The Bohemian Massif of Central Europe is a Variscan collage of lithospheric fragments that formed at the northern margin of Gondwana during the late Neoproterozoic. A key geodynamic process that shaped this margin before it became involved in the Variscan orogen was the Cambro–Ordovician rifting that opened the Rheic Ocean. This rifting event has been studied extensively, yet a number of issues remain unresolved, among which are its geodynamic causes. New U–Pb zircon ages of orthogneisses from the mid-crustal Moldanubian unit, in combination with available information on magmatism and basin subsidence in the upper-crustal Teplá–Barrandian unit of the Bohemian Massif, are here used to reconstruct in detail the mechanism of the Cambro–Ordovician rifting. We argue that extension occurred in three phases defined by (1) protracted ~524–480 Ma intermediate to felsic plutonism (including the dated ~490–480 Ma orthogneisses), (2) basaltic submarine volcanism at ca. 470 Ma, and (3) rapid subsidence at ca. 458–452 Ma. This relative timing is interpreted to reflect stretching of the lower lithosphere before upper lithospheric rifting. In a broader context, these inferences are compatible with contrasting, rheologically controlled modes of northern Gondwana break-up during the early Ordovician, in which the westerly Avalonian-type terranes were rifted away from Gondwana, whereas the easterly Cadomian-type terranes formed a hyperextended Gondwanan shelf.
Limestones in the highly deformed Tecomate Formation, uppermost unit of the Acatlán Complex, are latest Pennsylvanian—earliest Middle Permian in age rather than Devonian, the latter based on less diagnostic fossils. Conodont collections from two marble horizons now constrain its age to range from latest Pennsylvanian to latest Early Permian or early Middle Permian. The older collection contains Gondolella sp., Neostreptognathodus sp., and Streptognathodus sp., suggesting an oldest age limit close to the Pennsylvanian—Permian time boundary. The other collection contains Sweetognathus subsymmetricus, a short-lived species ranging only from Kungurian (latest Leonardian) to Wordian (earliest Guadelupian: 272 ± 4 to 264 ± 2 Ma). A fusilinid, Parafusulina c.f. P. antimonioensis Dunbar, in a third Tecomate marble horizon is probably Wordian (early Guadelupian, early Middle Permian). Furthermore, granite pebbles in a Tecomate conglomerate have yielded ~320-264 Ma U-Pb SHRIMP ages probably derived from the ~288 Ma, arc-related Totoltepec pluton. Collectively, these data suggest a correlation with two nearby units: (1) the Missourian—Leonardian carbonate horizons separated by a Wolfcampian(?) conglomerate in the upper part of the less deformed San Salvador Patlanoaya Formation; and (2) the clastic, Westphalian—Leonardian Matzitzi Formation. This requires that deformation in the Tecomate Formation be of Early—Middle Permian age rather than Devonian. These three formations are re-interpreted as periarc deposits with deformation related to oblique subduction. The revised dating of the Tecomate Formation is consistent with new data, which indicates that the unconformity between the Tecomate and the Piaxtla Group is mid-Carboniferous and corresponds to a tectonothermal event.
In the Cobequid Highlands of Nova Scotia, low-grade late Precambrian arc-related volcano-sedimentry rocks typical of the Avalon Composite Terrane overlie platformal metasedimentry rocks and are spatially associated with gneisses previously considered to be basement to both these units. U–Pb zircon dates of 580–587 Ma from an orthogneiss and an amphibolite are similar to the U–Pb zircon dates of 580–610 Ma from both syntectonic granites in ductile shear zones and high-level posttectonic plutons that intruded the Avalonian successions. Hence, the gneisses do not represent basement but are an integral part of the Avalonian orogenic cycle. The geochronological data indicate that penetrative fabrics in the gneisses, syntectonic granites, and volcano-sedimentary successions are penecontemporaneous (ca. 580–620 Ma) and not sequential, as previously interpreted. The gneisses have a metamorphic fabric (S1a), crystallized under amphibolite-facies conditions, and may represent the deeper roots of a late Precambrian magmatic arc. Fabrics within the deformed granite gneisses (S1b) are interpreted as reflecting crystallization within active ductile shear zones associated with intra-arc transtension and basin development. Fabrics in the volcano-sedimentary successions (S1c) are associated with deformation of the basin.
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.
Repeated amalgamation and subsequent breakup of continental lithosphere have profoundly affected Earth9s evolution since the Archean. Following breakup, distinctive rift and passive margin sequences along the trailing edges of dispersing continents have been used to identify such margins in the geologic past. Using western North America as an analogue, we show that the leading edges of dispersing continents have isotopic characteristics that can likewise be used to identify these margins. For example, the Sm-Nd isotopic signatures of Late Neoproterozoic and early Paleozoic igneous rocks along the northern margin of Gondwana indicate derivation from 0.7 to 1.1 Ga old mantle lithosphere. This lithosphere originated in the Mirovoi Ocean surrounding Rodinia. It subsequently accreted to northern Gondwana in response to Rodinia breakup, and provided a source for subsequent magmatism. Accretion and subsequent recycling of oceanic mantle lithosphere should be common along the leading edges of dispersing continents following supercontinent breakup, providing an additional aid in paleocontinental reconstructions.
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