Detrital zircon sources in the Ordovician metasedimentary rocks of the Moroccan Meseta: Inferences for northern Gondwanan passive-margin paleogeography

ABSTRACT Detrital zircon U-Pb geochronology has been widely used to constrain the pre-Carboniferous geography of the European and, to a lesser extent, the Moroccan Variscides. The latter have been generally considered as part of a long-lasting passive margin that characterized northern Gondwana from Ordovician to Devonian time, and was subsequently involved in the late Paleozoic Variscan orogeny. We report detrital zircon ages for three Early to Late Ordovician samples from the Beni Mellala inlier in the northeastern part of the Western Moroccan Meseta in order to discuss the temporal evolution of the sources of sediments in this region. The detrital zircon spectra of these samples, characterized by two main populations with mean ages of 630–610 Ma and 2170–2060 Ma, are typical of Cambrian–Devonian rocks from the Moroccan Variscides and confirm their link to the West African craton. A minor Stenian–Tonian population (peak at ca. 970 Ma) suggests the influence of a distant and intermittent NE African source (Sahara metacraton), which was probably interrupted after Ordovician time. Our data support previous interpretations of the Moroccan Meseta (and the entire northern Moroccan Variscides) as part of the northern Gondwana passive margin. The main sources of these sediments would have been the West African craton in the western regions of the passive margin (Moroc- can Meseta and central European Paleozoic massifs), and the Arabian-Nubian Shield and/or Sahara metacraton in the eastern areas (Libya, Egypt, Jordan, central and NW Iberian zones during Paleozoic time), where the 1.0 Ga detrital zircon population is persistent throughout the Ordovician–Devonian time span.

Supplemental Material: New detrital zircon U-Pb ages and Lu-Hf isotopic data from metasedimentary rocks along the western boundary of the composite Avalon terrane in the southeastern New England Appalachians

U-Pb LA-ICP-MS detrital zircon data for three samples collected along the western boundary of the Avalon terrane in southeastern New England

Supplemental Material: New detrital zircon U-Pb ages and Lu-Hf isotopic data from metasedimentary rocks along the western boundary of the composite Avalon terrane in the southeastern New England Appalachians

U-Pb LA-ICP-MS detrital zircon data for three samples collected along the western boundary of the Avalon terrane in southeastern New England

Regional geology and tectonic framework of the Southern Indian domain, Trans-Hudson Orogen, Manitoba

The collaborative federal-provincial Southern Indian Lake project in north-central Manitoba covered an area of more than 3500 km2 of the Trans-Hudson orogen. Regional-scale geological mapping, sampling, and lithogeochemical, isotopic and geochronological studies resulted in the identification of distinct assemblages of supracrustal rocks and varied episodes of plutonism. A granodiorite gneiss dated at ca. 2520 Ma is interpreted to represent the basement of the Southern Indian domain and is considered a separate crustal domain, named the Partridge Breast block. The Churchill River assemblage is composed of juvenile pillow basalt with intervening clastic sedimentary rocks, possibly a reflection of plume magmatism related to initial rifting of the Hearne craton margin. The Pukatawakan Bay assemblage consists mainly of massive to pillowed, juvenile metabasaltic rocks and associated basinal metasedimentary rocks. The Partridge Breast Lake assemblage is dominated by continental-arc volcanic and volcaniclastic rocks associated with basinal metasedimentary rocks. The Strawberry Island assemblage, consisting of arenite and polymictic conglomerate, is interpreted to have been deposited in a foreland-basin basin or intra-orogen pull-apart basin environment. The Whyme Bay assemblage is characterized by fluvial-alluvial orogenic sediments and is temporally linked to the Sickle Group rocks in the Lynn Lake greenstone belt. Granitoid rocks, dominantly monzogranite and granodiorite, range in age from ca. 1890 to 1830 Ma and occur throughout the Southern Indian domain, and intermediate and mafic intrusions of similar ages are also present. In this paper we integrate these new data into a tectonic framework for the Southern Indian domain of the Trans-Hudson orogen in Manitoba.

Multiple sediment incorporation events in a continental magmatic arc: Insight from the metasedimentary rocks of the northern North Cascades, Washington (USA)

The rheology and composition of arc crust and the overall evolution of continental magmatic arcs can be affected by sediment incorporation events. The exhumed Cretaceous–Eocene North Cascades arc exposes abundant metasedimentary rocks that were incorporated into the arc during multiple events. This study uses field relationships, detrital zircon geochronology, bulk rock geochemistry, geothermometry, and quartz­in­garnet geobarometry to distinguish approximate contacts and emplacement depths for different metasedimentary units to better understand their protolith incorporation history and impact on the arc. The Skagit Gneiss Complex is one of the main deep crustal units of the North Cascades arc. It includes metasedimentary rocks with distinct detrital zircon signatures: Proterozoic–Cretaceous (Group 1) or Triassic–Cretaceous (Group 2) zircon populations. Both metasedimentary groups achieved near­ peak metamorphic conditions of 640–800 °C and 5.5–7.9 kbar; several Group 2 samples reveal the higher pressures. A third group of metasedimentary rocks, which was previously interpreted as metamorphosed equivalents of backarc sediments (Group 3), exhibited unimodal Triassic or bimodal Late Jurassic–Early Cretaceous detrital zircon signatures and achieved near­peak conditions of 570–700 °C and 8.7–10.5 kbar. The combined field and analytical data indicate that protoliths of Group 1 and Group 2 metasedimentary rocks were successively deposited in a forearc basin and underthrusted into the arc as a relatively coherent body. Group 3 backarc sediments were incorporated into the arc along a transpressional step­over zone. The incorporation of both forearc and backarc sediments was likely facilitated by arc magmatism that weakened arc crust in combination with regional transpression.

Supplemental Material: New detrital zircon U-Pb ages and Lu-Hf isotopic data from metasedimentary rocks along the western boundary of the composite Avalon terrane in the southeastern New England Appalachians

U-Pb LA-ICP-MS detrital zircon data for three samples collected along the western boundary of the Avalon terrane in southeastern New England

Evolution of Neoproterozoic Shillong Basin, Meghalaya, NE India: implications of supercontinent break-up and amalgamation

Fragmentation and amalgamation of supercontinents play an important role in shaping our planet. The break-up of such a widely studied supercontinent, Rodinia, has been well documented from several parts of India, especially the northwestern and eastern sector. Interestingly, being located very close to the Proterozoic tectonic margin, northeastern India is expected to have had a significant role in Neoproterozoic geodynamics, but this aspect has still not been thoroughly studied. We therefore investigate a poorly studied NE–SW-trending Shillong Basin of Meghalaya from NE India, which preserves the stratigraphic record and structural evolution spanning the Neoproterozoic Era. The low-grade metasedimentary rocks of Shillong Basin unconformably overlie the high-grade Archean–Proterozoic basement and comprise a c. 4000-m-thick platform sedimentary rock succession. In this study, we divide this succession into three formations: lower Tarso, middle Ingsaw and upper Umlapher. A NW–SE-aligned compression event later caused the thrusting of these sedimentary rocks over the basement with a tectonic contact in the western margin, resulting in NE–SW-trending fold belts. The rift-controlled Shillong Basin shows a comparable Neoproterozoic evolution with the equivalent basins of peninsular India and eastern Gondwana. The recorded Neoproterozoic rift tectonics are likely associated with Rodinia’s break-up and continent dispersion, which finally ended with the oblique collision of India with Australia and the intrusion of Cambrian granitoids during the Pan-African Orogeny, contributing to the assembly of Gondwana. This contribution is the first to present a complete litho-structural evolution of the Shillong Basin in relation to regional and global geodynamic settings.