摘要
本文对出露于勉略缝合带南侧碧口地区的阳坝岩体进行了系统的岩石学、锆石U-Pb年代学和地球化学研究研究,重点讨论了阳坝岩体的岩石成因、成岩物质来源及其地质意义。岩体的主体岩性为花岗闪长岩,其中广泛发育代表岩浆混合作用的暗色微粒包体。锆石LA-ICP-MS定年结果表明阳坝花岗闪长岩的成岩年龄为215．4±8．3Ma,晚于秦岭造山带的主造山期。地球化学特征上,寄主花岗闪长岩显示部分埃达克岩的地球化学特征,具体表现为SiO2≥56％,Al2O3>15％, Na2O>K2O,Mg#(50．8-54．5)>47,富集LILE和LREE,Sr>900μg/g,Sr／Y比值(65-95)>65,负Eu异常不明显(δEu=0．84 -0．89),亏损HREE、Y(Y=9．51-14．5μg／g,Yb=0．74-1．20μg／g,Y／Yb=11．12-15．10),REE强烈分异((La／Yb)N= 22．18-29．51),但是,花岗闪长岩相对高的K2O含量和HREE相对平坦的特征更类似于中国东部中生代C型埃达克岩,暗示其可能是由加厚基性下地壳脱水部分熔融形成的,岩石的高Mg#暗示其受到地幔物质混染。暗色微粒包体显示钾玄岩的地球化学特征,具体表现为SiO2≤63％,σ(4．54-6．18)>3．3,K2O含量(4．22％-6．04％)高,大多数样品K2O／Na2O>1;在K2O -SiO2图中,所有样品均落入钾玄岩区域,暗色微粒包体强烈富集LILE和LREE及明显的Nb、Ta和Ti负异常暗示其可能起源于曾经受到俯冲流体交代的富集地幔。Harker图解上,寄主花岗闪长岩和暗色微粒包体的主量和微量元素表现出混合成因的演化趋势,表明岩体可能是富集的岩石圈地幔发生部分熔融产生的基性岩浆和其所诱发的加厚下地壳酸性岩浆混合的产物。结合秦岭地区已有研究成果,本文的研究认为阳坝岩体的形成可能代表了西秦岭地区在秦岭主造山晚期或造山期后发生的下地壳的拆沉作用和幔源岩浆的底侵作用。
The Yangba pluton, located in Bikou area, southern margin of Mianlue Suture,central China,were formed during middle Indosinian. The data of occurrence, petrography, geochemistry and zircon U-Pb chronology are reported, the source and geological significance of the pluton are discussed in this paper. Lithologically, this pluton consists mainly of granodiorite with mafic microgranular enclaves. Under microscope, acicular apatite in the mafic microgranular enclaves suggested quenching textures. Zircon LA-ICP-MS dating for the granodiorites yields an age of 215.4±8.3Ma, indicating they were formed during the late-orogenic or post-collisional stage（ ≤242±21Ma） of south Qinling Mountain Belt. Geochemically, the granodiorites have the partial characteristics of adakites,with SiO2≥56% , high Al2O3 content （ 〉 15% ） , Na2O 〉 K2 O, Mg^# （ 50.8 - 54. 5 ） 〉 47, and enriched in LREE and LILE, Sr 〉900μg/g, Sr/Y ratio（65 -95 ） 〉 65, slightly negative Eu anomaly（ δEu = 0.84 -0.89 ） , depleted in HREE, Y and high field strength element（HFSE） （Y =9.51 -14.5μg/g, Yb =0.74- 1.20μg/g, Y/Yb = 11. 12 -15. 10）, obvious differenatiation between LREE and HREE（ （La/Yb） N = 22.18 - 29.51 ）. However, the geochemical features of high concentration of K2O （ 3.22% - 3.84% ） and flatter HREE pattern are more similar to the C-type adakites from eastern China of Mesozoic, which imply they were originated from dehydration melting of thickened mafic lower continental crust（LCC） , the feature of high Mg^# demonstrates that they were contaminated by mantle-sourced materials. The mafic microgranular enclaves are characterised by shoshonite with SiO2 ≤63% , σ（4. 54 -6.18） 〉3.3, high K2O content（4.22% -6.04% ）, most of the samples＇ K2O/Na2O〉 1 ; in the K2O - SiO2 diagram, all the samples pohted in the shoshonite area, enriched in LILE and LREE, with obvious Nb, Ta negative anomaly, these geochemical features indicated that the dioritic magmas were derived from an enriched mantle, which maybe once metasomatized by subduction fluids. The variation trends of major and trace element between enclaves and host rocks suggest a mixing and mingling process in their petrogenesis. On a whole,Integrated geological and geochemical data suggested the Yangba granodiorite and their mific microgranular enclaves were generated by mixing of enriched mantle-derived magma and thickened lower crustal felsic magma which was triggered by the former. Combined with previous studies of Qinling Mountain Belt, the Yangba granodiorites were likely to represent underplated mantle-derived magmas activities and delamination of lower crust during the late orogenic stage in Western 0inling area.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2005年第3期697-710,共14页
Acta Petrologica Sinica
基金
本文由国家自然科学基金重点资助项目(批准号:40234041)西北大学地质学系国家基础科学人才培养基地创新基金资助项目(批准号:XDCX03-04)联合资助
