| Located on the border of Hunan province and Jiangxi province,the exposed area about 1400 km2,with massive,porphyritic or porphyraceous structure,Wanyangshan batholith is Caledonian multi-stage magmatism,with all the granites having massive structure and porphyritic texture.The granites are composed of biotite two-micmonzogranite(principal part),biotite granodiorite and two-mica monzogranite.Based on the investigation of field work,we make the best use of advanced technology and methods.We studied the items in detail,which include isotope geology,petrography,geochemistry,geochronology and other aspects of Wanyangshan batholiths and we discussed the geological background of Caledonian movement in Southern China.Based on analyzing the interior structures of zircons by the CL technology and in-situ zircon U-Pb dating of a biotite two-mica monzogranite sample,the authors obtained the 16 points of zircon 206Pb/238 U weighted average ages of(446±3.4)Ma(MSWD= 0.15)(95% confidence degree),suggesting crystallization age of the batholith.The main mineral components of granites are plagioclase,K-feldspar,quartz,and the secondary minerals are biotite.The result indicates that it formed during late Ordovician to Early Silurian the Southern China Caledonian magmatism.Geochemical analyses showed that the Wanyangshan batholith has a SiO2 content of 65.91% to 73.35%(an average 69.50%),a K2 O content of 3.19%~5.36%(an average 4.20%),(Na2O+K2O)content of 5.90 to 7.88%,the average 6.76%.The average of K2O/Na2 O,Al2O3 and ASI are 1.64,13.81% and 1.09%.The TFeO index(average 4.55%)content is relatively high and the magnesium iron ratio(F/F+M)change scope is releatively low,ranging from 0.67 to 0.73,mainly in about 0.70.The MALI index scope is much higher,ranging from 2.72% to 6.90%.The samples of Wanyangshan batholith belong to high potassium calc alkaline peraluminous granite.Trace elements such as Ba,K,Sr,P,Ti showed significant losses,but Rb,Th,U,Nb,Zr and Hf are relatively enriched.The ratio of Rb/Sr(average of 1.91%)and Rb/ Nb(average of 14.48%)are higher and the granite intrusion has moderate?REE(159.71×10-6~262.78×10-6).It is enriched in LREE and depleted in HREE(LREE/HREE=6.16~10.01),(La/Yb)N=6.37~12.17,with more obvious right deviation curve and negative Eu anomaly,ranging from 0.30 to 0.59(average 0.45).The Wanyangshan granite has a range of initial strontium from 0.71223 to 0.72509,and a range of ?Sr(t)from 117.5 to 299.9,?Nd(t)ranges from-9.39.1 to-7.30,with the depleted mantle model age of 1.77~1.95 Ga.Which is the same as the value of the background of granite in Hunan inland(1.8~2.4 Ga).All these geochemical characteristics point to S-type granitiod and suggest that the granites might have come from crust,whose source rock is from the middle upper crust metamorphic clastic rock.Multiple kinds oxide diagrams and trace elements diagrams for discrimination of structural environment show that the granites were formed in a postcollisional tectonic setting.Combined with the previous research results on the Southern China Caledonian movement,we conclude that the formation mechanism of Caledonian granites in Southern China that is affected by the global plate movement.At the same time Cathaysian plate was combined with the Yangtze plate along QinzhouHangzhou fault zone,which made the Southern China basin of shallow water or trough close.Then the movement put Cathaysian plate and the Yangtze plate together again,resulting in a large area of the crustal thickening.This crustal thickening can make the elements redistribute in space,which can produce moer calories,and centered on the thickening area.As a result,the upper crust melt partly in sufficient pressure and high temperature.Finally,the magma intruded in the intracontinental extension and the S type granite formed in the area of Southern China.Although the plate movement did not lead to magmatic incursion directly,whose remote effects of plate collision led to tectonic events within the plate indirectly. |
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