小莱河麻粒岩相铁建造的矿物组合──变质过程p、T变化的探讨  

MINERAL ASSEMBLAGES IN THE GRANULITE-FACIES IRON-FORMING OF LAIHE: A DISCUSSION OF p-T VARIATIONS DURING METAMORPHISM

作　　者：黄婉康 王明再[2] 龚国洪[2] 杜春辉 王岩国[4] 

机构地区：[1]中国科学院地质新技术研究所,广州510640 [2]中国科学院地球化学研究所,贵阳550002 [3]中国有色金属辽宁省地勘局地质研究所,沈阳110800 [4]中国科学院金属研究所,沈阳110015

出　　处：《矿物学报》1994年第3期247-256,T001,共11页Acta Mineralogica Sinica

摘　　要：小莱河太古宙麻粒岩相铁建造的矿物组合有：①斜方辉石（Fs87）＋钙铁辉石＋石英＋磁铁矿±莱河矿；②铁闪石＋铁浅闪石质角闪石＋石英＋磁铁矿＋碳酸盐±Fe－镁川石（云辉闪石）。辉石的出溶显示了它形成的不同阶段：片晶发育的OpxⅡ－CpxⅡ是最后稳定产物，计算的平衡温度是742℃，按相图获得的压力是7×108～7．8×108Pa；钙铁辉石中“001”片晶是易变辉石片晶转变的，Opx－Cpx－Pig（片晶）阶段据相图推测可能形成于近820℃，8×108Pa的条件下。原始均一相的OpxⅠ－CpxⅠ阶段矿物成分按估算的片晶含量计算，它们的形成温度接近820℃，可能的压力范围是11×108～13×108Pa。由此得到ITD型麻粒岩相p－T－t轨迹。莱河矿无氧化成因的证据，认为它是在麻粒岩相条件下生成的。云辉闪石的Si—O链重复周期是3×，其Fe／（Fe＋Mg）＝0．85，是接近富铁端元的Fe－镁川石，它与闪石一起在640到近700℃条件下交代了辉石。The Laihe iron-formation, formed as a result of regional metamorphism of ferruginous silicolites, is located in an Archean granulite-charnockite terrene in Qingyuan, Liaoning Province. Two mineral assemblages in the Laihe iron-formation are recognized: (1) eulite (Fs87) +hedenbergite +quartz + magnitite ±laihunite ; and (2) grunerite +ferro-edenite hornblende +quartz +magnetite ±carbonite±Fe-jimthompsonite. Both '100' and '001 'lamellae of eulite in hedenbergite were appeared and the '001' lamelki were derived from pigionites. There fore, there are the following continuous stages during the formation of pyrWnes: a. OpxⅠ-Cpx Ⅰ(homogeneous facies) ; b.Opx^Cpx-Pig (lamellae) ; and c. Opx Ⅱ-CpxⅡ (both with lamellae).The calculated temperature for OpxⅡ -CpxⅡ is 742℃ and the corresponding pressure shown in H. W. Jaffe's (1978) p-T diagram is 7 ×108~ 7. 8 ×108 Pa. In the same diagram, Opx-Cpx-Pig (lamellae) could appear at ~820℃ and ~ 8×108Pa. Assuming the lamellae account for 4% in hedenbergite and eulite, the calculated temperature for OpxⅠ -CpxⅠ is 800. 5℃, never higher than 815. 9℃ in case the lamellae account for 8%. By comparing the pressure of 13 ×108Pa for the Archean terrene having laihunite in Qianan and the subsolidus phase relations for pyroxenes, it is plausible to presume that the pressure for OpxⅠ -CpxⅠis 11 ×108~ 13×108Pa. There fore, an ITDtype p-T-t path for the granulite iron-formation in the Laihe would be constructed. It might represent the tectonic environment of an old basement, which was unconformably covered by the Archean granite-greenstone terrene in Liaoning Province.The homogeneous laihunite (without relict minerals) is not indirect contact with quartz; between them exists a rim of eulite and hedenbergite. lt is suggested that the laihunite might be formed under the same condition of granulite facies with T≥742℃ and p≥7×108~ 7. 8 ×108Pa.It is shown that the biopyribole with triple silicate chains in HRTEM [100]image and Fe/(Fe +Mg) = 0. 85 is a Fe

关 键 词：麻粒岩 岩相 铁建造 矿物组合 

分 类 号：P588.346[天文地球—岩石学]