东天山四顶黑山地区545Ma层状镁铁质-超镁铁质岩体的发现及其大地构造学和成矿学意义

Discovery of layered mafic-ultramafic intrusives formed in 545Ma in the Sidingheishan area,eastern Tianshan,China and its significances for tectonics and Cu-Ni mineralization

在东天山四顶黑山地区元古代深变质地层中发现层状镁铁质-超镁铁质岩体,研究了它的产状、岩石类型、结构构造、矿物组成、稀土微量元素地球化学、年代学等特征及其发现的地质意义。层状镁铁质-超镁铁质岩为后侵位的镁铁质杂岩体和花岗岩所穿切与破坏,其正堆晶结构发育,并可划分出两个结晶与演化旋回。四顶黑山层状钱铁质-超镁铁质岩的稀土微量元素含量和配分结构与该地区镁铁质杂岩、及图拉尔根镁铁质-超镁铁质岩体明显不同,具低∑REE、正δEu 异常、高 Sr 低Rh、高 Nb/U 值、及 Nb、Zr 和 Hf 亏损等地球化学特征,该特征与裂谷成因的攀枝花层状辉长岩和曹家村层状岩体非常相似,这意味着四顶黑山层状镁铁质-超镁铁质岩可能也形成于裂谷构造环境。选择层状镁铁质-超镁铁质岩体下部韵律层顶部辉石角闪石岩、镁铁质杂岩体闪长岩和角闪花岗岩中的角闪石开展^(39)Ar/^(40)Ar 定年,结果显示3种岩体分别形成于545±5Ma、263.8±2.6Ma 和200.8±0.91Ma。四顶黑山545Ma 裂谷成因层状镁铁质-超镁铁质岩体的发育,意味着早寒武世是天山地区镁铁质-超镁铁质岩型 Cu-Ni 矿的成矿期之一,天山地区在元古宙变质岩为基底的古大陆在早寒武世出现了雏形裂谷构造,545Ma可能是古天山洋形成的初始期。

We report a new discovery of layered mafic-ultramafic intrusives intruded and broken by later mafic complex and granite in Sidingheishan area, eastern Tianshan and their occurrence, petrologic, mineralogical, REE and trace elements, and chronological features and its tectonic implication in this paper. The layered mafic-uhramafic intrusive are characterized by normal cumulate structure and consisted of two crystallization and evolution units. Composition and patterns of REE and trace elements of the layered mafic- ultramafic intrusive in Sidingheishan area are different from that of mafic complex in Sidingheishan and the mafic-uhramafic rocks in Tulaergeng area, and have some distinctive features, including lower ∑ REE, normal δEu anomaly, enrichment of Sr, depletion of Rb, Nb, Zr and Hf, and high Nb/U value. These Geochemical features are similar to that of Panzhihua layered gabbro and Caojiacun layered intrusive formed in rift tectonics, implying a similar formation tectonic setting. ^39 Ar/^40 Ar dating results of hornblende from hornblendite located in top layer of the bottom unite of the layered mafic-ultramafic intrusives, diorite form the mafic complex, and hornblende-granite show that they are formed in about 545Ma BP, 263.8Ma BP and 200.8Ma BP respectively. Occurrences of 545Ma layered mafic-ultramafic intrusives formed in rift tectonics, suggest that early Cambrian is one metallogenic period of Cu-Ni deposits related to mafic-ultramafic rocks in eastern Tianshan, and 545Ma is possible the forming time of rifts in Tianshan paleo-continent characterized by a united Proterozoic metamorphic basements in early Cambrian, and the beginning time of formation of Tianshan paleoocean as well.