ALBERT

Description: 〈p〉 On multi-million-year timescales, Earth has experienced warm ice-free and cold glacial climates, but it is unknown if transitions between these background climate states were the result of changes in CO〈sub〉2〈/sub〉 sources or sinks. Low-latitude arc-continent collisions are hypothesized to drive cooling by uplifting and eroding mafic and ultramafic rocks in the warm, wet tropics, thereby increasing Earth’s potential to sequester carbon through chemical weathering. To better constrain global weatherability through time, the paleogeographic position of all major Phanerozoic arc-continent collisions was reconstructed and compared to the latitudinal distribution of ice-sheets. This analysis reveals a strong correlation between the extent of glaciation and arc-continent collisions in the tropics. Earth’s climate state is set primarily by global weatherability, which changes with the latitudinal distribution of arc-continent collisions.〈/p〉

Description: The study of petrology (fieldwork, petrography, and phase diagram modeling) and structural data of the metapelitic granulites and the southern, high-temperature exposed peridotites in the Beni Bousera massif (northern Morocco), combined with results from previous regional studies of the Alborán, suggest a new emplacement mechanism for the mantle rocks in the Betico-Rifean belt. We document two key metamorphic episodes in the granulites within a temperature window of 710–830 ± 50 °C: (1) An earlier prograde high-pressure period (from 9 ± 1.0 to 12 ± 1.0 kbar) characterized by the assemblage garnet + biotite + kyanite + K-feldspar + rutile. Pressure differences of ~3 kbar are found over a continuous crustal section of ~1.5 km of exposed granulites that indicate a significant crustal attenuation during exhumation of the ultramafic rocks; and (2) a later post-kinematic low-pressure (5 ± 0.8 kbar) symplectic assemblage of cordierite + spinel + plagioclase + sillimanite. At the scale of the entire Betico-Rifean belt, two main contacts are observed as mirror images in both sides of the Alborán Sea: (1) the long axis of the high-temperature ductile contact between granulites and peridotites occurs in the west side of the Beni Bousera and Ronda massifs, coupled with (2) the consistent high-angle, east-dipping normal fault in the east parts of the belt massifs. The integration of the petrologic results with information on the rotation of both contacts reveals ductile deformation in the lower crust related to the emplacement of the ultramafic rocks in the Betico-Rifean belt along deep-reaching normal faults. The presence of the early high-temperature contact suggests that it was originally a shallow, west-dipping detachment fault developed in a back-arc environment of the east-dipping, retreating subduction zone (current western part of the Gibraltar arc). This scenario is in concordance with the tectonic evolution in western Italy, where anticlockwise Pleistocene rotations associated with northeast-directed thrusting in the Apennines—and coeval with the southeastward motion in the Calabria-Peloritani terrane—were triggered by retreat and rollback of the Adriatic-Ionian slab toward the southeast during the northwest-directed subduction beneath the Calabrian arc.

Description: Intraoceanic volcanic arcs have long been recognized as sites where continental crust is created. Yet, despite their importance to understanding magmatic systems and the evolution of our planet, very little is known about their long-term rates of magma production and crust formation. Constraining both crustal construction and destruction processes at intraoceanic arcs allows for improved estimates of magma production. Our revised magma production rates for active intraoceanic arcs are consistent with those calculated for mid-ocean ridge segments that have slow to moderate spreading rates. This is surprising because magma production at intraoceanic arcs has traditionally been assumed to be significantly less than that at mid-ocean ridges.

Description: The origin of pyroxenites and their relation to melt migration in the mantle have been investigated in two pyroxenite-rich zones in the Beni Bousera massif. Based on combined field, microtextural, mineralogical and geochemical observations, the pyroxenites were separated into four types. Type-I Cr-diopside websterites contain bright green diopside and have primitive bulk Ni, Cr and Mg-number. Their trace element systematics are characterized by slight light rare earth element (LREE) enrichment compared with the middle (MREE) and heavy (H)REE, and negative high field strength element (HFSE) anomalies in bulk-rock and mineral compositions suggesting that they result from melting of metasomatized mantle. Trace element concentrations of melts calculated to be in equilibrium with Type-I cpx have a subduction-like signature and show a close similarity to certain lavas erupted in the Alboran Basin. Calculated mineral equilibration temperatures of ~1200 to 1350°C are close to the basalt liquidus and higher than for other pyroxenite types in Beni Bousera, which generally yield 〈1100°C. Type-II spinel websterites are also primitive, but contain augitic clinopyroxene; their whole-rock compositions are characterized by high Ti, Ni, and Mg-number, intermediate Cr and trace element patterns with LREE depletion over the MREE and HREE. Type-III garnet pyroxenites, which include the famous diamond-pseudomorph-bearing garnet pyroxenites, are more evolved than Types-I and -II and have low and variable Mg-number correlating with an Fe-enrichment trend. High bulk-rock and garnet HREE to LREE ratios result from high-pressure fractionation of garnet and augitic cpx at calculated pressures of 〉45 to 20–30 kbar. Type-III pyroxenites display strong variations of LREE and HFSE depletion and strong bulk Nb/Ta fractionation. Calculated melts in equilibrium with augitic cpx are variably enriched in incompatible trace elements similar to intraplate basalts. Type-IV pyroxenites are composed of green diopside, opx, garnet and plagioclase and/or spinel. Whole-rocks have high Na 2 O, CaO and Al 2 O 3 concentrations and high Mg-number, are HREE depleted, and have positive Eu and Sr anomalies. Garnets are characterized by low HREE/MREE and positive Eu anomalies. The absence of bulk-rock HREE enrichment indicates a metamorphic origin for this garnet, which is corroborated by the presence of Al-rich metamorphic spinels. Relict magmatic plagioclase indicates a shallower (〈10 kbar) crustal origin for these pyroxenites. Their metamorphic assemblage yields temperatures and pressures of 800–980°C and 14 kbar, indicating a pressure increase during the metamorphic overprint. The whole-rock geochemistry of Type-IV pyroxenites is comparable with that of rocks from the lower crustal section of the Kohistan (northern Pakistan) paleo-arc, indicating a possible origin of these rocks as cumulates in the deeper arc crust and subsequent delamination into the underlying mantle.

Description: 〈p〉Magmatic arcs associated with subduction zones are the dominant active locus of continental crust formation, and evolve in space and time towards magmatic compositions comparable to that of continental crust. Accordingly, the secular evolution of magmatic arcs is crucial to the understanding of crust formation processes. In this paper we present the first comprehensive U–Pb, Hf, Nd and Sr isotopic dataset documenting 〈i〉c.〈/i〉 120 myr of magmatic evolution in the Kohistan-Ladakh paleo-island arc. We found a long-term magmatic evolution that is controlled by the overall geodynamic of the Neo-Tethys realm. Apart from the post-collisionnal melts, the intra-oceanic history of the arc shows two main episodes (150–80 Ma and 80–50 Ma) of distinct geochemical signatures involving the slab and the sub-arc mantle components that are intimately linked to the slab dynamics.〈/p〉 〈p〉〈b〉Supplementary material:〈/b〉 All data and Concordia plots of data samples are available at 〈a href="https://doi.org/10.6084/m9.figshare.c.4234220"〉https://doi.org/10.6084/m9.figshare.c.4234220〈/a〉〈/p〉

Description: Magmatic arcs associated with subduction zones are the dominant active locus of continental crust formation, and evolve in space and time towards magmatic compositions comparable to that of continental crust. Accordingly, the secular evolution of magmatic arcs is crucial to the understanding of crust formation processes. In this paper we present the first comprehensive U–Pb, Hf, Nd and Sr isotopic dataset documenting c. 120 myr of magmatic evolution in the Kohistan-Ladakh paleo-island arc. We found a long-term magmatic evolution that is controlled by the overall geodynamic of the Neo-Tethys realm. Apart from the post-collisionnal melts, the intra-oceanic history of the arc shows two main episodes (150–80 Ma and 80–50 Ma) of distinct geochemical signatures involving the slab and the sub-arc mantle components that are intimately linked to the slab dynamics. Supplementary material: All data and Concordia plots of data samples are available at https://doi.org/10.6084/m9.figshare.c.4234220

Description: Broad areas of subcontinental lithospheric mantle are commonly exposed along ocean-continent transition zones in magma-poor rifts and are thought to be exhumed along lithospheric-scale detachment faults during the final stages of rifting. However, the nature of the transition from final rifting to seafloor spreading is controversial. We present the first high-precision U-Pb zircon geochronologic and Hf isotopic data from gabbros that intrude exhumed mantle at Ocean Drilling Program (ODP) Sites 1070 and 1277 in the Newfoundland-Iberia rift (North Atlantic). The sites are conjugate to one another within crust that is commonly considered to have been emplaced during early seafloor spreading. Magnetic data suggest that crustal accretion occurred at both sites during magnetic polarity chrons M3–M0 (130–126 Ma). However, our data indicate that asthenospheric melts were emplaced over brief intervals (≤1 m.y.) prior to or coeval with mantle exhumation at 124 Ma at ODP Site 1070 and 115 Ma at ODP Site 1277. We suggest that this discrepancy is the result of continued mantle exhumation along large, west-dipping detachment faults until lithospheric breakup. The breakup location is likely coincident with the large-amplitude magnetic J anomaly, and our 115 Ma date for magmatism within this anomaly provides the best available age constraint for breakup along the studied transect.

Description: Quantifying the time scales of magmatic differentiation is critical for understanding the rate at which silicic plutonic and volcanic rocks form. Directly dating this process is difficult because locations with both clear evidence for fractional crystallization and the accessory phases necessary for radiometric dating are rare. Early zircon saturation, however, appears to be characteristic of many high-K, arc-related melts due to their generally elevated initial Zr concentrations. Thus, high-K plutonic series are ideal candidates to study the time scales of magmatic differentiation using zircon U-Pb geochronology. This study focuses on the Dariv Igneous Complex in western Mongolia where early saturation of zircon in a suite of cogenetic, upper crustal (〈0.5 GPa) igneous rocks ranging from ultramafic cumulates to evolved granitoids allows us to date magmatic differentiation. Crystallization ages from six samples across the sequence indicate that magmatic fractionation from a basalt to high-silica (〉65 wt% SiO 2 ) melt occurred in ≤590 ± 350 k.y. This estimate is greater than modeled time scales of conductive cooling of a single intrusion and physical segregation of minerals from a melt, suggesting that continued influx of heat through magmatic activity in the complex may have prolonged cooling and thus time scales associated with the production of silica-enriched melts.

Description: 〈p〉On multimillion-year time scales, Earth has experienced warm ice-free and cold glacial climates, but it is unknown whether transitions between these background climate states were the result of changes in carbon dioxide sources or sinks. Low-latitude arc-continent collisions are hypothesized to drive cooling by exhuming and eroding mafic and ultramafic rocks in the warm, wet tropics, thereby increasing Earth’s potential to sequester carbon through chemical weathering. To better constrain global weatherability through time, the paleogeographic position of all major Phanerozoic arc-continent collisions was reconstructed and compared to the latitudinal distribution of ice sheets. This analysis reveals a strong correlation between the extent of glaciation and arc-continent collisions in the tropics. Earth’s climate state is set primarily by global weatherability, which changes with the latitudinal distribution of arc-continent collisions.〈/p〉