Tectonothermal history of the Mesoproterozoic Novillo Gneiss of eastern Mexico: support for a coherent Oaxaquia microcontinent

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 high-grade 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 g