The Carboniferous volcanic rocks in the Wulungu-Luliang area are mainly andesites of medium- to high-K calc-alkalic series. Volcanic rock samples have relatively high alkali(Na2O + K2 O = 4.7 % to 6.8 %) and low Ti O2...The Carboniferous volcanic rocks in the Wulungu-Luliang area are mainly andesites of medium- to high-K calc-alkalic series. Volcanic rock samples have relatively high alkali(Na2O + K2 O = 4.7 % to 6.8 %) and low Ti O2contents(0.7 % to 0.9 %), relatively high Mg O(2.5 % to 3.4 %) and Mg#(49.9 % to 67.1 %), high rare earth element(REE) contents, and relatively high K2 O contents(1.7 % to 3.1 %). Chondrite-normalized REE patterns show light REE enrichment((La/Yb)N= 4.15 to 5.19)with weak Eu anomalies(d Eu = 0.75 to 0.92). These samples are enriched in large-ion lithophile elements but relatively depleted in high field strength elements. The trace elements and REE patterns are similar to those of Setouchi and central Ryukyu high-Mg andesites, indicating a highMg andesite source. Relatively high Y contents(16.7 to24.4 ppm), and relatively low Sr/Y ratios(17.2 to 38.8) and Ti O2contents(0.7 % to 0.9 %) exclude the possibility of slab melting. Low Sr/Nd(16.6 to 42.8), Ba/Th(66.4 to266.8), and U/Th(0.2 to 0.3) indicate that the influence of slab-derived fluids is low. The ratios of Ce/Th(4.9–7.3), Ce/Pb(1.8–4.2), Ba/Rb(7.99–22.03), Ba/Th(66.4–266.8), and La/Sm(3.6–4.3) are similar to ratios found in subducting sediment melts. Relatively high ratios of K/Nb(1357–3258),Th/La(0.28–0.42), Zr/Nb(8.8–27.1), and especially Th/Nb(0.48–1.25) suggest that the magma was assimilated and contaminated by upper continental crust. These characteristics, along with the ratios of La/Yb, Sc/Ni, Th/Yb, Ta/Yb,Ce/P2O5, and Zr/Ti O2, demonstrate that the earlier Carboniferous volcanic rocks in the Wulungu-Luliang area were generated in a continental island-arc setting.展开更多
文摘The Carboniferous volcanic rocks in the Wulungu-Luliang area are mainly andesites of medium- to high-K calc-alkalic series. Volcanic rock samples have relatively high alkali(Na2O + K2 O = 4.7 % to 6.8 %) and low Ti O2contents(0.7 % to 0.9 %), relatively high Mg O(2.5 % to 3.4 %) and Mg#(49.9 % to 67.1 %), high rare earth element(REE) contents, and relatively high K2 O contents(1.7 % to 3.1 %). Chondrite-normalized REE patterns show light REE enrichment((La/Yb)N= 4.15 to 5.19)with weak Eu anomalies(d Eu = 0.75 to 0.92). These samples are enriched in large-ion lithophile elements but relatively depleted in high field strength elements. The trace elements and REE patterns are similar to those of Setouchi and central Ryukyu high-Mg andesites, indicating a highMg andesite source. Relatively high Y contents(16.7 to24.4 ppm), and relatively low Sr/Y ratios(17.2 to 38.8) and Ti O2contents(0.7 % to 0.9 %) exclude the possibility of slab melting. Low Sr/Nd(16.6 to 42.8), Ba/Th(66.4 to266.8), and U/Th(0.2 to 0.3) indicate that the influence of slab-derived fluids is low. The ratios of Ce/Th(4.9–7.3), Ce/Pb(1.8–4.2), Ba/Rb(7.99–22.03), Ba/Th(66.4–266.8), and La/Sm(3.6–4.3) are similar to ratios found in subducting sediment melts. Relatively high ratios of K/Nb(1357–3258),Th/La(0.28–0.42), Zr/Nb(8.8–27.1), and especially Th/Nb(0.48–1.25) suggest that the magma was assimilated and contaminated by upper continental crust. These characteristics, along with the ratios of La/Yb, Sc/Ni, Th/Yb, Ta/Yb,Ce/P2O5, and Zr/Ti O2, demonstrate that the earlier Carboniferous volcanic rocks in the Wulungu-Luliang area were generated in a continental island-arc setting.
