Diachronous Redistribution of Hf and Nd Isotopes at the Crystal Scale—Consequences for the Isotopic Evolution of a Poly-Metamorphic Crustal Terrane

In metamorphic rocks, mineral species react over a range of pressure–temperature conditions that do not necessarily overlap. Mineral equilibration can occur at varied points along the metamorphic pressure–temperature (PT) path, and thus at different times. The sole or dominant use of zircon isotopic compositions to constrain the evolution of metamorphic rocks might then inadvertently skew geological interpretations towards one aspect or one moment of a rock’s history. Here, we present in-situ U–Pb/Sm–Nd isotope analyses of the apatite crystals extracted from two meta-igneous rocks exposed in the Saglek Block (North Atlantic craton, Canada), an Archean metamorphic terrane, with the aim of examining the various signatures and events that they record. The data are combined with published U–Pb/Hf/O isotope compositions of zircon extracted from the same hand-specimens. We found an offset of nearly ca. 1.5 Gyr between U-Pb ages derived from the oldest zircon cores and apatite U–Pb/Sm–Nd isotopic ages, and an offset of ca. 200 Ma between the youngest zircon metamorphic overgrowths and apatite. These differences in metamorphic ages recorded by zircon and apatite mean that the redistribution of Hf isotopes (largely hosted in zircon) and Nd isotopes (largely hosted in apatite within these rocks), were not synchronous at the hand-specimen scale (≤~0.001 m3). We propose that the diachronous redistribution of Hf and Nd isotopes and their parent isotopes was caused by the different PT conditions of growth equilibration between zircon and apatite during metamorphism. These findings document the latest metamorphic evolution of the Saglek Block, highlighting the role played by intra-crustal reworking during the late-Archean regional metamorphic event.

GEOLOGICAL AND PETROGRAPHICAL CHARACTERISTICS OF SORAP MASSIF GABBROS, RAS-KOH OPHIOLITE, BALOCHISTAN, WESTERN PAKISTAN

This study discussed the Sorap Massif which is the ophiolitic fragment composed of the upper mantle and lower crustal section of an ophiolitic sequence. An ophiolitic sequence in Sorap Massif consists of harzburgite, dunite, large distant units of serpentinized wehrlite intrusion and outcrop of confined layered gabbro covered by the Quaternary sand dunes. The basal part of gabbroic intrusion is in contact with mantle rocks and the upper part is juxtaposed with the Kuchakki Volcanic Group. On outcrop-level and in hand specimen, the gabbros exhibit needle-like ferromagnesian minerals including hornblende associated with plagioclase and pyroxene. Petrographically the gabbros are classified into norite, gabbro norite and gabbro. The mineral constituents of norite are plagioclase, orthopyroxene and amphibole, gabbro norite is consists of equal constituents of orthopyroxene and clinopyroxene, plagioclase and amphibole and the gabbro is composed of clinopyroxene, plagioclase and amphibole. The high constituents of minerals such as pyroxene, amphibole and low constituents of olivine in the Sorap gabbros indicates that these gabbros are formed by the immature part of the oceanic plate with dehydration of the oceanic plate subduction.

3D-Microfabric reconstruction of Neoproterozoic diamictites from the Valjean Hills, California (USA)

<p>Neoproterozoic rocks from the Kingston Peak Formation (KPF) in the Valjean Hills (USA) expose a succession of diamictites associated with major glacial events taking place during the Cryogenian, approximately 700 million years ago. Within any glacial period, diamictites are widespread and in addition, their mechanics of deposition are highly variable. Some are massive in appearance at outcrop or in hand specimen, and apparently lacking any information that allows their mode of emplacement to be elucidated. Yet the correct interpretation for deep time successions in this area is especially important, since it is debated whether the diamictites have a tectonically driven, gravitational (Mrofka & Kennedy, 2011) or direct (sub)glacial origin (Le Heron et al. 2016).</p><p>In this contribution we determine the origin of the diamictites based on its internal microfabric and associated microstructures. We base our method on the technique of Philips et al. (2011) for Quaternary sediments, by mapping the apparent longest axes of skeleton grains (ranging from fine-grained sand to fine-grained pebbles) in oriented thin sections and reconstructing their fabric in a 3D space, we could identify a bimodal signal in the orientation of the longest axes. Contrary to gravitational deposition, clasts in subglacial diamictites tend to align themselves to a stress field, induced by the movement of the glacier. Macroscopic observations (<strong>Fig. 1A</strong>), microtexture- and structures (<strong>Fig. 1B</strong>) as well as the reconstructed microfabric domains (<strong>Fig. 1C</strong>) suggests a subglacial origin. These circumstances suggest temperate glacial conditions with wet based ice sheets during the deposition of the KPF. Moreover, the quantitative data allow confident flow directions to be extracted from seemingly chaotic diamictites.</p><p> </p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.d6720944370063784801161/sdaolpUECMynit/12UGE&app=m&a=0&c=72368bd26ba7123af5374457ad5cd0b9&ct=x&pn=gepj.elif&d=1" alt=""></p><p>Figure 1: (A) Valjean Hills Diamictite (label is 5x5 cm), (B) Rotational structure around bigger skeleton grain, (C) traced long axes of clasts (white lines) and interpreted  microfabric domains (blue, orange)</p><p> </p><p>References:</p><p>Le Heron, D.P., Tofaif, S., Vandyk, T. & Ali, D.O. (2017): https://doi.org/10.1130/G38460.1</p><p>Mrofka, D., Kennedy, M., (2011):  https://doi.org/10.1144/M36.40</p><p>Phillips, E. et al., (2011): https://doi.org/10.1016/j.quascirev.2011.04.024</p>

Taxonomic status and typification of the neglected name Calamagrostis vinealis (Poaceae, Agrostidinae)

The unresolved name Calamagrostis vinealis Steudel (Poaceae, Agrostidinae) is lectotypified and its taxonomic identity is discussed based on the discovery of Baumgarten’s collection at the P herbarium (P02142817), based on which this name was assigned. This collection appears to be a mixed gathering of Calamagrostis epigejos (left-hand specimen) and C. arundinacea (right-hand specimen). Here, the left-hand specimen is designated and labeled as lectotype of the name Calamagrostis vinealis, and in consequence, C. vinealis is a later heterotypic synonym of C. epigejos.

A possible serpulid tube worm of the genus Filograna from the upper Permian Cadeby Formation of South Yorkshire, UK

Simple, tubular clusters from a single boundstone hand specimen, collected from loose material in the Permian (Lopingian) Cadeby Formation at Hazel Lane Quarry, Doncaster Metropolitan Borough, are tentatively identified as the serpulid worm, Filograna sp. A. The tubes are calcareous, found in vertical clusters, with some short sections growing horizontally, and with external diameters between 0.6 mm and 1 mm. Recent work on the Permian Tethyan deposits of Italy have identified occurrences of the genus Filograna from before the Permian-Triassic extinction, but the importance of this example is its provenance in the Zechstein English Shelf biota, where reported occurrences of Serpulidae have been rare.

Clastic Sedimentary Rocks and Sedimentary Mélanges: Potential Naturally Occurring Asbestos Occurrences (Amphibole and Serpentine)

ABSTRACT Petrography of mélange matrix and clastic sedimentary rocks in coastal California reveals the occurrence of detrital serpentine and detrital asbestiform sodic amphibole (glaucophane). Many sandstones of the Franciscan Complex have small amounts of detrital serpentine, with amounts of up to several percent in some cases. Detrital amphibole, including asbestiform glaucophane, is also present in some sandstones. Whereas rare sandstones have so much detrital glaucophane that they appear blue in hand specimen (up to nearly half of the rock volume), most glaucophane-bearing sandstones lack blue color, and the detrital glaucophane is not apparent in hand specimen. Most of the occurrences of detrital glaucophane are in blueschist facies sandstones, some of which also contain neoblastic (grew in place) glaucophane, but a notable exception is a widespread prehnite-pumpellyite facies unit that crops out primarily in Sonoma and Marin Counties. The detrital mineralogy of sandstones mirrors the block and matrix compositions of Franciscan mélanges that can be thought of as scaled-up equivalents of these clastic sedimentary rocks (mega-conglomerates/sedimentary breccias). Franciscan mélanges range from having a detrital siliciclastic to a detrital serpentinite matrix, and interfingering and gradation of the two matrix types is common. These findings suggest that clastic sedimentary rocks associated with current or past active orogenic settings else-where in the world may contain naturally occurring asbestos (NOA) even if the NOA component minerals are not visible in hand specimen.

Lectotypification of the name Chenopodium hircinum, a wild relative of the pseudocereal crop species C. quinoa (Chenopodiaceae)

The name Chenopodium hircinum (Chenopodiaceae) applicable to a South American tetraploid species is lectotypified on a specimen deposited at LE (barcode LE00011694, right-hand specimen) that is associated with H.A. Schrader and his herbarium purchased for LE in 1841. A brief overview of relationships of the pseudocereal crop species C. quinoa withC. hircinum (considered to be a wild crop relative and/or progenitor) is also provided. Since the lectotype of C. hircinum in LE lacks mature fruits/seeds, which are important for morphology-based diagnostics of taxa of Chenopodium (and also of infraspecific taxa described within C. hircinum), designation of an epitype with well-developed fruits/seeds is desirable according to Art. 9.9 of the ICN. However, we argue that an epitype should be proposed later, after achieving a better understanding of the variability of C. hircinum and proper typification of its infraspecific taxa, because hasty epitypifications could be nomenclaturally confusing and disrupting. In our opinion, an epitype specimen for that taxon name should represent a plant corresponding to the lectotype and well characterized molecularly, genetically and morphologically.

The Distribution of Precious Metals in High-Grade Banded Quartz Veins from Low-Sulfidation Epithermal Deposits: Constraints from µXRF Mapping

High-grade ore zones in low-sulfidation epithermal deposits are commonly associated with the occurrence of banded quartz veins. The ore minerals in these veins are heterogeneously distributed and are mostly confined to ginguro bands, which can be identified in hand specimen based on their distinct dark gray to black color. Micro-X-ray fluorescence element maps obtained on representative samples of banded quartz veins show that Au occurs together with Ag minerals in some of the ginguro bands, but Au can also be present in quartz bands that are light gray to white and cannot be macroscopically distinguished from barren bands. The occurrence of compositionally distinct ginguro and gankin bands, the latter being a new term coined here for colloform quartz bands containing primarily electrum or native gold, can be explained by temporal changes in the composition of the ore-forming thermal waters or variations in the conditions of ore deposition. Textural relationships, including the dendritic shape of ore minerals that appear to have grown in a matrix of silica microspheres, suggest that the ginguro and gankin bands have formed as a result of rapid deposition associated with vigorous boiling or flashing of the thermal waters.