Metaultramafic schists and dismembered ophiolites of the Ashe Metamorphic Suite of northwestern North Carolina, USA
Metaultramafic schists and dismembered ophiolites of the Ashe Metamorphic Suite of northwestern North Carolina, USA
复制标题
DOI：
10.1080/00206814.2015.1129515
复制
发表时间：
2016-01
影响因子：
2.6
通讯作者：
L. A. Raymond;A. Merschat;R. Vance
中科院分区：
文献类型：
--
作者：
L. A. Raymond;A. Merschat;R. Vance
北卡罗来纳州西北部阿什变质岩系中的超镁铁质岩石(MUR)包括含石英±长石的QF角闪岩和局部缺乏石英的含滑石、绿泥石和/或含镁角闪石的TC角闪岩。一些工作人员根据矿物和地球化学的差异将TC角闪岩分为托德型和埃德蒙德型，我们暂时增加了第三种类型--富橄榄石±辉石的富山型岩石。在区域上，MUR天体的范围从Equant、Rich Mountain-到高度拉长的Todd-TC-角闪岩型天体。Mur显示出三到五个矿物组合，其中包含橄榄石、叶绿角闪石、镁角闪石、镁阳起岩、斜方辉石和蛇纹石等组合，代表了榴辉岩到绿片岩相变质程度的逐渐减少。MUR原岩很难确定。西南MUR体具残留的橄榄石+富辉石组合，代表超基性-基性、纯橄榄岩-辉石岩原岩。东北TC角闪岩Mur含有角闪岩和阳起闪长角闪岩以及绿泥石-铝质和钙质组合，这表明基性QF-角闪岩交代作用产生了所有TC-角闪岩。然而，许多TC角闪岩的镁钙铝和微量元素化学组成与斜长橄榄岩的成分相似。我们证明了少数AMS TC角闪岩具有玄武岩/辉长岩原岩，同时提出了反对交代假说适用于所有TC角闪岩的论点。我们确定MUR岩体在岩石学上是异石体，TC角闪岩与多种岩石类型接触；高铬、镍、镁角闪岩具有以橄榄石或辉石为主的原岩；大多数显示三种或三种以上变质矿物组合；被认为交代的接触是构造的。显然，不同的MUR岩体具有代表不同原岩的不同化学成分，具有不同的矿物组合，既反映了化学成分，也反映了变质历史。异石器MUR遗体的现场采样不能揭示MUR遗体的特征或历史，也不允许进行“类型”命名。我们建议摒弃将MUR小体细分为“类型”的做法，并谨慎应用交代假说。AMS、Mur和伴生的变质岩可能是来自各种蛇绿岩类型的解体蛇绿岩的碎片。
ABSTRACT Metaultramafic rocks (MUR) in the Ashe Metamorphic Suite (AMS) of northwestern North Carolina include quartz ± feldspar-bearing QF-amphibolites and quartz-deficient, locally talc-, chlorite-, and/or Mg-amphibole-bearing TC-amphibolites. Some workers divide TC-amphibolites into Todd and Edmonds types, based on mineral and geochemical differences, and we provisionally add a third type – olivine ± pyroxene-rich, Rich Mountain-type rocks. Regionally, MUR bodies range from equant, Rich Mountain- to highly elongate, Todd-TC-amphibolite-type bodies. The MURs exhibit three to five mineral associations containing assemblages with olivine, anthophyllitic amphibole, Mg-hornblende, Mg-actinolite, cummingtonite, and serpentine representing decreasing eclogite to greenschist facies grades of metamorphism over time. MUR protoliths are difficult to determine. Southwestern MUR bodies have remnant olivine ± pyroxene-rich assemblages representing ultrabasic-basic, dunite-peridotite-pyroxenite protoliths. Northeastern TC-amphibolite MURs contain hornblende and actinolitic amphiboles plus chlorites – aluminous and calcic assemblages suggesting to some that metasomatism of basic, QF-amphibolites yields all TC-amphibolites. Yet MgO-CaO-Al2O3 and trace element chemistries of many TC-amphibolites resemble compositions of plagioclase peridotites. We show that a few AMS TC-amphibolites had basaltic/gabbroic protoliths, while presenting arguments opposing application of the metasomatic hypothesis to all TC-amphibolites. We establish that MUR bodies are petrologically heterolithic and that TC-amphibolites are in contact with many rock types; that those with high Cr, Ni, and Mg have olivine- or pyroxene-dominated protoliths; that most exhibit three or more metamorphic mineral associations; and that contacts thought to be metasomatic are structural. Clearly, different MUR bodies have different chemistries representing various protoliths, and have different mineral assemblages, reflecting both chemical composition and metamorphic history. Spot sampling of heterolithic MUR bodies does not reveal MUR body character or history or allow ‘type’ designations. We recommend that the subdivision of MUR bodies into ‘types’ be abandoned and that the metasomatic hypothesis be carefully applied. AMS MURs and associated metamafic rocks likely represent fragments of dismembered ophiolites from various ophiolite types.