北祁连东段红土堡基性火山岩和陈家河中酸性火山岩地球化学特征及构造环境

Geochemical characteristics and tectonic environment of Hongtubu basalts and Chenjiahe intermediate-acid volcanic rocks in the eastern segment of North Qilian orogenic belt

北祁连东段天水-宝鸡地区早古生代火山岩包括红土堡基性熔岩和陈家河中酸性火山岩。地球化学分析结果表明,红土堡基性熔岩和陈家河中酸性火山岩中所夹基性火山岩的特征相似,属于拉斑玄武岩,两者TiO2含量较高(1.50%～2.73%),∑REE分别为65.97×10-6～133.46×10-6和78.04×10-6～175.55×10-6,LREE轻度富集[(La/Yb)N分别为2.00～4.40和2.71～4.40],Eu异常不明显或略具Eu负异常(δEu分别为0.85～1.10和0.85～0.99);两者LILE明显富集,HFSE(尤其是Ta、Nb)强烈亏损,Nb/La较低(0.28～0.43),显示出与岛弧拉斑玄武岩(IAT)的亲源性;同时,基性熔岩的εNd(t)均为正值(+2.22～+4.08),暗示其源区类似亏损地幔源区,Zr/Nb=17.21～36.33,Ce/Nb=5.73～8.17,又具有N-MORB的地球化学特征;两者的Sr、Nd、Pb同位素组成十分相似,显示出基性熔岩主要来自亏损地幔(DM)和富集地幔端员(EMⅠ或EMⅡ)混合源区,可能有少量地壳物质混染。陈家河中酸性火山岩属于钙碱性系列,∑REE=127.51×10-6～276.01×10-6,LREE显著富集[(La/Yb)N=4.79～13.51],多数具有弱负Eu异常(δEu=0.53～1.20);微量元素分配型式类似于岛弧花岗岩,具明显的Nb、Ta、Zr、Hf低谷。综合研究表明,北祁连最东段陈家河中酸性火山岩形成于岛弧环境,而其南部的红土堡玄武岩形成于弧间裂谷环境(或初始弧后盆地),是早古生代晚期北祁连东段岛弧带扩张向弧后盆地演化的早期产物。这些研究成果为秦祁结合部位沟弧盆体系的存在提供了佐证。

Early Paleozoic volcanic rocks in Tianshui and Baoji areas along the eastern segment of North Qilian orogenic belt consists of Hongtubu basaltic lavas and Chenjiahe intermediate-acid volcanic rocks. Geochemical analyses show that Hongtubu basaltic lavas are similar to the intercalated basalts in Chenjiahe intermediate-acid volcanic rocks in characteristics, both fallen in the tholeiite series with high TiO2 （1.50%- 2.73% ）. Their ∑REE are 65.97×10^-6-133.46×10^-6 and 78.04 ×10^-6-175.55×10^-6 respectively, both are slightly enriched in LREE [ （ La/Yb）N being 2.00 - 4.40 and 2.71 - 4.40 respectively ], and both have no obvious Eu anomaly or weak Eu negative anomaly（SEu being 0.85-1.10 and 0.85-0.99 respectively）. Basalts from two groups are typically characterized by selected enrichment of LILEs, low abundances of HFSEs relative to N- MORB, and prominent troughs of Nb and Ta, with low Nb/La ratio（0.28--0.43）, which indicates the affinity of these volcanic rocks to island arc tholeiite（IAT）. In addition, εNd（ t ）（ ＋ 2.22- ＋ 4.08）values of basalts suggest that their mantle sources are similar to the depleted mantle source. Zr/Nb = 17.21- 36.33 and Ce/Nb = 5.73- 8.17, implying geochemical characteristics of N-MORB. Sr, Nd and Pb isotopes from Hongtubu basalts are similar to those in Chenjiahe basalts in composition. In the diagrams of eNd （t）- （87 Sr/86 Sr）t, （^207pb/^204pb） t -（^206pb/^204pb）t, （^87Sr/^86Sr） t -（^206pb/^204pb） t and εNd（ t ）-（^206pb/^204pb） t for basalts, the data are fallen in the DM, EM I and EM ]1 areas, probably with a little crustal contamination, indicating that the magma of basalts might have had a mixed origin. The intermediate-acid volcanic rocks from Chenjiahe belong to the calc-alkaline series, they have relatively high abundances of REE（ ∑REE = 127.51× 10^-6～276.01 ×10^- 6）, and are significantly enriched in LREE[ （La/Yb） N = 4.79- 13.51 ]. Most intermediate-acid volcanic rocks show weak Eu negative anomaly （δEu= 0.53-1.20）. The trace element patterns of Chenjiahe acid volcanic rocks are similar to those of the ocean ridge granite （ORG）, with marked troughs of Nb, Ta, Zr and Hr. A synthetic study shows that the Chenjiahe intermediate-acid volcanic rocks in the eastern segment of North QilJan orogenic belt were formed in an island-arc setting, whereas the Hongtubu basalts were formed in an intra-arc rift setting （or an initial back-arc basin setting）, probably being products of the early spreading evolution of the island-arc system towards the back-arc basin in the eastern segment of North Qilian during late Early Paleozoic. The results obtained provide evidence for the existence of the trench-arc-basin system at the juncture of Qilian and North Qinling orogenic belts.