赣南古家营盆地英安岩时代、成因及对早古生代构造演化的制约

Age and genesis of dacite in Gujiaying Basin,southern Jiangxi Province,and its constraints on Early Paleozoic tectonic evolution

华南板块早古生代的构造属性是长期存在争议的重大地质问题。以赣南古家营盆地英安岩作为研究对象,通过岩石学、野外实测剖面的测量、SHRIMP锆石U-Pb年龄、主微量元素及Sr-Nd-Pb-O同位素分析,对其岩石类型、地层归属、成因及构造环境进行了研究。结果表明:该火山岩具有富硅、富钾、低钠、贫镁的特点,属于高钾钙碱性英安岩;轻重稀土元素分馏明显(La/Yb=5.52~15.08),富集大离子亲石元素Rb、Th、K,亏损高场强元素Ta、Nb及呈现明显的负Eu异常,显示出火山弧的特征;SHRIMP锆石U-Pb年龄分析结果为439.0±4.0 Ma,与南迳英安岩、安山岩年龄一致,均为晚奥陶世末期—早志留世初期岩浆活动的产物,古家营剖面与南迳盆地乔子山标准剖面及中寨标准剖面相似,为一套灰白色英安岩、流纹岩、火山碎屑岩、浅变质岩系组合,在地层上归属于南迳组;负的ε_(Nd)(t)值(-6.5~-6.3)、较小的(^(87)Sr/^(86)Sr)_(i)值(0.70624~0.70704)、较年轻的T_(DM2)(1685~1703 Ma),以及较高的(^(206)Pb/^(204)Pb)_(i)、(^(207)Pb/^(204)Pb)_(i)、(^(208)Pb/^(204)Pb)_(i)值,指示古家营英安岩为中元古代地壳物质部分熔融形成。古家营英安岩形成于与俯冲有关的岛弧环境,赣南—粤北地区在晚奥陶世—早志留世存在洋陆俯冲作用。

The Early Paleozoic tectonic property of the South China Plate has been a major dispute for a long time.The dacite intrusive in the Gujiaying Basin in southern Jiangxi was selected as a case to study its rock type,strata attribution,genesis and structural environment based on petrology,field-measured profile measurement,SHRIMP zircon U-Pb chronology,major trace elements and Sr-Nd-Pb-O isotopic analysis.The analysis results show that the volcanic rock is rich in silicon and potassium,low in sodium,and poor in magnesium,and belongs to the high-K calc-alkaline dacite.Its light and heavy REE show obvious fractionation(La/Yb=5.52~15.08),and it is rich in lithophile elements Rb,Th,K,loss of high field strength elements Ta,Nb,with obvious negative Eu anomalies,showing the characteristics of volcanic arc.SHRIMP zircon U-Pb dating yields 439.0±4.0 Ma,similar to that of Nanying dacite and andesite,suggesting products of magmatic activity from the Late Ordovician to the Early Silurian.The Gujiaying section is similar to the Qiaozishan standard section and the Zhongzhai standard section in the Nanjing Basin.It is a set of gray-white dacite,rhyolite,volcanic clastic rock,and shallow metamorphic rock series,which belongs to the Nanjing Formation.The negative ε_(Nd)(t)value(-6.5~-6.3),lower(^(87)Sr/^(86)Sr)_(i) value(0.70624~0.70704),younger T_(DM2)(1685~1703 Ma),and higher values(^(206)Pb/^(204)Pb)_(i),(^(207)Pb/^(204)Pb)_(i )and(^(208)Pb/^(204)Pb)_(i),indicating that the Gujiaying dacite was formed by partial melting of the Mesoproterozoic crustal material.It is inferred that the Gujiaying dacite was formed in an island-arc environment related to subduction,and ocean-continent subduction existed in southern Jiangxi-northern Guangdong during the Late Ordovician-Early Silurian period.