阿尔金山南缘长沙沟镁铁-超镁铁质层状杂岩体的发现与地质意义——岩石学和地球化学初步研究

Discovery of layered maflc-ultramafic intrusion in Changshagou, Altyn Tagh, and its geological implication:A pilot study on its petrological and geochemical characteristics.

阿尔金山南缘地区,侵入于新太古界—新元古界变质地层中的长沙沟镁铁-超镁铁质岩由四个呈断层接触的镁铁-超镁铁质岩块组成,形成年龄为462～470Ma。不同岩块内岩石的地球化学特征虽有差异,但均以LREE及强不相容元素的富集和高的Zr/Y比值(>4.1)为特征,形成于大陆裂谷环境。其中,清水泉北段可以划分出3～4个主体由辉橄岩-角闪辉长岩构成的岩浆旋回,具有层状岩体的特征,其母岩浆的Mg~#=53.7～55.9,为演化型母岩浆,暗示其经历了富镁矿物的分离结晶,且(Th/Nb)_N>1.0、Nb/La比值<1.0、以及发育的矿物逆序包裹现象等表明经历了明显的地壳混染和岩浆混合作用;而清水泉南和长沙沟中段镁铁-超镁铁质岩的(Th/Nb)_N<1.0、Nb/La比值>1.0,基本未遭受地壳混染,并且此Mg~#与FeO^T、TiO_2负相关、与SiO_2正相关,呈现良好的Fenner演化趋势特征;清水泉南纯橄岩.辉橄岩具有极高的Mg~#(90.6～84.5),而赋存有钛-磁铁矿工业矿体的长沙沟中段镁铁-超镁铁岩的Mg~#值较低(75.8～49.2),推测它们是同一母岩浆(Mg~#=78.2)经Fenner演化趋势后分别形成的早期富Mg矿物堆晶相和稍晚期的富Fe-Ti残余岩浆相。长沙沟中奥陶世裂谷型层状镁铁-超镁铁质杂岩体的发现,意味着这一时期阿尔金山南缘地区处于伸展背景下,具有形成岩浆型PGE-Cu-Ni硫化物矿床和V-Ti-Fe氧化物矿床的地质背景和重要的成矿物质载体。作为该地区一种新的找矿思路,该地区同一构造带内其它镁铁-超镁超镁铁质岩体的性质及可能的金属矿化作用等也是值得进一步研究探索的。

The Changshagou layered intrusion are comprised of some mafic-ultramafic blocks with ages of 462～470 Ma, the Northern Qingshuiquan (NQ) , Southern Qingshuiquan (SQ) and Middle section of Changshagou (MC) , comparted by faults. They intruded in the Neoarchean and Neoproterozoic metamorphic strata on the southern margin of Altyn Tagh. The geochemical characteristics of the mafic-ultramafic rocks from these blocks are not uniform, but they all formed in continental rift tectonic setting according to the enrichments in LREEs and strong incompatible elements and higher Zr/Y ratios(>4.1). The NQ mafic-ulmafic block consists of three to four magmatic cycles and exhibits the feature of layered mafic-ultramafic intrusion; higher ( Th/Nb)_N ratios (>1.0) , lower Nb/La ratios (<1.0) and reverse mineral enwrapping of these rocks indicate the crust contamination and mingling of magma; lower Mg (53. 7～55. 9) of their parental magma suggest they are from evolved magma undergone by significant fractional crystallization. Whereas the lower (Th/Nb)_N ratios (<1.0) and higher Nb/La ratios(>1.0) of macfic-ultrmafic rocks from SQ and MC indicate they were not contaminated by crust. The increasing FeO^T and TiO_2, and decreasing SiO_2 with decreasing Mg~# of these macfic-ultrmafic rocks from SQ and MC present the distinct Fenner trend for magma evolution. Accordingly, we inferred that the maficultramafic rocks with higher Mg~# (90.6～84.5) from SQ were the magnesium-enriched cumulus phase from the parental ( primary) magma with Mg~# = 78.2, while the mafic-ultramafic rocks with lower Mg~# (75.8～49.2) from MC intrusion containing Ti-magnetite orebodies, were the residual magma phase by Fenner-trend of fractional crystallization from this parental magma. Occurrences of the Changshagou continental rift-type layered mafic-ultramafic intrusion indicate the south margin of Altyn Tagh were in the continental extending setting in middle Ordovician. As a new idea for geologic exploration in this area, the distribution and potential magmatic PGE-Cu-Ni sulfide and V-Ti-Fe oxide metallization of the same type intrusions in this area are worthy of further study.