J.D. Keppie

Geochemistry and U–Pb protolith ages of eclogitic rocks of the Asís Lithodeme, Piaxtla Suite, Acatlán Complex, southern Mexico: tectonothermal activity along the southern margin of the Rheic Ocean

Recent data indicating that the Piaxtla Suite (Acatlán Complex, southern Mexico) underwent eclogite-facies metamorphism and exhumation during the Devono-Carboniferous suggest an origin within the Rheic Ocean rather than the Iapetus Ocean. The Asís Lithodeme (Piaxtla Suite) consists of polydeformed metasediments and eclogitic amphibolites that are intruded by megacrystic granitoid rocks. U–Pb (zircon) data indicate that the metasediments were deposited after c. 700 Ma and before intrusion of c. 470–420 Ma quartz-augen granite. The metasedimentary rocks contain abundant Mesoproterozoic detrital zircons (c. 1050–1250 Ma) and a few zircons in the range of c. 900–992 and c. 1330–1662 Ma. Their geochemical and Sm–Nd isotopic signature is typical of rift-related, passive margin sediments derived from an ancient cratonic source, which is interpreted to be the adjacent Mesoproterozoic Oaxacan Complex. Megacrystic granites were derived by partial melting of a c. 1 Ga crustal source, similar to the Oaxacan Complex. Amphibolitic layers exhibit a continental tholeiitic geochemistry, with a c. 0.8–1.1 Ga source (TDM age), and are inferred to have originated in a rift-related environment by melting of lithospheric mantle in the Ordovician. This rifting may be related to the Early Ordovician drift of peri-Gondwanan terranes (e.g. Avalonia) from Gondwana and the origin of the Rheic Ocean.

Palaeozoic within-plate volcanic rocks in Nova Scotia (Canada) reinterpreted: isotopic constraints on magmatic source and palaeocontinental reconstructions

Palaeozoic volcanism in the Avalon Terrane of northern Nova Scotia occurred during three time intervals: Cambrian-early Ordovician, late Ordovician-early Silurian and middle-late Devonian. In the Meguma Terrane of southern Nova Scotia, Palaeozoic volcanism is limited to the middle Ordovician. Geochemical data show that most of these volcanic rocks are bimodal, within-plate suites. Initial e Nd signa- tures range from +5.4 to -1.9 in the rhyolites and +6.8 to +2.7 in the basalts, a difference attributable to the absence or presence, respectively, of a significant crustal component. The data and regional tectonic settings of the Avalon and Meguma terranes suggest that the volcanism was generated in three different within-plate settings: (1) Cambrian-early Ordovician volcanism related to thermal decay of late Proterozoic arc magma- tism during transtensional deformation; (2) middle Ordovician-early Silurian volcanism during sinistral tele- scoping between Laurentia and Gondwana where extensional bends in the Appalachians produced rifting; and (3) Devonian volcanism resulting from lithospheric delamination during dextral transpression and tele- scoping. In each setting, active faults served as conduits for the magmas. Nd isotopic data indicate that the source of the Palaeozoic felsic volcanic rocks is isotopically indistinguishable beneath southern and northern Nova Scotia and did not substantially change with time. This crustal source appears to have separated from the mantle during the Proterozoic, a conclusion consistent with the hypothesis that the Palaeozoic rocks in Nova Scotia were deposited upon a late Proterozoic oceanic-cratonic volcanic arc terrane. The Nd data, when combined with published faunal, palaeomagnetic and U-Pb isotopic data, suggest that the Avalon Terrane was peripheral to Gondwana off northwestern South America during Neoproterozoic and early Palaeozoic times.

Superposed Neoproterozoic and Silurian magmatic arcs in central Cape Breton Island, Canada: geochemical and geochronological constraints

Isotopic and geochemical data indicate that intrusions in the eastern Creignish Hills of central Cape Breton Island, Canada represent the roots of arcs active at ~ 540–585 Ma and ~ 440 Ma. Times of intrusion are closely dated by (1) a nearly concordant U–Pb zircon age of 553 ± 2 Ma in diorites of the Creignish Hills pluton; (2) a lower intercept U–Pb zircon age of 540 ± 3 Ma that is within analytical error of 40 Ar/ 39 Ar hornblende plateau isotope-correlation ages of 545 and 550 ± 7 Ma in the River Denys diorite; and (3) an upper intercept U–Pb zircon age of 586 ± 2 Ma in the Melford granitic stock. On the other hand, ~ 441–455 Ma 40 Ar/ 39 Ar muscovite plateau ages in the host rock adjacent to the Skye Mountain granite provide the best estimate of the time of intrusion, and are consistent with the presence of granitic dykes cutting the Skye Mountain gabbro–diorite previously dated at 438 ± 2 Ma. All the intrusions are calc-alkaline; the Skye Mountain granite is peraluminous. Trace element abundances and Nb and Ti depletions of the intrusive rocks are characteristic of subduction-related rocks. The ~ 540–585 Ma intrusions form part of an extensive belt running across central Cape Breton Island, and represent the youngest Neoproterozoic arc magmas in this part of Avalonia. Nearby, they are overlain by Middle Cambrian units containing rift-related volcanic rocks, which bracket the transition from convergence to extension between ~ 540 and 505/520 Ma. This transition varies along the Avalon arc: 590 Ma in southern New England, 560–538 Ma in southern New Brunswick, and 570 Ma in eastern Newfoundland. The bi-directional diachronism in this transition is attributed to northwestward subduction of two mid-ocean ridges bordering an oceanic plate, and the migration of two ridge–trench–transform triple points. Following complete subduction of the ridges, remnant mantle upwelling along the subducted ridges produced uplift, gravitational collapse and the high-temperature/low-pressure metamorphism in the arc in both southern New Brunswick and central Cape Breton Island. The ~ 440 Ma arc magmatism in the Creignish Hills extends through the Cape Breton Highlands and into southern Newfoundland, and has recently been attributed to northwesterly subduction along the northern margin of the Rheic Ocean.

Birth of the Avalon arc in Nova Scotia, Canada: geochemical evidence for [similar]700–630 Ma back-arc rift volcanism off Gondwana

Central Cape Breton Island in Nova Scotia, Canada, is host to ∼700–630 Ma felsic and associated mafic volcanic rocks that are relatively rare in other parts of the Avalon Composite Terrane, occurring elsewhere only in the Stirling Block of southern Cape Breton Island and in parts of eastern Newfoundland. The mafic rocks of central Cape Breton Island are typically intraplate tholeiitic basalts generated by melting of a garnet-bearing mantle source. They lack a continental trace element and e Nd imprint although they were emplaced on continental crust; they resemble oceanic island basalts. Contemporaneous volcanism in the Stirling Block is calc-alkaline and formed in a volcanic arc setting. In the absence of evidence for an intervening trench complex or suture, it may be inferred that the central Cape Breton tholeiites formed in a back-arc setting relative to the Stirling Block. This rifting may represent the initial stages of separation of an Avalonian arc from western Gondwana. The arc rifted further between ∼630–610 Ma when the younger Antigonish-Cobequid back-arc basin formed. Subsequently, the extensional arc became convergent, telescoping the back-arc basin. Northwestward migration of calc-alkaline arc magmatism may be related to shallowing of the associated Benioff zone through time.

The 186 Ma dashibalbar alkaline granitoid pluton in the North-gobi rift of central Mongolia: Evidence for melting of neoproterozoic basement above a plume

The Jurassic Dashibalbar granitoid pluton (∼300 km2) crops out in the Triassic North-Gobi rift of central Mongolia, just south of the 230 to 195 Ma Khentei batholith. The granitoids are shallow-seated dominantly, amphibole-bearing alkali feldspar granite that contain quartz-syenite/syenite enclaves. They are all composed of megacrystic mesoperthite, quartz, Ca-Na amphibole altered to biotite and rarely with pyroxene cores, magnetite and ilmenite. The pluton yielded a concordant U-Pb zircon age of 186 ± 1 Ma, which is similar to a published 189 ± 3 Ma 40Ar/39Ar amphibole age, and indicates rapid cooling through ca. 550 °C. This age is ca. 10 my younger than the 196 ± 4 Ma age of the bimodal volcanic complex intruded by the pluton. The volcanic complex is composed of augite-phyric transitional basalt and rhyolite/comendite. Both basalts and rhyolites/comendites are evolved within-plate varieties with positive εNd(t) (∼ +2.5) values. The granitoids are evolved alkaline, A-type granites and quartz-syenites/syenites that are enriched in light REEs, but show a distinct depletion in Eu, Sr and Ba, indicative of feldspar fractionation. The data are consistent with derivation of the granites from the syenites by an assimilation-fractional crystallization process involving a silicic crustal contaminant. The granitic rocks have εNd(t) values of ∼ +0.8 to +1.2, which are slightly lower than εNd(t) values +1.3 to +1.6 in the syenites, although both have similar TDM model ages (∼800-970 Ma). The ∼800 Ma model ages of the basalt and rhyolite/comendite are comparable to those of the intrusion and enclaves. The compositions of all these rocks, including εNd(t) and TDM, are within the range of A-type granites and volcanic complexes of the Early Mesozoic Mongolian-Transbaikalian igneous province. The results suggest derivation of a parent magma of the granitoids and felsic volcanic rocks from underplated, enriched, Neoproterozoic mantle-derived basaltic rocks in the lower crust, whereas the Dashibalbar basalts were derived from Neoproterozoic subcontinental lithospheric mantle; Neoproterozoic megablocks crop out in adjacent parts of the Central Asian Orogenic Belt. Melting of lower crust and subcontinental lithospheric mantle implies a rising heat source. Although such a heat source is consistent with both rifting and passage over the Mongolian mantle plume, only the latter explains the west-to-east migration of the magmatism and rifting.

Tectonic Plates Come Apart at the Seams

At a rate of a few centimeters per year, the movement of continents is imperceptible to transient beings like ourselves. But over geologic time, the land masses that define the world of our senses have cruised around the globe, smashing together and ripping apart. Pangea, the supercontinent that broke up more than 100 million years ago, was only the most recent union of Earths landmasses. Supercontinents and superoceans have been forming and disappearing for 3 billion years. But why do supercontinents split at one site and not another? The answer to this question is several hundred million years older than Pangea, dating back to the breakup of the previous supercontinent. The mountainous sutures of old continental collisions, it seems, carry the seeds of the next continental dispersion.