Marc K. Reichow

Drilling to gabbro in intact ocean crust

Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.

Petrogenesis of syenite-granite suites from the Bryansky Complex (Transbaikalia, Russia): Implications for the origin of A-type granitoid magmas

Abstract Two syenite–granite suites, metaluminous and peralkaline, that form the Bryansky Complex in Transbaikalia, Russia, have been studied with the aim to constrain the existing models of A-type granitoid magma generation. The Bryansky Complex is a large intrusive body of about 1600 km2 emplaced in the central part of the Mongolian–Transbaikalian granitoid belt, which extends for more than 2000 km and is 200–300 km wide. The Belt comprises about 350 A-type granitoid plutons and numerous volcanic fields. U–Pb and Rb–Sr isotope dating revealed that all the intrusive rocks of the Complex and closely associated comendites were emplaced within a narrow time span, 279–285 Ma. The isotope characteristics are rather similar for all main rock types. The metaluminous suite has a (87Sr/86Sr)T value of 0.7050±0.001, eNd(T) from −1.9 to −3.0, and the peralkaline suite has (87Sr/86Sr)T=0.7053±0.0008, eNd(T)=−2.1 and −2.4. Comendites and trachyandesites have similar eNd(T) values (from −2.2 to −3.5), but a slightly higher (87Sr/86Sr)T value of 0.7062±0.0002. The systematic change in chemical and mineralogical composition from syenitic to granitic rocks in both suites and the similar isotopic ratios suggest that the granites were formed by fractional crystallization of the syenite magmas. Several lines of evidence suggest that metaluminous syenite is the parental magma for the whole Bryansky Complex. Study of melt inclusions in quartz phenocrysts from the peralkaline granite and in pyroxene from the nordmarkite indicates that fractional crystallization has resulted in significant F enrichment in the granitic magma (up to 1.5–1.7 wt.%). The syenite magmas crystallized at rather high temperature >940 °C whereas the near-liquidus temperature of the peralkaline granite was lower, 760–790 °C. Very high homogenization temperatures of the melt inclusions in quartz phenocrysts from comendites (1000–1100 °C) suggest that the alkali-rich silicic magma formed at a depth of 50–60 km (?) far exceeding the normal crust thickness. The Sr–Nd isotope data advocate the main role of mantle-derived material in the source region from which the alkali-rich syenitic and granitic magmas were produced.

Age and evolution of late Mesozoic metamorphic core complexes in southern Siberia and northern Mongolia

Numerous Cretaceous metamorphic core complexes (MCCs) extend from Transbaikalia in Russia to northern Mongolia within the Central Asian Orogenic Belt. We investigated the Buteel and Zagan MCCs in detail. Shear sense indicators in mylonitized rocks show footwall-to-the-NW tectonic transport. Single zircon dating of footwall rocks in the Buteel MCC establishes the emplacement of granitoid orthogneiss precursors at 240–211 Ma, a felsic metavolcanic rock at 265.0 ± 1.2 Ma, a syenite at 265.5 ± 1.2 Ma and a metarhyolite of the pre-granitoid basement at 553.6 ± 2.9 Ma. A peralkaline granite intruding orthogneisses of the Zagan MCC has a new U–Pb zircon age of 151.6 ± 0.7 Ma. 40Ar/39Ar ages of 133.5 ± 1.8 Ma of hornblende from amphibolite and 122.6 ± 1.8 Ma of biotite from mylonitized gabbro–dolerite of the Buteel MCC are interpreted as cooling ages representing the time of deformation in the footwall. Geological data suggest that the MCCs in Transbaikalia and northern Mongolia formed as a result of extension in a crust that had previously been thickened by abundant calc-alkaline magmatism in an Andean-type setting on the border of the closing Mongol–Okhotsk ocean, by widespread collisional to post-collisional thrusting, and by extensive alkaline–peralkaline magmatism.

Mantle domains in the lithosphere beneath the French Massif Central: trace element and isotopic evidence from mantle clinopyroxenes

Lenoir et al. [Earth Planet. Sci. Lett. 181 (2000) 359] have suggested the existence of a lithospheric domain boundary at approximately 45°30′N within the French Massif Central because of differences in bulk rock geochemistry between mantle peridotite xenoliths from localities north and south of this latitude. New laser ablation ICP–MS analyses of trace element contents in clinopyroxenes from xenoliths from the northern domain, combined with Sr and Nd isotopic data for the same clinopyroxenes, clearly indicate significant differences between the northern and southern domains. LA–ICP–MS results show that the northern domain peridotites have experienced significantly more depletion than ones from the southern domain, having undergone almost twice as much fractional melting. Although they do not contain garnet at the present day, the northern domain peridotites appear to have contained garnet when they underwent melting. They can have extremely high Sm/Nd ratios (>0.5), indicating unusually strong depletion in Nd relative to Sm. However, they are also more enriched in LREE and Sr, and show significant fractionation of Zr from Hf, and of U from Th (Zr/Hf and Th/U≪chondritic values). This is due to metasomatism, possibly related to carbonitite infiltration. The Sr–Nd isotopic compositions of northern domain xenoliths are also unusual, in that many have high 143Nd/144Nd or high 87Sr/86Sr values which do not occur in samples from the southern domain. In contrast, the southern domain mantle shows low (MORB source) 87Sr/86Sr values not found in the northern domain. This confirms that significant differences exist between the two domains and offers strong support to the concept of the existence of lithospheric blocks with separate histories beneath the Massif Central. A remarkable similarity exists between the mantle beneath the northern domain of the Massif Central and that beneath the Rhon volcanic area (Germany) 1000 km to the northeast.

Mid-Miocene record of large-scale Snake River–type explosive volcanism and associated subsidence on the Yellowstone hotspot track: The Cassia Formation of Idaho, USA

The 1.95-km-thick Cassia Formation, defined in the Cassia Hills at the southern margin of the Snake River Plain, Idaho, consists of 12 refined and newly described rhyolitic members, each with distinctive field, geochemical, mineralogical, geochronological, and paleomagnetic characteristics. It records voluminous high-temperature, Snake River−type explosive eruptions between ca. 11.3 Ma and ca. 8.1 Ma that emplaced intensely welded rheomorphic ignimbrites and associated ash-fall layers. One ignimbrite records the ca. 8.1 Ma Castleford Crossing eruption, which was of supereruption magnitude (∼1900 km 3 ). It correlates regionally and exceeds 1.35 km thickness within a subsided, proximal caldera-like depocenter. Major- and trace-element data define three successive temporal trends toward less-evolved rhyolitic compositions, separated by abrupt returns to more-evolved compositions. These cycles are thought to reflect increasing mantle-derived basaltic intraplating and hybridization of a midcrustal region, coupled with shallower fractionation in upper-crustal magma reservoirs. The onset of each new cycle is thought to record renewed intraplating at an adjacent region of crust, possibly as the North American plate migrated westward over the Yellowstone hotspot. A regional NE-trending monocline, here termed the Cassia monocline, was formed by synvolcanic deformation and subsidence of the intracontinental Snake River basin. Its structural and topographic evolution is reconstructed using thickness variations, offlap relations, and rheomorphic transport indicators in the successive dated ignimbrites. The subsidence is thought to have occurred in response to incremental loading and modification of the crust by the mantle-derived basaltic magmas. During this time, the area also underwent NW-trending faulting related to opening of the western Snake River rift and E-W Basin and Range extension. The large eruptions probably had different source locations, all within the subsiding basin. The proximal Miocene topography was thus in marked contrast to the more elevated present-day Yellowstone plateau.

Magnetic anisotropy in rhyolitic ignimbrite, Snake River Plain: Implications for using remanent magnetism of volcanic rocks for correlation, paleomagnetic studies, and geological reconstructions

Some of the field and laboratory expenses for this work were covered by the Natural Environment Research Council (NERC) grant NE/G005372/1. The salary and school fees for the first author were covered in part by a National Science Foundation NSF grant (EAR 443549–81179) awarded to X. Zhao.

Cyanobacterial blooms tied to volcanism during the 5 m.y. Permo–Triassic biotic crisis: COMMENT

40 Ar/ 39 Ar ages obtained on volcanic ash layers from South China, and volcanic rocks of the Siberian Traps (ST), respectively, Xie et al. argue that the volcanism associated with the ST is predominantly younger than the P–Tr boundary age. Xie et al. note that the majority of ST 40 Ar/ 39 Ar ages (e.g., Reichow et al., 2009) are similar to U/Pb zircon ages for two Triassic boundaries, and consequently that ST volcanism was likely responsible for the prolonged stress in the Early Triassic ecosystems. However, the suggested age correlation is fl and the purpose of this Comment is to challenge the comparison based on ages obtained by different methodologies, and demonstrate that one of the conclusions drawn by Xie et al. is invalid. Ar/

Weak Vertical Surface Movement Caused by the Ascent of the Emeishan Mantle Anomaly

Prevailing mantle plume models reveal that the roles of plume-lithosphere interactions in shaping surface topography are complex and controversial, and also difficult to test. The exposed and complete strata in the Emeishan large igneous province (LIP) recorded abundant paleoenvironmental information associated with preeruptions and syneruptions, attracting numerous workers to this province to test these models. Despite intensified research these models are still strongly debated. This study represents an extensive field investigation combining new and previously published data from the Emeishan LIP to further seek information on plume-induced topographic variations. Our results indicate that there are inconspicuous vertical motions of the surface topography during the ascent of mantle plume, and a significant surface subsidence occurred at the early stage of the volcanism that has a significantly positive correlation with the thickness of local lavas, and the topographic uplift emerged in the late stage of the volcanism. Our studies provide key geological and geochemical evidence that the ascent of the Emeishan plume is unable to drive a significant surface uplift, owing to the plume containing numerous entrained bodies of dense recycled oceanic crust (10–20%) that can significantly reduce plume buoyancy. The significant surface subsidence maybe linked to a significant loss of thermal buoyancy due to the release of heat, which, accompanied by rapid loading of numerous dense erupted lava and a strong lithospheric flexure, also lead to a later synchronous and significant surface subsidence in the Emeishan LIP.

Distinguishing and correlating deposits from large ignimbrite eruptions using paleomagnetism: The Cougar Point Tuffs (mid-Miocene), southern Snake River Plain, Idaho, USA

In this paper, we present paleomagnetic, geochemical, mineralogical, and geochronologic evidence for correlation of the mid-Miocene Cougar Point Tuff (CPT) in southwest Snake River Plain (SRP) of Idaho. The new stratigraphy presented here significantly reduces the frequency and increases the scale of known SRP ignimbrite eruptions. The CPT section exposed at the Black Rock Escarpment along the Bruneau River has been correlated eastward to the Browns Bench escarpment (six common eruption units) and Cassia Mountains (three common eruption units) regions of southern Idaho. The CPT records an unusual pattern of geomagnetic field directions that provides the basis for robust stratigraphic correlations. Paleomagnetic characterization of eruption units based on geomagnetic field variation has a resolution on the order of a few centuries, providing a strong test of whether two deposits could have been emplaced from the same eruption or from temporally separate events. To obtain reliable paleomagnetic directions, the anisotropy of anhysteretic remanence was measured to correct for magnetic anisotropy, and an efficient new method was used to remove gyroremanence acquired during alternating field demagnetization.