Geochemical, Isotopic and Petrological Constraints on the Origin and Evolution of the Recent Silicic Magmatism of the Greater Caucasus

Significant volumes of rhyolites and granites of the Pliocene-Pleistocene age are exposed in the collision zone of the Greater Caucasus, Russia. The volcanic history of the region includes ignimbrites and lavas associated with the Chegem caldera (2.9 Ma) and Elbrus volcano (1.98 and 0.7 Ma) and rhyolitic necks and granites in Tyrnyauz (1.98 Ma). They are characterized by a similar bulk and mineral composition and close ratios of incompatible elements, which indicates their related origin. The 1.98 Ma Elbrus ignimbrites, compared to the 2.9 Ma Chegem ignimbrites, have elevated concentrations of both compatible (Cr, Sr, Ca, Ni) and incompatible elements (Cs, Rb, U). We argue that the Elbrus ignimbrites were produced from magma geochemically similar to Chegem rhyolites through fractionation crystallization coupled with the assimilation of crustal material. The 1.98 Ma Eldjuta granites of Tyrnyauz and early ignimbrites of the Elbrus region (1.98 Ma) are temporally coeval, similar mineralogically, and have comparable major and trace element composition, which indicates that the Elbrus ignimbrites probably erupted from the area of modern Tyrnyauz; the Eldjurta granite could represent a plutonic reservoir that fed this eruption. Late ignimbrites of Elbrus (0.7 Ma) and subsequent lavas demonstrate progressively more mafic mineral assemblage and bulk rock composition in comparison with rhyolites. This indicates their origin in response to the mixing of rhyolites with magmas of a more basic composition at the late stage of magma system development. The composition of these basic magmas may be close to the basaltic trachyandesite, the flows exposed along the periphery of the Elbrus volcano. All studied young volcanic rocks of the Greater Caucasus are characterized by depletion in HSFE and enrichment in LILE, Li, and Pb, which emphasizes the close relationship of young silicic magmatism with magmas of suprasubduction geochemical affinity. An important geochemical feature is the enrichment of U up to 8 ppm and Th up to 35 ppm. The trace element composition of the rocks indicates that the original rhyolitic magma of Chegem ignimbrites caldera was formed at >80%–90% fractionation of calc-alkaline arc basalts with increased alkalinity. This observation, in addition to published data for isotopic composition (O-Hf-Sr) of the same units, shows that the crustal isotopic signatures of silicic volcanics may arise due to the subduction-induced fertilization of peridotites producing parental basaltic magmas before a delamination episode reactivated the melting of the former mantle and the lower crust.

The Petrology and Petrochemistry of Andesite and Dacite Volcanoes in Eastern Bay of Plenty, New Zealand

<p>The volcanic rocks of Edgecumbe, Whale Island, White Island and Manawahe are andesites and dacites, which are collectively termed the Bay of Plenty volcanics. Edgecumbe is a comparatively young volcano, being active between 1700 and 8000 years B.P.; Whale Island has probably been inactive for at least the last 36,000 years; White Island has probably been active for much of the late Pleistocene, and is still in a stage of solfataric activity with intermittent tephra eruptions; and Manawahe is probably of the order of 750,000 year old (K-Ar date by J.J. Stipp). The geology of Edgecumbe, Whale Island and White Island is discussed, and the petrography and mineralogy of the Bay of plenty volcanics is discussed and compared. The rocks of Edgecumbe and Whale Island are extremely similar petrographically, but the rocks of White Island and Manawahe are sufficiently different that they can be distinguished both from one another and from Edgecumbe and Whale Island rocks. Most of the Bay of Plenty volcanics are plagioclase andesites or plagioclase dacites. New total rock analyses for 28 elements in 44 samples of the Bay of Plenty volcanics are presented, together with analyses of 4 samples from elsewhere in the Taupo Volcanic Zone. Three samples were analysed for an additional 17 elements. The Bay of Plenty volcanics are calc-alkaline and are predominantly dacites (greater than or equal to 63% SiO2) by Taylor et al.'s (1969) definition, but there is chemical continuity from samples with about 61% SiO2 to samples with about 66% SiO2. Major and trace element variation trends cannot be explained entirely by a crystal fractionation hypothesis, and assimilation of upper crustal material of rhyolitic composition best explains the variation trends for Edgecumbe and Whale Island. The variation trends and certain element abundances in White Island rocks suggest the assimilation of marine sediments, and introduction of seawater into the magma. Taken as a whole, the Bay of Plenty volcanics fit the chemical trends which have been established for the Taupo Zone by earlier workers (e.g. Steiner, 1958; Clark, 1960). The apparent geochemical 'gap' or discontinuity between about 68% and 71.5% SiO2 noted by Steiner (1958) is further substantiated by the new geochemical data presented here. It is considered likely that basalt, andesite and rhyolite are all primary magmas in the Taupo Volcanic Zone. Their possible origins, and the origins of Taupo Zone dacites are discussed.</p>

Lithospheric Xenoliths from the Marie Byrd Land Volcanic Province, West Antarctica

<p>Studies of the Earths lithosphere, and particularly the lower crust, have in the past relied on geophysical methods, and on geochemical studies of granulite terrains exposed at the surface. Geophysical studies can not evaluate the compositions to any large extent. Granulite terrains typically represent ancient rather than present day sections, have invariably suffered retrograde metamorphism, and have been affected by fluids during uplift. More recently, studies of lithospheric xenoliths (fragments of the lithosphere brought to the surface by entraining (typically alkaline) melts) have been used to study the composition of, and processes influencing, the lithosphere. Xenoliths have the advantage of representing relatively unaltered and young fragments of the lithosphere, and together with other studies have added much to our understanding of the Earths composition and processes. The study of the lithosphere in Marie Byrd Land (MBL), West Antarctica, is complicated by the difficult access and harsh climate of the region. Geophysical studies are limited, and deep crustal exposures are entirely absent. In an attempt to study the composition and structure of the MBL lithosphere, xenoliths were collected from various volcanic edifices in MBL, including the volcanoes of the Executive Committee Range (ECR), and the USAS Escarpment in central MBL, and Mount Murphy on the Walgreen coast. The xenolith suite consists of peridotites, pyroxenites and granulites, spanning a vertical section from upper mantle to lower crust, that are in pristine condition, due to the arid Antarctic conditions. The peridotite suite from MBL consists of spinel Iherzolites from Mounts Hampton and Cumming in the ECR, the USAS Escarpment, and Mount Murphy. Cr-diopside rich peridotites also occur at Mounts Hampton and Murphy, indicating a more chemically diverse upper mantle in these regions (e.g. Mg# 75-92 in Cr-diopside rich peridotites compared to Mg# 87-92 in spinel Iherzolites). REE contents of the peridotites vary from LREE-depleted (up to 0.293 (La/Yb)n in USAS Escarpment peridotites) to LREE-enriched (up to 10.015 (La/Yb)n in Mount Hampton peridotites), further indicating the extreme heterogeneity of the MBL upper mantle. Lower crustal xenoliths from Mounts Sidley and Hampton in the ECR, and from Mount Murphy have meta-igneous textures ranging from pyroxenite to gabbro. They consist of varying amounts of olivine, clinopyroxene, orthopyroxene, plagioclase and spinels; garnet is entirely absent. Orthopyroxene is absent in Mount Sidley xenoliths, whereas olivine is rare in Mount Hampton xenoliths. Mineral P-T equilibria indicate crystallisation of Mounts Sidley and Murphy pyroxenites at lower levels (7-11 kb and 6.5-12 kb respectively) than the granulites (3-5.5 kb and 3-9 kb), with Mount Hampton pyroxenites (6-7.5 kb) and granulites (5.5-8.5 kb) crystallising at similar crustal levels. High temperatures of equilibration (> 1000 [degrees] C) are consistent with a rift-like geotherm in the MBL lithosphere. Whole rock composition of the lower crustal xenoliths is controlled by the mineral assemblage, reflecting their origin as mafic cumulate rocks. Elements that partition readily into the xenolith mineral assemblage are present in higher abundances (e.g. up to 1700 ppm Sr in plagioclase rich xenoliths, and 3745 ppm Cr in clinopyroxene rich pyroxenites) than elements that do not (e.g. Rb < 6 ppm in all lower crustal xenoliths). 87Sr/86Sr (0.702861 [plus or minus] 7 to 0.704576 [plus or minus] 15) and 143Nd/144Nd (0.512771 [plus or minus] 6 to 0.512870 [plus or minus] 5) ratios indicate that the melts were primitive magmas, that did not assimilate any isotopically evolved crustal material prior to or during crystallisation. The single-pyroxene mineral assemblage of Mount Sidley (and possibly Mount Murphy) xenoliths crystallised from an alkaline melt, whereas the two-pyroxene assemblage of Mount Hampton xenoliths crystallised from a sub-alkaline melt. Xenoliths from Mount Sidley reveal petrographic and geochemical evidence for melt-fluid interaction at lower crustal depths. This interaction is inferred to be associated with late Cenozoic plume-related volcanism. It is manifested by high-temperature oxidation of olivine, replacement of clinopyroxene by kaersutite, traces of alkaline mafic glass, and the growth of apatite, Fe-Ti oxides and plagioclase. The xenolith suite has been enriched in elements that readily partition into these mineral phases (e.g. Ti, K, P, Sr, Ba), as well as in mobile elements (e.g. LILEs and LREEs). Pb isotopic ratios (e.g. 206Pb/204Pb from 18.005 - 19.589) and REEs define mixing lines between unradiogenic lower crust (206Pb/204Pb = 18.005) and small volume melts (206Pb/204Pb > 19.53) approaching HIMU composition, sourced from the inferred mantle plume. The composition of the infiltrating melts has also evolved, by percolative fractional crystallisation in the lower crust. The chemical heterogeneity detected in the MBL lower crust indicates a lower crustal discontinuity in the ECR, between Mount Sidley and Mount Hampton, here termed the ECR lower crustal discontinuity. Granulites from Mount Sidley are similar in composition to granulites from the Transantarctic Mountains (TM) in the McMurdo Sound region, Mount Ruapehu and Fiordland (New Zealand). Granulites from Mount Hampton are similar in composition to granulites from Mount Murphy, and the Ross Embayment (RE). These groups have been termed the TM Group and the RE Group respectively. The compositional similarity of granulites in each group may indicate the derivation of the lower crust in these regions from similar melts, and possibly indicate their juxtaposition as parts of the Gondwana supercontinent. The mafic cumulate character of the xenolith suite is inferred to represent original oceanic crust, and a model for the growth of the crust since its formation in latest pre-Cambrian - early Cambrian is presented here.</p>

The Petrology and Petrochemistry of Andesite and Dacite Volcanoes in Eastern Bay of Plenty, New Zealand

<p>The volcanic rocks of Edgecumbe, Whale Island, White Island and Manawahe are andesites and dacites, which are collectively termed the Bay of Plenty volcanics. Edgecumbe is a comparatively young volcano, being active between 1700 and 8000 years B.P.; Whale Island has probably been inactive for at least the last 36,000 years; White Island has probably been active for much of the late Pleistocene, and is still in a stage of solfataric activity with intermittent tephra eruptions; and Manawahe is probably of the order of 750,000 year old (K-Ar date by J.J. Stipp). The geology of Edgecumbe, Whale Island and White Island is discussed, and the petrography and mineralogy of the Bay of plenty volcanics is discussed and compared. The rocks of Edgecumbe and Whale Island are extremely similar petrographically, but the rocks of White Island and Manawahe are sufficiently different that they can be distinguished both from one another and from Edgecumbe and Whale Island rocks. Most of the Bay of Plenty volcanics are plagioclase andesites or plagioclase dacites. New total rock analyses for 28 elements in 44 samples of the Bay of Plenty volcanics are presented, together with analyses of 4 samples from elsewhere in the Taupo Volcanic Zone. Three samples were analysed for an additional 17 elements. The Bay of Plenty volcanics are calc-alkaline and are predominantly dacites (greater than or equal to 63% SiO2) by Taylor et al.'s (1969) definition, but there is chemical continuity from samples with about 61% SiO2 to samples with about 66% SiO2. Major and trace element variation trends cannot be explained entirely by a crystal fractionation hypothesis, and assimilation of upper crustal material of rhyolitic composition best explains the variation trends for Edgecumbe and Whale Island. The variation trends and certain element abundances in White Island rocks suggest the assimilation of marine sediments, and introduction of seawater into the magma. Taken as a whole, the Bay of Plenty volcanics fit the chemical trends which have been established for the Taupo Zone by earlier workers (e.g. Steiner, 1958; Clark, 1960). The apparent geochemical 'gap' or discontinuity between about 68% and 71.5% SiO2 noted by Steiner (1958) is further substantiated by the new geochemical data presented here. It is considered likely that basalt, andesite and rhyolite are all primary magmas in the Taupo Volcanic Zone. Their possible origins, and the origins of Taupo Zone dacites are discussed.</p>

Lithospheric Xenoliths from the Marie Byrd Land Volcanic Province, West Antarctica

<p>Studies of the Earths lithosphere, and particularly the lower crust, have in the past relied on geophysical methods, and on geochemical studies of granulite terrains exposed at the surface. Geophysical studies can not evaluate the compositions to any large extent. Granulite terrains typically represent ancient rather than present day sections, have invariably suffered retrograde metamorphism, and have been affected by fluids during uplift. More recently, studies of lithospheric xenoliths (fragments of the lithosphere brought to the surface by entraining (typically alkaline) melts) have been used to study the composition of, and processes influencing, the lithosphere. Xenoliths have the advantage of representing relatively unaltered and young fragments of the lithosphere, and together with other studies have added much to our understanding of the Earths composition and processes. The study of the lithosphere in Marie Byrd Land (MBL), West Antarctica, is complicated by the difficult access and harsh climate of the region. Geophysical studies are limited, and deep crustal exposures are entirely absent. In an attempt to study the composition and structure of the MBL lithosphere, xenoliths were collected from various volcanic edifices in MBL, including the volcanoes of the Executive Committee Range (ECR), and the USAS Escarpment in central MBL, and Mount Murphy on the Walgreen coast. The xenolith suite consists of peridotites, pyroxenites and granulites, spanning a vertical section from upper mantle to lower crust, that are in pristine condition, due to the arid Antarctic conditions. The peridotite suite from MBL consists of spinel Iherzolites from Mounts Hampton and Cumming in the ECR, the USAS Escarpment, and Mount Murphy. Cr-diopside rich peridotites also occur at Mounts Hampton and Murphy, indicating a more chemically diverse upper mantle in these regions (e.g. Mg# 75-92 in Cr-diopside rich peridotites compared to Mg# 87-92 in spinel Iherzolites). REE contents of the peridotites vary from LREE-depleted (up to 0.293 (La/Yb)n in USAS Escarpment peridotites) to LREE-enriched (up to 10.015 (La/Yb)n in Mount Hampton peridotites), further indicating the extreme heterogeneity of the MBL upper mantle. Lower crustal xenoliths from Mounts Sidley and Hampton in the ECR, and from Mount Murphy have meta-igneous textures ranging from pyroxenite to gabbro. They consist of varying amounts of olivine, clinopyroxene, orthopyroxene, plagioclase and spinels; garnet is entirely absent. Orthopyroxene is absent in Mount Sidley xenoliths, whereas olivine is rare in Mount Hampton xenoliths. Mineral P-T equilibria indicate crystallisation of Mounts Sidley and Murphy pyroxenites at lower levels (7-11 kb and 6.5-12 kb respectively) than the granulites (3-5.5 kb and 3-9 kb), with Mount Hampton pyroxenites (6-7.5 kb) and granulites (5.5-8.5 kb) crystallising at similar crustal levels. High temperatures of equilibration (> 1000 [degrees] C) are consistent with a rift-like geotherm in the MBL lithosphere. Whole rock composition of the lower crustal xenoliths is controlled by the mineral assemblage, reflecting their origin as mafic cumulate rocks. Elements that partition readily into the xenolith mineral assemblage are present in higher abundances (e.g. up to 1700 ppm Sr in plagioclase rich xenoliths, and 3745 ppm Cr in clinopyroxene rich pyroxenites) than elements that do not (e.g. Rb < 6 ppm in all lower crustal xenoliths). 87Sr/86Sr (0.702861 [plus or minus] 7 to 0.704576 [plus or minus] 15) and 143Nd/144Nd (0.512771 [plus or minus] 6 to 0.512870 [plus or minus] 5) ratios indicate that the melts were primitive magmas, that did not assimilate any isotopically evolved crustal material prior to or during crystallisation. The single-pyroxene mineral assemblage of Mount Sidley (and possibly Mount Murphy) xenoliths crystallised from an alkaline melt, whereas the two-pyroxene assemblage of Mount Hampton xenoliths crystallised from a sub-alkaline melt. Xenoliths from Mount Sidley reveal petrographic and geochemical evidence for melt-fluid interaction at lower crustal depths. This interaction is inferred to be associated with late Cenozoic plume-related volcanism. It is manifested by high-temperature oxidation of olivine, replacement of clinopyroxene by kaersutite, traces of alkaline mafic glass, and the growth of apatite, Fe-Ti oxides and plagioclase. The xenolith suite has been enriched in elements that readily partition into these mineral phases (e.g. Ti, K, P, Sr, Ba), as well as in mobile elements (e.g. LILEs and LREEs). Pb isotopic ratios (e.g. 206Pb/204Pb from 18.005 - 19.589) and REEs define mixing lines between unradiogenic lower crust (206Pb/204Pb = 18.005) and small volume melts (206Pb/204Pb > 19.53) approaching HIMU composition, sourced from the inferred mantle plume. The composition of the infiltrating melts has also evolved, by percolative fractional crystallisation in the lower crust. The chemical heterogeneity detected in the MBL lower crust indicates a lower crustal discontinuity in the ECR, between Mount Sidley and Mount Hampton, here termed the ECR lower crustal discontinuity. Granulites from Mount Sidley are similar in composition to granulites from the Transantarctic Mountains (TM) in the McMurdo Sound region, Mount Ruapehu and Fiordland (New Zealand). Granulites from Mount Hampton are similar in composition to granulites from Mount Murphy, and the Ross Embayment (RE). These groups have been termed the TM Group and the RE Group respectively. The compositional similarity of granulites in each group may indicate the derivation of the lower crust in these regions from similar melts, and possibly indicate their juxtaposition as parts of the Gondwana supercontinent. The mafic cumulate character of the xenolith suite is inferred to represent original oceanic crust, and a model for the growth of the crust since its formation in latest pre-Cambrian - early Cambrian is presented here.</p>

Light oxygen isotopes in mantle-derived magmas reflect assimilation of sub-continental lithospheric mantle material

Oxygen isotope ratios in mantle-derived magmas that differ from typical mantle values are generally attributed to crustal contamination, deeply subducted crustal material in the mantle source or primordial heterogeneities. Here we provide an alternative view for the origin of light oxygen-isotope signatures in mantle-derived magmas using kimberlites, carbonate-rich magmas that assimilate mantle debris during ascent. Olivine grains in kimberlites are commonly zoned between a mantle-derived core and a magmatic rim, thus constraining the compositions of both mantle wall-rocks and melt phase. Secondary ion mass spectrometry (SIMS) analyses of olivine in worldwide kimberlites show a remarkable correlation between mean oxygen-isotope compositions of cores and rims from mantle-like 18O/16O to lower ‘crustal’ values. This observation indicates that kimberlites entraining low-18O/16O olivine xenocrysts are modified by assimilation of low-18O/16O sub-continental lithospheric mantle material. Interaction with geochemically-enriched domains of the sub-continental lithospheric mantle can therefore be an important source of apparently ‘crustal’ signatures in mantle-derived magmas.

Early Mesozoic Rare-Metal Granites and Metasomatites of Mongolia: Mineral and Geochemical Features and Hosted Ore Mineralization (Baga Gazriin Chuluu Pluton)

—We present results of petrographic, mineralogical, and geochemical study of all types of rocks of a multiphase pluton and consider the chemical evolution of igneous and metasomatic rocks of the Baga Gazriin Chuluu pluton, based on new precise analytical data. At the early stage of their formation, the pluton granites were already enriched in many trace elements (Li, Rb, Cs, Be, Nb, Ta, Th, and U), F, and HREE relative to the upper continental crust. They show strong negative Ba, Sr, La, and Eu anomalies, which is typical of rare-metal Li–F granites. The geochemical evolution of the Baga Gazriin Chuluu multiphase pluton at the postmagmatic stage was marked by the most intense enrichment of greisens and microclinites with lithophile and ore elements (Sn, W, and Zn) and the formation of ore mineralization. In the permeable rift zone where the Baga Gazriin Chuluu pluton is located, the fluid–magma interaction took place under the impact of a mantle plume. High-temperature mantle fluids caused melting of the crustal substratum, which determined the geochemical specifics of Li–F granite intrusions. Genesis of granitic magma enriched in Li, F, Rb, Sn, and Ta is possible at the low degrees of melting of the lower crustal substratum. The Baga Gazriin Chuluu pluton formed in the upper horizons of the Earth’s crust, where magma undergoes strong differentiation and the saturation of fluids with volatiles can lead to the postmagmatic formation of metasomatites of varying alkalinity (zwitters (greisens), microclinites, and albitites) producing rare-metal mineralization. By the example of the early Mesozoic magmatism area of Mongolia, it is shown that the formation of granites and associated rare-metal minerals is due to the interaction of mantle fluids with the crustal material and the subsequent evolution of granitic magmas.

A geochemical study of the Crown Formation and Bird Member lavas of the Mesoarchaean Witwatersrand Supergroup, South Africa

The ca. 2.97 to 2.80 Ga Witwatersrand Supergroup, South Africa, represents the oldest intracontinental sedimentary basin of the Kaapvaal craton. Two volcanic units occur in this supergroup: the widespread Crown Formation lavas in the marine shale-dominated West Rand Group and the more geographically restricted Bird Member lavas, intercalated with fluvial to fluvio-deltaic sandstone and conglomerate of the Central Rand Group. These units remain poorly studied as they are rarely exposed and generally deeply weathered when cropping out. We report whole-rock major and trace elements, Hf and Nd-isotope whole-rock analyses of the lavas from core samples drilled in the south of the Witwatersrand basin and underground samples from the Evander Goldfield in the northeast. In the studied areas, both the Crown Formation and Bird Member are composed of two units of lava separated by sandstone. Whereas all the Crown Formation samples show a similar geochemical composition, the upper and lower volcanic units of the Bird Member present clear differences. However, the primitive mantle-normalized incompatible trace element concentrations of all Crown Formation and Bird Member samples show variously enriched patterns and marked negative Nb and Ta anomalies relative to Th and La. Despite the convergent geodynamic setting of the Witwatersrand Supergroup suggested by the literature, the Crown Formation and Bird Member are probably not related to subduction-related magmatism but more to decompression melting. Overall, the combined trace element and Sm-Nd isotopic data indicate melts from slightly to moderately depleted sources that were variably contaminated with crustal material. Greater contamination, followed by differentiation in different magma chambers, can explain the difference between the two signatures of the Bird Member. Finally, despite previous proposals for stratigraphically correlating the Witwatersrand Supergroup to the Mozaan Group of the Pongola Supergroup, their volcanic units are overall geochemically distinct.

Crustal structure of the East African Limpopo margin, a strike-slip rifted corridor along the continental Mozambique Coastal Plain and North Natal Valley

Coincident wide-angle and multi-channel seismic data acquired within the scope of the PAMELA Moz3-5 project allow us to reconsider the formation mechanism of East African margins offshore of southern Mozambique. This study specifically focuses on the sedimentary and deep-crustal architecture of the Limpopo margin (LM) that fringes the eastern edge of the Mozambique’s Coastal Plain (MCP) and its offshore southern prolongation the North Natal Valley (NNV). It relies primarily on the MZ3 profile that runs obliquely from the northeastern NNV towards the Mozambique basin (MB) with additional inputs from a tectonostratigraphy analysis of industrial onshore–offshore seismic lines and nearby or crossing velocity models from companion studies. Over its entire N–S extension the LM appears segmented into (1) a western domain that shows the progressive eastward crustal thinning and termination of the MCP/NNV continental crust and its overlying pre-Neocomian volcano-sedimentary basement and (2) a central corridor of anomalous crust bounded to the east by the Mozambique fracture zone (MFZ) and the oceanic crust of the MB. A prominent basement high marks the boundary between these two domains. Its development was most probably controlled by a steep and deeply rooted fault, i.e., the Limpopo fault. We infer that strike-slip or slightly transtensional rifting occurred along the LM and was accommodated along this Limpopo fault. At depth we propose that ductile shearing was responsible for the thinning of the continental crust and an oceanward flow of lower crustal material. This process was accompanied by intense magmatism that extruded to form the volcanic basement and gave the corridor its peculiar structure and mixed nature. The whole region remained at a relative high level during the rifting period and a shallow marine environment dominated the pre-Neocomian period during the early phase of continent–ocean interaction. It is only some time after break-up in the MB and the initiation of the MFZ that decoupling occurred between the MCP/NNV and the corridor, allowing for the latter to subside and become covered by deep marine sediments. A scenario for the early evolution and formation of the LM is proposed taking into account both recent kinematic and geological constraints. It implies that no or little change in extensional direction occurred between the intra-continental rifting and subsequent phase of continent–ocean interaction.

Neodymium isotope mapping a polygenetic TTG batholith: Failed back-arc rifting in the Central Metasedimentary Belt, SW Grenville Province

Fifty-five new Nd isotope analyses are presented for plutonic orthogneisses from the Grimsthorpe domain in the marble-rich segment of the Grenvillian Central Metasedimentary Belt (CMB) to test the back-arc aulacogen model for its origin. Nd isotope analyses from the Weslemkoon batholith, Elzevir batholith, Lingham Lake complex and Canniff tonalite are used to probe the crustal formation age of their source rocks. Despite its concentric foliation, the Weslemkoon batholith displays a complex geochemical pattern consisting of several NE trending domains with older TDM ages, surrounded by juvenile crustal material. The new Nd isotope results, coupled with geochemistry for the Weslemkoon and Elzevir batholiths depict the fragmentation of a block of old crust that formed a screen between en echelon segments of a mid-Mesoproterozoic back-arc rift zone. The isotope boundaries identified within the Weslemkoon batholith delineate magma pulses sampling two distinct sources, interpreted as Laurentian basement and juvenile basaltic underplate. Underplating could be attributed to slab rollback under the pre-Grenvillian continental margin arc. The intensification of rift-related magmatism in the CMB is demonstrated by its bimodal petrological character. A modern analogue for the tectonic context of the CMB is the Gulf of California, where subduction-related magmatism has transitioned to rift-related magmatism. However, the Gulf of California exhibits more transcurrent motion than is evidenced by the geometry of the CMB rift. A geometrical analogue for the break-up of the Elzevir block between two rift segments is provided by the Danakil block of the Red Sea, which is currently undergoing similar tectonic fragmentation.