ABSTRACT Mesoproterozoic crust is widely exposed in the Grenville Province portion of northeastern Laurentia, where it is interpreted as an assemblage of two continental-arc segments separated by a composite arc belt (Quebecia) with island-arc remnants. A synthesis of the geologic context, types, and geochemical patterns of 1.5–1.35 Ga granitoids reveals a regional distribution in each segment, with dioritic to granitic plutonism variably associated with arc-related volcano-sedimentary belts in the south and inboard monzonitic to granitic plutonism in the north. In addition, belts of dioritic to granitic orthogneisses occupy intermediate positions in Quebecia and in the west. The inboard granites are consistently old in all segments (1.5–1.45 Ga), but the preserved volcano-sedimentary belts are older in the east and in Quebecia (1.5–1.45 Ga) and younger in the west (1.39? and 1.36 Ga), while the belts of orthogneisses show a large spread of ages at 1.45–1.37 Ga. Granitoids in the volcano-sedimentary belts and the orthogneisses include magnesian, calcic to calc-alkalic components to ferroan, alkali-calcic components. In contrast, the inboard plutons are dominantly ferroan and alkali-calcic to alkalic in the continental-arc segments, where they are locally associated with anorthosite-mangerite-charnockite-granite (AMCG) suites. Collectively, the different types of granitoid magmatism can be linked to an active margin, with subduction under northeastern Laurentia, involving arc building, arc rifting, back-arc opening and inboard extension, and amalgamation processes variably operating at different parts of the margin and at different times. In addition, the data provide a basis for comparison with other parts of the eastern to southwestern Laurentian margin in the 1.5–1.35 Ga time frame.

ABSTRACT The Paleozoic plate boundary zone between Laurussia and Gondwana in western Pangea hosts major magmatic and hydrothermal Sn-W-Ta, Au, and U mineralization. Individual mineral deposits represent the results of the superposition of a series of exogenic and endogenic processes. Exogenic processes controlled (1) the enrichment of the ore elements in sedimentary protoliths via residual enrichment during intense chemical weathering and via climatically or tectonically controlled redox traps, (2) the spatial distribution of fertile protoliths, and, thus, eventually (3) the spatial distribution of mineralization. Endogenic processes resulting in metamorphism and crustal melting controlled the mobilization of Sn-W, Au, and U from these enriched protoliths and, thus, account for the age distribution of Sn-W and Au mineralization and U-fertile granites. It is the sequence of exogenic and endogenic processes that eventually results in the formation of mineralization in particular tectonic zones. Whereas the endogenic processes were controlled by orogenic processes during the assembly of western Pangea itself, the exogenic processes were linked to the formation of suitable source rocks for later mineralization. The contrasting distribution of magmatic and hydrothermal Sn-W-Ta, Au, and U mineralization on the Laurussia and Gondwana sides of the plate boundary zone reflects the contrasting distribution of fertile protoliths and the contrasting tectonic situation on these margins. The Laurussian margin was an active margin during most of the Paleozoic, and the distribution of different mineralization types reflects the distribution of terranes of contrasting provenance. The Gondwanan margin was a passive margin during most of the Paleozoic, and the similar distribution of a wide range of different metals (Sn, W, Ta, Au, and U) reflects the fact that the protoliths for the various metals were diachronously accumulated on the same shelf, before the metals were mobilized during Acadian, Variscan, and Alleghanian orogenic processes.

ABSTRACT Early Ordovician to late Permian orogenies at different plate-boundary zones of western Pangea affected continental crust derived from the plates of North America (Laurentia), Europe (East European Craton including Baltica plus Arctida), and Gondwana. The diachronic orogenic processes comprised stages of intraoceanic subduction, formation and accretion of island arcs, and collision of several continents. Using established plate-tectonic models proposed for different regions and time spans, we provide for the first time a generic model that explains the tectonics of the entire Gondwana-Laurussia plate-boundary zone in a consistent way. We combined the plate kinematic model of the Pannotia-Pangea supercontinent cycle with geologic constraints from the different Paleozoic orogens. In terms of oceanic lithosphere, the Iapetus Ocean is subdivided into an older segment (I) and a younger (II) segment. Early Cambrian subduction of the Iapetus I and the Tornquist oceans at active plate boundaries of the East European Craton triggered the breakup of Pannotia, formation of Iapetus II, and the separation of Gondwana from Laurentia. Prolonged subduction of Iapetus I (ca. 530 –430 Ma) culminated in the Scandian collision of the Greenland-Scandinavian Caledonides of Laurussia. Due to plate-tectonic reorganization at ca. 500 Ma, seafloor spreading of Iapetus II ceased, and the Rheic Ocean opened. This complex opening scenario included the transformation of passive continental margins into active ones and culminated in the Ordovician Taconic and Famatinian accretionary orogenies at the peri-Laurentian margin and at the South American edge of Gondwana, respectively. Rifting along the Avalonian-Cadomian belt of peri-Gondwana resulted in the separation of West Avalonian arc terranes and the East Avalonian continent. The vast African/Arabian shelf was affected by intracontinental extension and remained on the passive peri-Gondwana margin of the Rheic Ocean. The final assembly of western Pangea was characterized by the prolonged and diachronous closure of the Rheic Ocean (ca. 400–270 Ma). Continental collision started within the Variscan-Acadian segment of the Gondwana-Laurussia plate-boundary zone. Subsequent zipper-style suturing affected the Gondwanan Mauritanides and the conjugate Laurentian margin from north to south. In the Appalachians, previously accreted island-arc terranes were affected by Alleghanian thrusting. The fold-and-thrust belts of southern Laurentia, i.e., the Ouachita-Marathon-Sonora orogenic system, evolved from the transformation of a vast continental shelf area into a collision zone. From a geodynamic point of view, an intrinsic feature of the model is that initial breakup of Pannotia, as well as the assembly of western Pangea, was facilitated by subduction and seafloor spreading at the leading and the trailing edges of the North American plate and Gondwana, respectively. Slab pull as the plate-driving force is sufficient to explain the entire Pannotia–western Pangea supercontinent cycle for the proposed scenario.