挥发份对高硅岩浆演化趋势的制约:以东南沿海白垩纪晚期花岗岩类岩石为例

The role of volatile in the evolution of high silicic magma:Insights from the Late Cretaceous granitoids in the coastal area of southeastern China

东南沿海分布大面积的白垩纪晚期侵入岩。这些岩石可分为两期:其中115~100Ma以钙碱性系列岩石为主,岩石组合为辉长岩-闪长岩-花岗闪长岩-二长花岗岩-碱性长石花岗岩;而100~86Ma的岩石为碱性系列,岩石组合为石英二长斑岩-正长斑岩-碱性长石花岗岩。115~100Ma的辉长岩以角闪辉长岩为主,具有极高的CaO、MgO和Al_(2)O_(3)含量,具有极低的SiO_(2)(42.9%~53.8%)、全碱(K_(2)O+Na_(2)O:0.86%~5.28%)、Ba、Nb、Th、Rb和Zr含量,也具有极低的FeO^(T)/MgO、La/Yb和Zr/Hf比值,较高的Eu/Eu^(*)、Sr/Y比值和Sr含量,为基性-超基性堆晶岩。与辉长岩同期的闪长岩和细粒暗色包体具有较高的SiO_(2)(50.34%~63.68%),较低的CaO、P_(2)O_(5)、MgO、Al_(2)O_(3)含量,相对低的Eu/Eu^(*)和Sr/Y比值,变化较大的La/Yb和Zr/Hf比值,代表了从基性岩浆储库中抽取的富硅熔体。115~100Ma的花岗闪长岩和二长花岗岩类岩石为准铝质岩石,SiO_(2)含量变化较大(61.7%~75.3%),具有较低的FeO^(T)/MgO、Ga/Al比值和Nb、Zr及Nb+Zr+Ce+Y元素含量,显示出典型I型花岗岩的特征。这些花岗岩具有相对高的La/Yb、Eu/Eu^(*)和Zr/Hf比值和高的Sr、Ba和Zr含量。结合岩相学特征,这些花岗岩为堆晶花岗岩。而115~100Ma的碱性长石花岗岩具有极高的SiO_(2)含量(大于75%),低的Eu/Eu^(*)、La/Yb、Zr/Hf和Sr/Y比值,具有低的Ba、Sr和Zr含量和高的Rb、Nb、Y和Th含量和Rb/Sr比值,表明这些花岗岩是由富硅岩浆储库中抽离的高硅熔体侵入地壳形成。100~86Ma期间形成的二长斑岩和正长斑岩具有极高的全碱含量,可以达到8%~12%,其SiO_(2)主要集中在60%~70%,具有极高的Zr、Sr和Ba含量和Eu/Eu^(*)、La/Yb和Sr/Y比值,显示出堆晶花岗岩的特征。而100~86Ma期间形成的大部分碱性长石花岗岩具有极高的SiO_(2)含量(大于75%),并显示出A型花岗岩的特征,具有高的Rb/Sr比值和高的Rb、Y和Th和低的Ba、Sr含量和低的Zr/Hf、La/Yb、Eu/Eu^(*)和Sr/Y比值,表明它们是由富硅岩浆储库抽离的高硅熔体侵入浅部地壳形成。东南沿海高硅花岗岩的形成和穿地壳岩浆系统密切相关,高硅花岗岩是由浅部地壳内晶体-熔体分异产生的熔体侵入地壳所形成,而高硅花岗岩的地球化学特征与岩浆储库的水及挥发份含量密切相关。115~100Ma期间,从富水的岩浆储库抽离的熔体形成具有低高场强元素含量和低Rb/Sr比值的高硅花岗岩,这一过程与古太平洋板块俯冲有关;100~86Ma期间,从富挥发份的岩浆储库抽离的熔体形成碱性特征、富含高场强元素和具有高的Rb/Sr比值的高硅花岗岩,这一过程和古太平洋板块回撤软流圈上涌有关。

The Late Cretaceous intrusive rocks are widely distributed in the coastal area of southeastern China.These rocks can be divided into two groups:the calc-alkaline gabbro-diorite granodiorite-monzogranite-alkali feldspar granite suite aged in 115~100 Ma and the alkaline porphyritic monzonite-porphyritic syenite-alkali feldspar granite suite formed in 100~86 Ma.The 115~100 Ma gabbros which are mainly composed of hornblende gabbros,have relatively low SiO_(2)(42.9%~53.8%),alkaline,Ba,Nb,Th,Rb and Zr contents,high CaO,MgO,Al_(2)O_(3),and Sr contents,low FeO^(T)/MgO,La/Yb and Zr/Hf ratios,and high Eu/Eu^(*)and Sr/Y.Petrographic observation and geochemical characteristics suggested that these gabbros were cumulated rocks.The diorites and microgranular mafic enclaves,which were simultaneous with the gabbros,have high SiO_(2)(50.34%~63.68%)and low CaO,P_(2)O_(5),MgO,Al_(2)O_(3)contents and low Eu/Eu^(*)and Sr/Y ratios and variable La/Yb and Zr/Hf ratios.They represent the silicic melts that were extracted from a mafic crystal-rich mush.The 115~100 Ma monzogranites and granodiorites are metaluminous,have variably SiO_(2)(61.7%~75.3%)content.They have low FeO^(T)/MgO and Ga/Al ratios and low Nb,Zr and Nb+Zr+Ce+Y contents,displaying the features of I-type granite.They have high La/Yb,Eu/Eu^(*)and Zr/Hf ratios and Sr,Ba and Zr contents.Together with the petrographic features of the granodiorites and monzogranites,these geochemical characteristics indicates that they are the residual silicic cumulate of the crystal mush column.The 115~100 Ma alkali feldspar granites have extremely high SiO_(2)(>75%)contents,low Eu/Eu^(*),La/Yb,Zr/Hf and Sr/Yb ratios,low Ba,Sr and Zr contents,high Rb,Nb,Y and Th contents,and high Rb/Sr ratio.These features suggest that they were extracted from a shallow crustal crystal-rich mush.The 100~86 Ma porphyritic rocks have high alkali contents(8%~12%),low SiO_(2),high Zr,Sr and Ba,and low Eu/Eu^(*),La/Yb and Sr/Y ratios.Together with the petrographic features of the porphyritic rocks,these geochemical features suggest that these porphyritic rocks are cumulated rocks.The 100~86 Ma alkali feldspar granites have extremely high SiO_(2)(>75%)contents and show the features of A-type granite.They have extremely high Rb/Sr,high Rb,Y and Th,low Ba and Sr,and low Zr/Hf,La/Yb,Eu/Eu^(*)and Sr/Y ratios.These geochemical features suggest the high silicic granites were extracted from a shallow crystal mush.The formation of high silicic granites in the coastal area of southeastern China were closely related to the transcrustal magmatic system.The high silicic granites were extracted from a shallow crustal magma reservoir and their geochemical features were genetic related to water and volatile contents of the magma reservoir.The 115~100 Ma high silicic granites with low Rb/Sr and low high field strength elements contents were extracted from a water-rich magma reservoir,which was related to the subduction of paleo-Pacific plate;while the 100~86 Ma high silicic granites with extremely high Rb/Sr and high field strength elements were extracted from a volatile-rich magma reservoir,which was related to the upwelling of asthenosphere formed by back-arc extension.