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A User's Guide to Basalt Discrimination Diagrams

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... The data was interpreted with the aid of geochemical classification diagrams. Following the recommendations of Pearce (1996) the elements Ti, Y, Zr, and Nb were used to determine the petrologic character of the igneous lithics, and to interpret them with the aid of the Pearce WF and Pearce and Cann (1973) diagrams (see Richards, 2019;Pearce, 1996). A Principal Components Analysis (PCA) plot was also built using JMP software (version 15.2.0), although this was secondary to the classification systems of Pearce (1996) and Pearce and Cann (1973). ...
... The data was interpreted with the aid of geochemical classification diagrams. Following the recommendations of Pearce (1996) the elements Ti, Y, Zr, and Nb were used to determine the petrologic character of the igneous lithics, and to interpret them with the aid of the Pearce WF and Pearce and Cann (1973) diagrams (see Richards, 2019;Pearce, 1996). A Principal Components Analysis (PCA) plot was also built using JMP software (version 15.2.0), although this was secondary to the classification systems of Pearce (1996) and Pearce and Cann (1973). ...
... Following the recommendations of Pearce (1996) the elements Ti, Y, Zr, and Nb were used to determine the petrologic character of the igneous lithics, and to interpret them with the aid of the Pearce WF and Pearce and Cann (1973) diagrams (see Richards, 2019;Pearce, 1996). A Principal Components Analysis (PCA) plot was also built using JMP software (version 15.2.0), although this was secondary to the classification systems of Pearce (1996) and Pearce and Cann (1973). The Herron (1988) SandClass diagram (as modified by McCabe, 2021) was used to classify the non-igneous lithics and compare them with the sandstone reference samples (see Supplementary information 2 for more information). ...
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The geochemical sourcing and characterisation of lithic assemblages is surprisingly rare in Australian archaeology. The studies that have been undertaken have overwhelmingly focused on recent Holocene material and on ethnographic artefacts in museum collections with little attention paid to Pleistocene assemblages. Additionally, no work has been conducted on changes in raw material procurement over long time scales, despite many Australian sites having lengthy occupation histories. This paper presents the results of a portable X-Ray Fluorescence (pXRF) geochemical analysis of a sizeable sample (n = 760) of lithic artefacts and ochre from the site of Widgingarri Shelter 1, one of the earliest sites in northwest Australia with discontinuous occupation from at least ~50 ka to the contact period. This represents the first geochemical characterisation and sourcing study conducted on an Australian archaeological assemblage of this age and demonstrates how raw material procurement may have been influenced by climatic, sea-level, and landscape changes from the Late Pleistocene to the recent Holocene.
... Elevated Nb and reduced Zr levels indicate mantle sources that vary from depleted to transitional (Pearce et al. 1996). Specifically, Nb levels below 10 ppm and Zr levels below 200 ppm characterize depleted mantle, whereas higher concentrations suggest transitional to enriched sources. ...
... Debates on the tectonic origins of the analyzed metavolcanics have linked them to island arc collisions and rift mechanisms (Lahtinen 2000;Kirs et al. 2009;Bogdanova et al. 2015;Soesoo et al. 2020). HFSEs and HREEs, due to their stability, are critical for identifying the tectonic settings of extrusive rocks (Pearce et al. 1996;Sifeta et al. 2005;Saccani et al. 2018). Unlike typical midocean ridge basalt (MORB) from asthenospheric mantle melting, which exhibits lower LREEs and LILEs, Alutaguse metavolcanics display an arclike sig nature with notable HFSE depletion, suggesting subduction influences (Faisal et al. 2020;Ma et al. 2021). ...
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This research focuses on the geochemical analysis of Paleoproterozoic metasedimentary and metavolcanic units in the Alutaguse region of northern Estonia, shedding light on the geodynamic evolution during the Svecofennian orogeny in eastern Fennoscandia. The metasedimentary units consist of micaceous gneisses (± Grt ± Crd ± Sil), and the metavolcanic units include amphibolites and pyroxenic gneisses. Geochemical analyses utilized both historical and new whole-rock geochemical data. Weathering indices indicated their applicability for provenance studies and tectonic setting analyses. Metasediments are classified by their silica content: high-SiO2 (>63 wt%) metasediments resemble litharenites, implying higher maturity and felsic origins akin to the upper continental crust reference; low-SiO2 (<63 wt%) metasediments align with graywackes and shales, indicative of mafic to intermediate origins, similar to the post-Archean Australian shale, with TiO2–Ni suggesting sedimentary trends. Discriminant tectonic parameters associated these metasedimentary groups with a continental rift domain. Total alkali-silica classified the metavolcanics as subalkaline units. Geochemical ratios, such as La/Yb vs. Zr/Nb and La/Sm vs. Sm/Yb, crossing the spinel-lherzolite trend, were closest to the primitive mantle reference. The Th/Nb and Th/Zr ratios revealed asthenospheric mantle origins for the basaltic magma sources in Alutaguse. Tectonic settings derived from Y/15–La/10–Nb/8 and TiO2–10(MnO)–10(P2O5) ratios suggested a predominant oceanic arc affinity. It is proposed here that the Alutaguse structural zone developed as the back-arc of the Tallinn–Uusimaa belt(s), following the accretion of the Bergslagen microcontinent at 1.9–1.87 Ga, concluding with the closure of the paleo-Svecofennian ocean.
... The Red Cross Lake Intrusive Suite spans a compositional range between picro-basalt and andesite, whereas mafic dykes along the Valentine Lake Shear Zone corridor and the Mount Peyton Intrusive Suite range between basalt and andesite and have higher alkaline contents than the Red Cross Lake Intrusive Suite (Fig. 10A). On the Zr/Ti vs. Nb/Y plot (Pearce, 1996), the quartz diorite is basalt and the quartz gabbros range from basalt to rhyolite + dacite, whereas the Red Cross Lake Intrusive Suite, mafic dykes, and Mount Peyton Intrusive Suite cluster along the basalt-basaltic andesite/ andesite boundary (Fig. 10B). ...
... Compositions of representative magnesio-ferri-hornblende-andesine et al., 1986). B) Zr/Ti vs. Nb/Y plot (Pearce, 1996). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) ...
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The Howley Islands intrusions consist of three coarse-grained amphibole-phlogopite/biotite quartz gabbro dykes and one medium-grained amphibole-biotite quartz diorite body that cut rocks of the Exploits Subzone in central Newfoundland along strike from the multi-million-ounce Valentine gold deposits. The petrogeneses and ages of these rocks were investigated to better constrain the process evolution of the orogenic gold belt that extends for more than 200 km across central Newfoundland. The quartz gabbro dykes are composed of magnesio-ferri-hornblende-cummingtonite-phlogopite/biotite macrocrysts mantled by plagioclase (labradorite to oligoclase)-quartz coronas. The gabbros are LILE- and LREE-enriched, transitional arc-like rocks that formed from a different melt source and parental magma than the quartz diorite body. The quartz diorite is plagioclase-rich (50 modal % andesine), contains only trace cummingtonite, lacks phlogopite, and preserves rare diopside overgrown by magnesio-ferri-hornblende. This intrusion is more alkaline and OIB-like than the quartz gabbros and exhibits the influence of a deeper, more enriched mantle component, although both melts variably interacted with deep lithosphere. The quartz gabbro dykes and quartz diorite body may represent melts of the lower lithosphere and upper asthenosphere, respectively. The abundance of coarse- to medium-grained amphibole and phlogopite/biotite in the samples is consistent with crystallization of hydrous magmas and rapid, water-enhanced crystal growth, with cooling paths recorded by chemically zoned grains of magnesio-ferri-hornblende and plagioclase. One quartz gabbro displays reverse core to rim chemical zoning of plagioclase from andesine to labradorite, which may reflect decreasing pressure during magma ascent and crystallization, magma mixing of evolved and more primitive magmas, and/or fluctuations in H2O content. The presence of cummingtonite suggests crystallization at relatively low temperatures in shallow, low-pressure, upper crustal magma chambers. The quartz gabbros may represent melts equivalent to the nearby Howley Islands gabbro body, whereas the quartz diorite may represent a plagioclase cumulate along the margin of another melt chamber. U-Pb CA-ID-TIMS zircon geochronology yielded ages of ca. 400.3 Ma for the gabbro dykes and 399.9 Ma for the quartz diorite intrusion, within the ca. 410–377 Ma age range for mineralization of the nearby Valentine gold deposits. The ca. 400 Ma intrusions, when considered in conjunction with regional models, reflect melting and hydrous magmatism in the mantle wedge above a retreating Avalonian slab that was dehydrating during the Acadian orogenic cycle. The coincidence of Pridoli (ca. 422 Ma) to Emsian (ca. 400 Ma) bimodal magmatism and orogenic gold mineralization in central Newfoundland reflects more than twenty million years of high geothermal gradients and fluid flow, which when combined with structural focusing along faults, was apparently optimal for the formation of orogenic gold deposits of significant economic potential.
... (Colour online) (a, b) Classification of volcanic rocks in the SiO 2 versus Zr/TiO 2 diagram ofWinchester & Floyd (1977), and the Zr/Ti versus Nb/Y diagram of the same authors with revised fields ofPearce (1996). Coloured fields show samples that probably represent plutonic rocks or their detritus. ...
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The relationship of the Late Cretaceous amphibolite-facies Akrotiri and Vari subunits on the Greek islands of Tinos and Syros to similar occurrences in the Aegean is not fully understood, but a correlation with the Asterousia Crystalline Complex on Crete and corresponding rocks in the Upper Cycladic Unit on other islands of the archipelago is a plausible interpretation. There is currently no clear evidence that the Akrotiri subunit represents a fragment of the metamorphic sole of the nearby Tsiknias ophiolite, as there are differences in field appearance, geochemistry, metamorphic grade and retrograde overprint. Felsic rocks from amphibolite-gneiss sequences of the Akrotiri and Vari subunits are interpreted as reworked igneous rocks with minor admixture of terrigenous material. The Triassic U–Pb zircon age (c. 240 Ma) of such a gneiss from Syros indicates the magmatic crystallization age of the original source rocks. A similar interpretation is suggested for the studied Akrotiri gneiss, but is less clear due to the not fully clarified significance of the presumed Pb loss thought to be responsible for the considerable U–Pb age range of the dated zircons (c. 256–114 Ma). The Vari gneiss sensu strictu is closely associated with metadioritic rocks containing a Triassic zircon population (c. 238 Ma). The protolith is probably plutonic, but a mixture of volcanic and plutonic detritus cannot be excluded. Riebeckitic amphiboles occur in all rock types of the Akrotiri subunit, indicating late overprinting at elevated pressures, which is not known from similar occurrences in the southern Aegean.
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The forearc and back-arc region of the Andaman-Nicobar-Sumatra Subduction Zone are mainly submarine and poorly understood. To gain new insights sixty representative volcanic rocks were collected from the Andaman Sea region. Rock types range from basalt, basaltic andesite, andesite, dacite to rhyolite reflecting calc-alkaline, intermediate to acidic magma compositions. The rocks exhibit pronounced LREE enrichment with significant negative Nb, Ta and Ti anomalies and relative HREE depletion indicative of subduction zone related volcanism. The calc-alkaline rocks were generated by fractional crystallization along with crustal assimilation of older crustal rocks. A lower level of contamination is recorded in Barren Island samples, and mafic rocks from the Central Andaman Trough and Southern arc. Miocene zircon 238U/206Pb ages (10.8 ± 0.3 Ma, 19.8 ± 0.3 Ma and 19.5 ± 0.2 Ma) are recorded by West Sewell Ridge samples. Previous suggestions that hyperextended Malayan crust (Sundaland) existed below much of the Northern arc, Southern arc, Alcock and Sewell Rises and were later covered by volcanic crustal material is supported by the presence of inherited zircon cores and xenocrysts with ages spanning the Phanerozoic to Neoproterozoic (250, 320, 460–690, 800–1100, and 1400 Ma). These fit with the Malay-Sundaland as the main source of crustal contamination.
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ABSTRACT The timing and mechanism of the crustal extension that caused blocks of continental crust to rift and drift northwards from the northern Gondwana margin during the Early Palaeozoic are currently debated. Here, we present new field, geochemical and U-Pb zircon data from peri-Gondwanan Central Sakarya terrane in NW Turkey to shed light on the timing and petrogenesis of extensional magmatism and subsequent passive margin development of the northern Gondwana margin. Our 1/25.000 scale mapping of a selected area (250 km2) has revealed south-vergent thrust stack comprising three slices. The Middle Slice of the thrust stack consists of a gneiss-amphibolite complex that has a basement-cover type stratigraphy (before regional metamorphism). The basement unit consists of meta-granitoids, intruded by meta-syenite and banded amphibolites, with geochemical characteristics reflecting I&S-type, A-type and within plate alkaline/transitional-type magmatism, respectively. The cover meta-clastic sediments have amphibolite lenses with MORB and back-arc basin-type geochemistry. Meta-granitoid bodies yielded zircon U-Pb ages of 507 ± 16 Ma, 503 ± 12 Ma, and 488 ± 2.6 Ma (Late Cambrian-Early Ordovician), whereas the meta-syenites gave ages of 473.9 ± 5.7 Ma and 471.4 ± 2.5 Ma (late Early Ordovician); also, age of the banded amphibolite was dated as 473.3 ± 4 Ma. The youngest detrital zircon population in the pebbly quartzite sample within cover meta-sediments (458 ± 3 Ma) constrains the maximum depositional age to around early Late Ordovician. The late Early Ordovician bimodal magmatism, a key focus of our study, is interpreted to have formed in response to the Rheic Ocean rifting along the northern margin of Gondwana. The Late Cambrian granitic magmatism, a precursor to bimodal magmatism, can be explained by lithospheric thinning and upwelling of the asthenosphere to form anatectic silicic melts. The unconformity between the basement and cover units may represent the initiation of a new phase of extension at the N Gondwana margin that culminated in the northward drifting of the Central Sakarya and the opening of Palaeotethys, associated with back-arc basic magmatism in Late Ordovician-Silurian.
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