Tectonothermal history of the Mesoproterozoic Novillo Gneiss of eastern Mexico: support for a coherent Oaxaquia microcontinent
Tectonothermal history of the Mesoproterozoic Novillo Gneiss of eastern Mexico: support for a coherent Oaxaquia microcontinent
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墨西哥东部中元古代诺维洛片麻岩的构造热史：支持瓦哈基亚微大陆
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发表时间：
2011
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通讯作者：
J. Keppie
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作者：
R. Trainor;R. Nance;J. Keppie
诺维洛片岩是墨西哥东部中元古代（约1.0-1.2 Ga）基底的几个暴露之一，被解释为单个地壳块体（瓦克西亚）的露头，该地壳块体在晚前寒武纪-古生代大陆重建中占据突出地位。Novillo片麻岩位于维多利亚城附近的Sierra Madre Oriental，由两个主要的中元古代火成岩套组成，这些火成岩套侵入了稀有的变质沉积岩。较老的套组，之前的年代为1235-1115 Ma，主要由石榴石钾长石长片麻岩和花岗岩片麻岩组成，具有弧/弧后地球化学亲和力。较年轻的套（绿绿质片麻岩，斜长质变长岩）的年代为1035-1010 Ma，被解释为斜长岩-锰矿-绿绿岩-花岗岩组合的一部分。两套岩套均被两套角闪岩岩脉侵入，其中早于麻粒岩相条件下变质作用的角闪岩岩脉约990±5 Ma，后一套角闪岩岩脉品位较低，侵位时间约546 Ma。两个主要火成岩套的新构造资料主要为北西向陡倾角变质带/片理（S1）复合轴向平面至罕见等斜褶皱，以及在高变质条件下发育的以拉伸、12:2:1、钾长石奥长石和石英带为特征的线理。白色小体和中核小体边界之间的不规则接触可能是早期s1前的杂化作用的残余。这些地层上覆有紧密等斜的F2鞘褶皱，与偏东南斜斜的斜辉石矿物线理相关联，将S形和z形不对称F2褶皱轴的大圆分布分两半，表明斜左旋、顶向北西的相对运动。L1条线也有一个大圆分布，表明片理中的不均匀拉伸使L1向L2旋转。同构造组构被花岗麻粒岩相矿物学覆盖，其次是逆行角闪岩和绿片岩相组构，绿片岩相组构与诺维略片麻岩和古生代格兰杰诺片岩并置的断裂有关。沿该断层侵入的约350 Ma的浅花岗岩在约313 Ma时发生变形。这一构造热历史与瓦哈卡杂岩（墨西哥南部）的构造热历史非常相似，支持了一个连贯的瓦哈卡的存在。瓦哈卡杂岩中晚期角闪岩脉的缺失可能反映了其在脉就位时的相对古地理。
The Novillo Gneiss is one of several exposures of Mesoproterozoic (ca. 1.0–1.2 Ga) basement in eastern Mexico interpreted to be outcrops of a single crustal block (Oaxaquia) that has figured prominently in continental reconstructions for the late Precambrian-Paleozoic. Exposed within the Sierra Madre Oriental near Ciudad Victoria, the Novillo Gneiss comprises two major Mesoproterozoic igneous suites that intrude rare metasedimentary rocks. The older suite, previously dated at 1235–1115 Ma, principally comprises garnet K-feldspar augen gneiss and granite gneiss with arc/back-arc geochemical affinities. The younger suite (charnokitic gneiss, anorthositic metagabbro) has been dated at 1035–1010 Ma and is interpreted to be part of an anorthosite-mangerite-charnockite-granite assemblage. Both suites are intruded by two sets of amphibolite dikes, the earlier of which predates metamorphism under granulite facies conditions at ca. 990±5 Ma, whereas the later set is of low grade and was emplaced at ca. 546 Ma. New structural data in both major igneous suites are dominated by the presence of a composite NW-trending, steeply dipping metamorphic banding/foliation (S1) axial planar to rare isoclinal folds, and a lineation defined by stretched, 12:2:1, K-feldspar augen and quartz ribbons developed under highgrade metamorphic conditions. The irregular contacts between leucosome and mesosome boundaries may be a remnant of earlier, pre-S1 migmatization. These are overprinted by tight-isoclinal F2 sheath folds associated with a moderately ESE-plunging clinopyroxene mineral lineation that bisects the great circle distribution of S- and Z-shaped asymmetrical F2 fold axes, indicating oblique sinistral, top-to-NW relative movement. The L1 lineations also have a great circle distribution suggesting that inhomogeneous stretching in the foliation rotated L1 towards L2. Syntectonic fabrics are overprinted by a granoblastic granulite facies mineralogy followed by retrograde amphibolite and greenschist facies fabrics, the latter associated with the fault that juxtaposes the Novillo Gneiss and the Paleozoic Granjeno Schist. A ca. 350 Ma leucogranite intruded along this fault was deformed at ca. 313 Ma. This tectonothermal history is closely comparable to that of the Oaxacan Complex (southern Mexico), supporting the existence of a coherent Oaxaquia. The absence of the later amphibolite dikes in the Oaxacan Complex likely reflects its relative paleogeography at the time of dike emplacement.