Phase equilibrium modelling and implications for P-T determinations of medium-temperature UHP eclogites, North Qaidam terrane, China

In contrast to low-T eclogites (garnet growth zoning preserved) or high-T eclogites (garnet diffusionally homogenized at peak conditions), medium-temperature eclogites pose additional challenges to P-T determinations due to the partial preservation of garnet zoning. The Dulan area, in the southeastern part of North Qaidam ultrahigh-pressure terrane, exposes minor eclogites hosted by ortho- and paragneisses. Four fresh, medium-temperature eclogites contain the paragenesis Grt+Omp+Rt+Qz/Coe+Ph +/- Ky +/- Zo. In all samples, garnet X-Mg shows little zoning, suggesting diffusional modification, and precludes the use of pyrope+almandine+grossular isopleth intersections to determine a P-T path. However, in one sample, sharp zoning in grossular content suggests that grossular growth compositions are preserved. Since garnet pyrope+almandine compositions appear to be modified, we instead use the intersections of grossular and garnet volume isopleths to define a prograde P-T path. This approach yields a path from similar to 17 kbar and similar to 410 degrees C to similar to similar to 35 kbar and similar to 625 degrees C with a gradient of similar to 5-9 degrees C/km through the lawsonite stability field. Peak P-T conditions determined from the intersection between Si pfu in phengite and Zr-in-rutile isopleths are similar to 26-33 kbar and similar to 625-700 degrees C for the four eclogites. These conditions are close to the limit of the lawsonite stability field, suggesting that fluid released from lawsonite breakdown may have promoted re-equilibration at these conditions. These peak conditions are also in good agreement (within 3 kbar and 50 degrees C) with garnet-omphacite-phengite (+/- kyanite) thermobarometry in three of the four samples. We regard the phengite-rutile constraints as more reliable, because they are less sensitive to uncertainties associated with ferric iron compared to conventional thermobarometry. Phase equilibrium modelling predicts that the retrograde assemblage of amphibole+zoisite formed at similar to 60 km. Infiltration of external fluids was likely required for the growth of these hydrous minerals. Based on the comparison of P-T estimation methods applied in this study, we propose that the garnet grossular+volume isopleths can recover the prograde P-T path of medium-temperature eclogites, and that the combination of phengite+rutile isopleths represents a more robust approach to constrain peak P-T conditions

Size and Exhumation Rate of Ultrahigh-Pressure Terranes Linked to Orogenic Stage

A growing set of data indicates a stark contrast between the evolution of two types of ultrahigh-pressure (UHP) terranes: large terranes that evolved slowly (over 10–30 Myr), and small terranes that formed and were exhumed on timescales of \u3c 10 Myr. Here we compare the characteristics – area, thickness, formation rate, exhumation rate, age, and tectonic setting – of these two endmember types of UHP terrane worldwide. We suggest that the two UHP terrane types may form during different orogenic stages because of variations in the buoyancy and traction forces due to different proportions of subducting crust and mantle lithosphere or to different rates of subduction. The initial stages of continent collision involve the subduction of thin continental crust or microcontinents, and thus tectonic forces are dominated by the density of the oceanic slab; subduction rates are rapid and subduction angles are initially steep. However, as collision matures, thicker and larger pieces of continental material are subducted, and the positive buoyancy of the down-going slab becomes more prominent; subduction angles become gentle and convergence slows. Assessing the validity of this hypothesis is critical to understanding the physical and chemical evolution of Earth\u27s crust and mantle. Included here is the post-print copy of this article. The final publication is available via ScienceDirect at http://www.sciencedirect.com/science/article/pii/S0012821X1100756

High- and Ultrahigh-Pressure Metamorphism in the North Qaidam and South Altyn Terranes, Western China

The North Qaidam and South Altyn terranes extend approximately 1000 km across the northern Tibetan Plateau, and five localities preserve evidence of Early Paleozoic high-pressure (HP) or ultrahigh-pressure (UHP) metamorphism, including the presence of coesite, coesite pseudomorphs, and diamond. A review of the geology, petrology, and geochronology collected over the past 10 years since these localities were discovered supports a correlation of the North Qaidam and South Altyn terranes, offset 350-400 km across the Altyn Tagh fault. Geochronology interpreted to reflect eclogite-facies metamorphism yields ages between 500 and 420 Ma; detailed geochronology from one locality supports a protracted (tens of m.y.) history of HP/UHP metamorphism. Rock associations and geochronology support a passive-margin origin for the protolith of the HP/UHP rocks, which received sediments from a Proterozoic-Late Archean source, and was intruded by Neoproterozoic granites derived from crustal melting. Included here is the post-print copy of this article. The final publication is available from Taylor & Francis via http:// www.tandfonline.com/doi/abs/10.2747/0020-6814.49.11.969

Coesite in Eclogite from the North Qaidam Mountains and Its Implications

Coesite provides direct evidence for ultrahigh pressure metamorphism. Although coesite has been found as inclusions in zircon in paragneiss of the north Qaidam Mountains, it has never been identified in eclogite. In this contribution, based on petrographic observations and in situ Raman microprobe spectroscopy, coesite was identified as inclusions in garnet of eclogite from the Aercituoshan, Dulan UHP metamorphic unit, north Qaidam Mountains. Coesite is partly replaced by quartz, showing a palisade texture. This is the first report on coesite in eclogite from the north Qaidam Mountains, and is also supported by garnet-omphacite-phengite geothermobarometry (2.7–3.25 GPa, 670–730°C). Coesite and its pseudomorphs have not been found in eclogites and associated rocks of other units of the north Qaidam Mountains. Further studies are required to confirm if all metamorphic units in the north Qaidam Mountains underwent the ultrahigh-pressure metamorphism

Paragneiss Zircon Geochronology and Trace Element Geochemistry, North Qaidam HP/UHP Terrane, Western China

In the southeastern part of the North Qaidam terrane, near Dulan, paragneiss hosts minor peridotite and UHP eclogite. Zircon geochronology and trace element geochemistry of three paragneiss samples (located within a ∼3 km transect) indicates that eclogite-facies metamorphism resulted in variable degrees of zircon growth and recrystallization in the three samples. Inherited zircon core age groups at 1.8 and 2.5 Ga suggest that the protoliths of these rocks may have received sediments from the Yangtze or North China cratons. Mineral inclusions, depletion in HREE, and absence of negative Eu anomalies indicate that zircon U-Pb ages of 431 ± 5 Ma and 426 ± 4 Ma reflect eclogite-facies zircon growth in two of the samples. Ti-in-zircon thermometry results are tightly grouped at ∼660 and ∼600 °C, respectively. Inclusions of metamorphic minerals, scarcity of inherited cores, and lack of isotopic or trace element inheritance demonstrate that significant new metamorphic zircon growth must have occurred. In contrast, zircon in the third sample is dominated by inherited grains, and rims show isotopic and trace element inheritance, suggesting solid-state recrystallization of detrital zircon with only minor new growth

Geochronology and Tectonic Significance of Middle Proterozoic Granitic Orthogneiss, North Qaidam HP/UHP Terrane, Western China

Amphibolite-facies para- and orthogneisses near Dulan, in the southeast part of the North Qaidam terrane, enclose minor ultra-high pressure (UHP) eclogite and peridotite. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. Ion microprobe U–Pb and REE analyses of zircons from two granitic orthogneisses indicate magmatic crystallization at 927 ± Ma and 921 ± 7 Ma. Zircon rims in one of these samples yield younger ages (397–618 Ma) compatible with partial zircon recrystallization during in-situ Ordovician-Silurian eclogite-facies metamorphism previously determined from eclogite and paragneiss in this area. The similarity between a 2496 ± 18 Ma xenocrystic core and 2.4–2.5 Ga zircon cores in the surrounding paragneiss suggests that the granites intruded the sediments or that the granite is a melt of the older basement which supplied detritus to the sediments. The magmatic ages of the granitic orthogneisses are similar to 920–930 Ma ages of (meta)granitoids described further northwest in the North Qaidam terrane and its correlative west of the Altyn Tagh fault, suggesting that these areas formed a coherent block prior to widespread Mid Proterozoic granitic magmatism. Included here is the post-print copy of this article. The final publication is available at Springer via http://dx.doi.org/10.1007/s00710-006-0149-1

Late Cretaceous to Paleocene Metamorphism and Magmatism in the Funeral Mountains Metamorphic Core Complex, Death Valley, California

Amphibolite-facies Proterozoic metasedimentary rocks below the low-angle Cenozoic Boundary Canyon Detachment record deep crustal processes related to Mesozoic crustal thickening and subsequent extension. A 91.5 ± 1.4 Ma Th-Pb SHRIMP-RG (sensitive high-resolution ion microprobe–reverse geometry) monazite age from garnet-kyanite-staurolite schist constrains the age of prograde metamorphism in the lower plate. Between the Boundary Canyon Detachment and the structurally deeper, subparallel Monarch Spring fault, prograde metamorphic fabrics are overprinted by a pervasive greenschist-facies retrogression, high-strain subhorizontal mylonitic foliation, and a prominent WNW-ESE stretching lineation parallel to corrugations on the Boundary Canyon Detachment. Granitic pegmatite dikes are deformed, rotated into parallelism, and boudinaged within the mylonitic foliation. High-U zircons from one muscovite granite dike yield an 85.8 ± 1.4 Ma age. Below the Monarch Spring fault, retrogression is minor, and amphibolite-facies mineral elongation lineations plunge gently north to northeast. Multiple generations of variably deformed dikes, sills, and leucosomal segregations indicate a more complex history of partial melting and intrusion compared to that above the Monarch Spring fault, but thermobarometry on garnet amphibolites above and below the Monarch Spring fault record similar peak conditions of 620–680 °C and 7–9 kbar, indicating minor (\u3c3–5 km) structural omission across the Monarch Spring fault. Discordant SHRIMP-RG U-Pb zircon ages and 75–88 Ma Th-Pb monazite ages from leucosomal segregations in paragneisses suggest that partial melting of Proterozoic sedimentary protoliths was a source for the structurally higher 86 Ma pegmatites. Two weakly deformed two-mica leucogranite dikes that cut the high-grade metamorphic fabrics below the Monarch Spring fault yield 62.3 ± 2.6 and 61.7 ± 4.7 Ma U-Pb zircon ages, and contain 1.5–1.7 Ga cores. The similarity of metamorphic, leucosome, and pegmatite ages to the period of Sevier belt thrusting and the period of most voluminous Sierran arc magmatism suggests that both burial by thrusting and regional magmatic heating contributed to metamorphism and subsequent partial melting

Nanotools for Neuroscience and Brain Activity Mapping

Neuroscience is at a crossroads. Great effort is being invested into deciphering specific neural interactions and circuits. At the same time, there exist few general theories or principles that explain brain function. We attribute this disparity, in part, to limitations in current methodologies. Traditional neurophysiological approaches record the activities of one neuron or a few neurons at a time. Neurochemical approaches focus on single neurotransmitters. Yet, there is an increasing realization that neural circuits operate at emergent levels, where the interactions between hundreds or thousands of neurons, utilizing multiple chemical transmitters, generate functional states. Brains function at the nanoscale, so tools to study brains must ultimately operate at this scale, as well. Nanoscience and nanotechnology are poised to provide a rich toolkit of novel methods to explore brain function by enabling simultaneous measurement and manipulation of activity of thousands or even millions of neurons. We and others refer to this goal as the Brain Activity Mapping Project. In this Nano Focus, we discuss how recent developments in nanoscale analysis tools and in the design and synthesis of nanomaterials have generated optical, electrical, and chemical methods that can readily be adapted for use in neuroscience. These approaches represent exciting areas of technical development and research. Moreover, unique opportunities exist for nanoscientists, nanotechnologists, and other physical scientists and engineers to contribute to tackling the challenging problems involved in understanding the fundamentals of brain function

Terrestrial Plant Fossils from the Mississippian Diamond Peak Formation, White Pine Range, Eastern Nevada

Land plant remains are preserved in shallow water turbidite deposits within the upper Mississippian Diamond Peak Formation in the northern White Pine Range, White Pine County, Nevada. These deposits represent debris shed eastward from the Antler Mountains into the Antler Basin. Poor preservation limits the precision of identification, but six forms can be recognized including Lepidodendron cf. aculeatum and Lycopodites sp., Archacocalamites radiatus, A. species, and Sphenophyllum. An axis with a spiral branching pattern may represent a seed plant. Several specimens exhibit a consistent morphology of whorls or bracts with a cruciate cross section; these may be reproductive organs. This assemblage indicates that a Euramerican-type swamp flora existed on the eastern flank of the Mississippian Antler Mountains, typical of low-latitude tropical floras of the Carboniferous

Epidote Minerals in High P/T Metamorphic Terranes: Subduction Zone and High- to Ultrahigh-pressure Metamorphism

This chapter focuses mainly on modes of occurrences of zoisite, clinozoisite-epidote and piemontite in various rock types from selected subduction zones and HP to UHP metamorphic terranes. The amount of information on these rocks has increased dramatically during the last 20 years and it is virtually impossible to completely review all. We therefore restrict this review mainly to those localities where we have our own experience, and add some information from other localities. Epidote minerals in metamorphic rocks that followed classical barrovian- or Buchan-type metamorphism are reviewed by Grapes and Hoskin (2004). Nevertheless, as the P-T conditions encountered during subduction zone and Barrovian-type metamorphism are not thus distinct in the P-T range of the greenschist, blueschist, to epidote-amphibolite facies transition, some overlaps between Grapes and Hoskin (2004) and this chapter were found to be unavoidable. As subduction zone metamorphism represents also a potential prograde path of many HP and UHP rocks we will first review some selected and well studied examples of subduction zone metamorphism and then prograde into the field of the peculiar HP and UHP metamorphic rocks. Specifically, this chapter deals with the following major subjects: (1) Prograde sequences from lawsonite to epidote zone blueschists and the corresponding epidote-forming reactions are described from the Franciscan, New Caledonian and Sanbagawa metamorphic rocks; these examples are used to illustrate the role of epidote minerals in typical subduction zone metamorphic successions. (2) Phase relations of zoisite-bearing assemblages in calcareous metasediments are briefly reviewed for eclogite facies rocks in the European Alps. (3) The control of bulk rock composition on paragenesis and compositions of UHP phases is exemplified by various zoisite-bearing UHP eclogites from Dabie (central China) with an AFM diagram. (4) The common break down of plagioclase in HP and UHP rocks in the presence of H2O to form the assemblage zoisite + kyanite + sodic clinopyroxene + quartz is illustrated in some HP to UHP coronitic metagranites and metagabbros. (5) The common but selective occurrence of epidote minerals in Sulu-Dabie eclogite, quartzite and gneiss may be related to the oxidation of supracrustal rocks by a fossil hydrothermal system. (6) The pseudomorphic replacements of epidote/zoisite + amphibole + biotite + sodic plagioclase after garnet and of zoisite + albite after kyanite as indicators for a metasomatic and retrograde overprint during exhumation are described using kyanite-phengite-coesite eclogites from Dabie. (7) The coexistence of lawsonite and epidote in some HP metabasites is related to bulk composition, Æ’O2 and metastable persistence. (8) Composition of epidote minerals from buffered assemblages is sensitive to P, T and Æ’O2, and its use as a geobarometer or geothermometer is discussed. (9) Epidote minerals are a key to discuss fluid-rock interaction and partial melting of metamorphic rocks at high-P conditions. (10) Unusually high Sr and REE contents in epidote minerals including allanite suggest that they are the main reservoirs of these elements in subduction zones and HP to UHP environments. (11) Sector-zoning of zoisite and epidote and their petrological and mineralogical significance is also briefly discussed