摘要
长期以来,对碧口群的时代归属、碧口群的总体构造形态、碧口群的基底及上覆层等问题众说纷纭,争论不休。经笔者通过寻找碧口群基底、剥开碧口群的上覆层等项研究,把广义的碧口群解体为3部分:下伏基底晚太古代鱼洞子群、中元古代碧口群(狭义) 和上覆上震旦统一下寒武统。并进一步将碧口群内部区分出具有不同特征的3个构造分区,分别论述各区总体构造特征、层序建立的依据及碧口群上、下亚群间不整合关系的特征和划分原则,着重论述了与此问题紧密相关的广泛分布于碧口群中的“片理化凝灰质砂砾岩”的产出特征、层序位置及其成因,指出该“片理化凝灰质砂砾岩”。为海沟重力流相沉积,而非冰碛岩。最后论述了碧口群分布区新建立的晚太古代地层单位鱼洞子群。
Due to the complex geological structure and absence oryctocoeose, the sequence and age of Bikou Group have not been solved for long time. Now, Bikou Group (broad sense) has been disintegrated by the authors into three parts, that is, Yudongzi Group in Late Archaeozoic Era, being an underlying basement; Bikou Group (narrow sense) in Middle Proterozoic Era; and overlying Upper Sinian-Lower Cambrian Epoch, all marked by angular unconformity contact which are summarized as follows: Overlying, Upper Sinian-Lowre Cambrian Epoch Angular unconformity Upper rock formation in Upper Subgroup of Bikou Group(Pt_2b^(b-2)), slightly metamorphic stromatolite-bearing dolomite, metamorphic age of the rock being 1044±22 Ma. Continuity Lower rock formation in Upper Subgroup of Bikou Group(Pt_2b^(b-1)), slightly metamorhpic marine pyroclastic rock, being graviational current facies sediments. Parallel or slightly angular unconformity Upper rock formation in Lower Subgroup of Bikou Group(Pt_2b^(a-3)), slightly metamorphic marine pyroclastic rock, being graviational current facies sediments, with diagenetic age of 1 565±16 Ma. Continuity Middle rock formation in Lower Subgroup of Bikou Group(Pt_2b^(a-2)), slightly metamorphic marine volcanic rock (mainly spilite-keratophyre). Continuity Lower rock formation in Lower Subgroup of Bikou Group(Pt_2b^(a-1)), slightly metamorphic marin pyroclastic rock. Anguler unconformity Yudongzi Group(Ar_sy): Banded plagioclase hornblendite, leptite migmatized leptite, migmatized plagioclase horblendite and banded magnetite quartzite and so on. Ziron from migmatized cataclastic plagioclase hornblendite has been dated by U-Pb concordia at 2657±9 Ma. Hornblendite veins intrued in Yudongzi Group have yielded an K-Ar whole-rock dilation age of 2 038±30 Ma. The basis for setting up the sequenece of Bikou Group has been discussed in this professional paper and three structural divisions with different character within Bikou Group has been distinguished by the authors as follows: 1. Mian (Mianxian) -Lue (Lueyang County)-Ning (Ningqiang County) regions of Shaanxi Province, being an anticlinorium bundle of fold. The axis of anti- cline and syncline, the strike of strata and rift within the region have been converged from W (Baoji-Chengdu Railway) to E (Mianxian), and lower horizon of Bikou Group oppears in the axis position of Zengjiahe anticline. 2.Wenxian-Bikou regions of Gansu Province, being an anticlinorium overall, synthetic dip plunging S and southern wing being inversed, the sequence of strata becomer young from S to N. The nuclear position of anticlinorium is located in Hengdan-Yangtian regions. Autogeny pyrite selected from argillaceous slate with grey black colour within Pt_2b^a-3 yields an age (^(40)Ar/ ~89Ar) of 1565±16Ma, which approximates to diagenetic age of upper strata of lower sub-group of Bikou Group.3.Pingwu-Qingchuan regions, Sichuan Province. Setting down Qingxi-Maoxiangba as a boundary, its eastern district is complex monoclinal stratum dipping N, with inversion occurrence its axis is situated at Huangyang-Mupi. Anticlinorium is in west of Qingxi-Maoxiangba with synthetic dip of two wings plunging S and northern wing being inversed strata becoming young from the axis to N. Besides,contact relationship between upper and Lower Subgroup of Bikou Group has been described by representative five sections in this paper. Much attention was paid to 'schistosis tuffaceous sandy conglomerate rock' which was colsely interrelated with the contact relationship. Some consider that the original conglomerate rock was glacial rock, and that the horizon could contrast with Nan-tuo Formation, Lower Sinian Epoch in Sanxia region of Yangtz. Considering the intergrowth or transitional character with particle current facies and turbidity current facies in graviational current facies, the schistosis, tuffaceous sandy conglomerate rock interpreted as debris flow and fragment flow facies, instead of glacial rock. Angular counconformity contact of slightly metamorphic Volcanic rock of Bikou Group with migmatized pawdite of Yudongzi Group has been found in Lesuhe, along Baoji-Chengdu Railway. Yudongzi Group, being a granite-gree- stone terrain has been further ascertained. Bikou Group, being a acoretion belt of the terrain, both groups often constitutes splitting and shifting terrain in Late Jinning Movement, which put together on northern edge of Yangtz plate. The age of greestone of Yudoagzi Group is Late Archaeozoic Era. The slightly metamorphic volcanic rock system of Lower Subgroup of Bikou Group can contrast with Changcheng System, North China (age at 1800-1400 Ma). The slightly metamorphic pyroclast and dolomite system containing stromatolite of Upper Subgroup of Bikou Group can contrast with Jixian System, North China(age of 1 400-1 000 Ma).The type of metamorphic formation of Yudongzi Group differs from that of Bikou Group, the former being a hight greenschist-lower amplibolite facies, white the later being greenschist facies. Furthermore, the type of volcanic magma of Yudongzi Group is unlike that of Bikou Group as well, the former belonging to Fe-riched type calc-alkali series of magmatic evolution and its diagenetic structural environment being an island arc or edge of active continent, while the later belongs to partial calcium calcalkali series of tholeiite magmatic evolution, forming in island arc environment of orogenic zone. The difference of the magmatic sources and magmatic type between Yudongzi and Bikou Group has been indicated by ∑REE, HREE ratio and europium anormaly.
