摘要
青藏高原冈底斯斑岩成矿带不同于经典的产于岛弧和大陆边缘的斑岩铜矿,而形成于后碰撞挤压向伸展转变期,显示了极好的成矿前景。本文对冈底斯中段尼木矿田白容、厅宫和冲江斑岩铜矿区斑岩体进行了系统研究,确定出斑岩体演化和侵入序列为:似斑状二长花岗岩→成矿二长花岗斑岩→石英闪长玢岩→花岗闪长斑岩。K-Ar和^(40)Ar/^(39)Ar年代学研究获得白容矿区似斑状二长花岗岩中角闪石的K-Ar年龄为16.9±2.4Ma;石英闪长玢岩中黑云母的K-Ar年龄为12.3±0.2Ma、^(40)Ar/^(39)Ar坪年龄为12.5±0.2Ma;花岗闪长斑岩中黑云母的K-Ar年龄为11.5±0.2Ma、^(40)Ar/^(39)Ar坪年龄为12.4±0.2Ma;厅宫矿区石英闪长玢岩中黑云母的K-Ar年龄为13.8±0.2Ma、^(40)Ar/^(39)Ar坪年龄为14.9±0.2Ma;花岗闪长斑岩中黑云母的K-Ar年龄为13.5±0.3Ma、^(40)Ar/^(39)Ar坪年龄为14.2±0.2Ma,这些年龄表明:石英闪长玢岩晚于似斑状二长花岗岩,略早于花岗闪长斑岩。成矿与二长花岗斑岩有关,其侵位时间晚于似斑状二长花岗岩,早于石英闪长玢岩和花岗闪长斑岩。尼木斑岩铜矿田这种复式杂岩体较充分的分异演化有利于含矿热液的集中与逐渐富集成矿。白容斑岩铜矿蚀变矿化二长花岗斑岩的蚀变绢云母的K-Ar年龄为11.8±0.2Ma,^(40)Ar/^(39)Ar坪年龄为12.0±0.1Ma,代表了中低温蚀变和矿化末期的年龄。白容矿区绢云母化带的蚀变年龄与石英闪长玢岩和花岗闪长斑岩的黑云母^(40)Ar/^(39)Ar年龄基本一致,与厅宫矿区辉钼矿Re-Os年龄及石英闪长玢岩和花岗闪长斑岩的黑云母^(40)Ar/^(39)Ar年龄同样基本一致,暗示两个矿区石英闪长玢岩和花岗闪长斑岩的岩浆结晶冷却与成矿二长花岗斑岩后期热液成矿时间上有重叠。结合前人年龄数据大致确定出白容矿区岩浆-热液活动时限为0.5～5Ma,厅宫为4Ma,冲江为4.5Ma。尼木矿田成矿斑岩^(40)Ar/^(39)Ar年龄晚于冈底斯碰撞后第一次快速隆升时间≈21Ma,15Ma冈底斯中段NS向正断层开始活动,表明含矿斑岩体可能侵位于地壳加厚、冈底斯山大规模隆升到一定程度后出现弱伸展环境的构造背景下,即斑岩铜矿形成于从南北向挤压隆升到东西向伸展初始发育的过渡构造背景。
Gangdese porphyry copper metallogenetic belt, distinct from classical porphyry copper deposits that were occurred in island arc and continent margin setting, were formed in post-collision setting and showed considerable prospective. This article systematically researches the porphyries of Bairong and Tinggong porphyry deposits from Nimu metallogenetic cluster, and confirms its evolving and intrusive sequence from monzogranite to ore-forming monzogranite porphyry to quartz diorite porphyrite to granodiorite porphyry. The K-Ar age for phenocrystic hornblende in porphyroid monzogranite from Bairong copper deposit is 16. 9 ± 2. 4Ma. The K-Ar ages for phenocrystic biotites in quartz diorite porphyrite and granodiorite porphyry are 12. 3 ± 0. 2Ma and 11.5 ± 0. 2Ma from Bairong porphyry deposit, 13. 8 ± 0. 2Ma and 13.5 _± 0. 3Ma from Tinggong porphyry deposit, correspondingly, its 4~Ar/39Ar plateau ages are 12. 5 ± 0. 2Ma and 12.4 ± 0. 2Ma, 14. 9 _± 0. 2Ma and 14. 2 ± 0. 2Ma respectively. These ages indicate quartz diorite porphyrite formed after porphyroid monzogranite, whereas before granodiorite porphyry. The ore-forming process is related to monzogranite porphyry, which the forming age is later than porphyroid monzogranite and slightly earlier than quartz diorite porphyrite and granodiorite porphyry. Sufficient differentiation and evolvement of the complexes contribute to the concentration of ore-forming fluid and gradually enrichment to ore in Nimu metallogenetic cluster. The K-Ar age and ^40Ar/^39Ar plateau age for hydrothermal sericites from altered monzogranite porphyry respectively yield 1 1.8 ± 0. 2Ma and 12.0± 0. 1Ma in Bairong porphyry copper deposit, and represent the forming age of medium-low temperature alteration and late-mineralization, and its alteration age was consistent with the ^40Ar/^39Ar plateau ages for phenocrystic biotites in quartz diorite porphyrite and granodiorite porphyry from Bairong porphyry copper deposits and published molybdenite Re-Os age was consistent with the ^40Ar/^39Ar plateau ages for phenocrystic biotites in quartz diorite porphyrite and granodiorite porphyry from Tinggong porphyry copper deposit, which imply that the magma cooling and crystallization of quartz diorite porphyrite and granodiorite porphyry from Bairong and Tinggong porphyry copper deposits overlapped temporally with hydrothermal mineralization. Combined with the published ages, the duration limit of Bairong porphyry deposit approximately range from 0. 5 to 5Ma, and -4Ma for Tinggong porphyry deposit, - 5Ma for Chongjiang. The emplacement of porphyries sequence from Nimu metallogenetic cluster was later than post-collisional rapid uplift of Gangdese -21Ma and - 15Ma the normal fault began to form in Central Gangdese, indicated ore-forming porphyries possibly emplaced the tectonic setting from thickened crust and the rapid uplift of Gangdese mountain to initial extension, namely, the porphyry copper deposits possibly occur in a transition tectonic setting between contractional deformation associated with Gangdese uplift and the initial onset of east-west striking extension.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2007年第5期953-966,共14页
Acta Petrologica Sinica
基金
青藏973项目斑岩铜矿课题(2002CB412605)
国家自然科学基金项目(40672068
40334044)资助成果
