amounts of felsic to mafic igneous rocks with long timespan were emplaced in the Gongpoquan arc and their closely associations make Gongpoquan arc an ideal place to study the evolution of arc and the crust-mantle interaction.By analyzing the geochronology,geochemistry of the Paleozoic granitoids and combining with regional tectonic,sedimentary and other geological records,we can define the arc magmatic evolution,explore the crust-mantle interaction and reconstruct the schematic tectonic evolution of the BOC.Based on our field work,granitic plutons located in Jianshan,Shibanjing,Baitoushan and Haergen areas and Mazongshan mafic rocks in the Gongpoquan arc are choosed for study.The Baitoushan muscovite granite and Haergen two-mica granite are two of peraluminous granitic plutons in the Gongpoquan arc.Zircon LA-ICPMS U-Pb dating yields emplacement ages of ca.409–395 Ma and 408.5 ± 3.1 Ma for the Baitoushan and Haergen plutons,respectively.Both the granitic plutons are strongly peraluminous with A/CNK ratios of 1.10~1.20,indicative of S-type affinities.The rocks are characterized by high Si O2(>70.0 wt.%)and K2O(>4.27 wt.%)contents with high Ca O/Na2 O ratios(0.2~0.4).Moreover,the rocks possess low Mg O contents(0.25~0.82 wt.%),Rb/Sr(1.3~2.1)and Rb/Ba(0.2~0.5)ratios,together with their relatively high initial 87Sr/86 Sr ratios(0.7139–0.7152)and less radiogenic ?Nd(t)values(-3.15 to-5.17),implying a clay-poor and plagioclase-rich crustal source.Zircon U-Pb dating yields Early Devonian emplacement age of 403.8±2.6 Ma for the Mazongshan mafic rocks.The pluton consists of coarse grain troctolite in the central while fine grain complex with inhomogeneous structure outside.Combining with the structure and composition of minerals,the coarse troctolites with equilibrium composition are probably formed in a stable setting.Furthermore,the coarse troctolites have evolved Sr-Nd isotope compositions((87Sr/86Sr)i = 0.7079~0.7080;?Nd(t)=-2.56 ~-2.76).In contrast,the petrography of the fine grain complex shows that the olivines,pyroxenes are xenolitic origins,and the fine grain complex probably underwent rapid crystallization,which led to the caputure of the olivines(Fo = 63~97),clinopyroxenes(Mg# = 51~89),orthopyroxenes(Mg# = 89~93)by the plagioclase-rich melt(plagioclase composition: An = 30~82).In addition,the variable Sr-Nd isotope compostions with crustal and mantle signatures((87Sr/86Sr)i = 0.7042~0.7066;?Nd(t)=-0.55 ~ +5.22)suggest a strong crust-mantle interaction.Zircon LA-ICPMS U-Pb ages of Jianshan pluton and Shibanjing pluton are 363.4 ± 2.9 Ma and 362.6 ± 3.0 Ma,respectively.Jianshan granodiorites show similar characteristics to classical adakites,e.g.,high Si O2(> 56 wt.%),Al2O3(> 15 wt.%),Sr contents(> 400 ppm)and Na2O(Na2O > K2O)values,Sr/Y(60~88),(La/Yb)N(7.73~15.5)ratios,but low Y(8.15~11.2 ppm)and Yb(0.57~1.01 ppm)values with weak positive Eu anomalies(?Eu = 1.04~1.27),while Shibanjing granites don't show adakitic characteristics.Both of the two plutons are enriched in large ion lithophile elements(LILEs),depleted in high field strength elements(HFSEs,e.g.,Nb,Ta).By synthesizing regional data,the author suggests that the formation of Jianshan and Shibanjing plutons are related to the southward subduction of Xiaohuangshan ocean,and Jianshan adakitic rocks are formed by melting of oceanic crust.The two plutons have evolved Sr-Nd isotope compositions(Jianshan:(87Sr/86Sr)i(0.7057~0.7058)and ?Nd(t)(-0.4 ~-1.1);Shibanjing:(87Sr/86Sr)i(0.7070~0.7071)and ?Nd(t)(-2.9 ~-2.6),indicating some crustal contamination.Based on the rock assemblages and geochemical compositions,three pulses of magmatism have been recognized in the Gongpoquan and Huaniushan arcs,i.e.,(1)the Middle Ordovician to Late Silurian basalts and I-type granitoids,(2)the Early Devonian S-and A-type granitoids and(3)the Late Devonian to Early Carboniferous I-type granitoids.Compared with earlier pulse of arc-related magmatism(ca.470–423 Ma),the latter pulse of magmatism(ca.424–395 Ma)consists mainly of “normal granite” characterized by higher Si O2(> 66 wt.%)and K2 O contents,weaker fractionated REE patterns and lower ?Eu values,and gabbroic to dioritic intrusions are only sporadic.Moreover,the granitoids of the latter pulse show variable but more crust-like Sr-Nd isotopic compositions((87Sr/86Sr)i = 0.7038–0.7327;?Nd(t)=-6.70 ~ +0.33)than the earlier ones((87Sr/86Sr)i = 0.7024–0.7080;?Nd(t)=-2.56 ~ +8.86),indicating that the Early Devonian(ca.424–395 Ma)experienced extensive crustal melting with minor involvement of mantle materials.Considering Early Devonian geological evidence,the transition from I-to S-and A-type magmatism probably reflects the Early Devonian amalgamation between the Gongpoquan and Huaniushan arcs,which caused not only regional unconformity and strong deformation-metamorphism,but also extensive melting of the accreted crustal materials in the BOC.By the Middle to Late Devonian,Beishan was likely under extensional tectonic setting which led to the mafic magmatic activity and was followed by the Late Devonian to Early Carboniferous subduction. |
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