Mesoproterozoic Granites Research Articles

Contrasting rare earth element concentrations and mixing behaviors in the St. Lawrence Estuary and Saguenay Fjord

Rare earth elements (REEs) including Yttrium (Y) are commonly used as tracers of estuarine and oceanic mixing. The lanthanide series and yttrium are usually referred to as REYs. The geochemical behavior of REYs in estuarine environments is generally described as being non-conservative, with large-scale removal by particle scavenging. During mixing, partitioning of these elements occurs according to their source function and the stability of natural complexes, with heavy REEs typically forming more stable complexes than light REEs in solution. In this study, we compare the concentrations and partitioning of the 0.7 μm-filtered and 0.05 μm-filtered fractions of the dissolved REYs collected during the summers of 2017 and 2021 in the surface waters (< 3 m) of the St. Lawrence estuarine system (river, estuary and gulf) with those of the Saguenay Fjord, a tributary of the latter that drains the Mesoproterozoic rocks of the Canadian Shield. Whereas REYs do not mix conservatively in the St. Lawrence Estuary (SLE) in the summer, they nearly do so in the Saguenay Fjord (SF). REY concentrations are 2.5 to 6 times greater in the surface waters of the SF than those of the SLE at the same salinity and, in contrast to most estuaries including the SLE, the fjord waters are enriched in LREEs. The 0.05 μm-filtered REY concentrations are positively correlated with dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) concentrations in the SF but independent of both DOC and CDOM concentrations in the SLE. The CDOM in the fjord differs from that of the estuary as it is more aromatic and has a higher molecular weight. The formation of strong REE-humate complexes stabilizes REY ions in the SF surface waters and impedes their adsorption to and scavenging by solid surfaces during estuarine mixing. The LREE enrichment in the SF surface waters most likely reflects the geology of the fjord's drainage basin, more specifically the exposed Mesoproterozoic granites and gneisses of the Canadian Shield that are enriched in LREE relative to the younger Paleozoic sedimentary rocks exposed along the St. Lawrence Lowlands.

Magmatic-Hydrothermal Fluid Processes of the Sn-W Granites in the Maniema Province of the Kibara Belt (KIB), Democratic Republic of Congo

The Kibara belt (KIB) in the Maniema province hosts orebodies bearing cassiterite-wolframite, which are associated with equigranular to pegmatitic late Mesoproterozoic (1094–755 Ma) granites and Sn-W bearing quartz veins that cut through metasedimentary country rocks. Alteration assemblages of muscovite-quartz (±topaz-fluorite-tourmaline) occur in the granites, and muscovite-sericite-quartz occurs in Sn-W quartz veins. Petrographic analyses, including cathodoluminescence (SEM-CL) on cassiterite grains, reveal two types of cassiterite: yellow transparent cassiterite (lighter under SEM-CL: type I) and dark translucent cassiterite (darker under SEM-CL: type II). These types are organized in micro-textures as oscillatory (growth) zones and replacement zones (type II replaces type I). Unlike cassiterite, wolframite is texturally homogenous. LA-ICP-MS results reveal that type II cassiterite is relatively enriched in Fe, Al, Ga, In, As, Pb, Zn, and U, whereas type I is enriched in V, Ti, Zr, Ta, Hf, and Nb. Contrasting Ce anomaly values in the cassiterite types suggest a transition of redox potentials during the Sn precipitation. Fluid inclusion assemblages (FIAs) in quartz, fluorite, and cassiterite are dominantly aqueous, liquid- or vapor-rich, and rarely carbonic-bearing aqueous inclusions. These often texturally coexist in a single “boiling” assemblage in granites. Raman spectroscopy on the bubble part of fluid inclusions in quartz and cassiterite shows various gas species, including CO2, CH4, N2, and H2. Boiling assemblages in the granites suggest that fluid phase separation occurred at about 380–610 bars, which is about 1–2 km (lithostatic) or 3–5 km (hydrostatic) in apparent paleodepth. FIAs in the granites show ranges of salinities of 4–23 wt.% (NaCl equivalent) and homogenization temperatures (Th) of 190–550 °C. FIAs hosted in cassiterite displayed distinctively lower and narrower ranges of salinities of 2–10 wt.% and Th of 220–340 °C compared to the FIAs hosted in quartz in the granites (salinity of 4–23 wt.%, Th of 190–550 °C) and the quartz veins (salinity of 1–23 wt.%, Th of 130–350 °C). This suggests a less salinized and cooler fluid during the cassiterite precipitation. We suggest that magmatic-derived Sn-W bearing fluids be mixed with less saline and cooler aqueous fluids, possibly meteoric water, during the major cassiterite and possibly wolframite depositions in the KIB. This is based on (1) temperature and salinities, (2) hydrothermal alterations, (3) cassiterite micro-textures, and (4) trace element distributions.

Late Quaternary–Recent intraplate topographic uplift of the St. Francois Mountains (Ozark Plateau), Missouri (USA), driven by differential density and erosion

AbstractThe origin of relatively high topography in intraplate settings is elusive. The St. Francois Mountains, a tectonically inactive region with high relief, a Mesoproterozoic granite and rhyolite core, and Paleozoic sedimentary flanks, provide insights into the evolution of intraplate landscapes. We determined 14 in situ10Be exposure ages, 23 10Be catchment-wide denudation rates (CWDRs), and geomorphic indices using a geographic information system, and we modeled surface uplift using an isostatic-flexure model. CWDRs varied from ~5 to 10 ± 1.7 mm/k.y. and averaged 1.5 times greater in streams draining granite and rhyolite than in those draining flanking sedimentary rocks. In situ results from bedrock strath terraces gave Quaternary incision ages and incision rates that were tenfold higher than CWDRs. Major knickpoints (locally called shut-ins), all at 200 ± 70 m elevation, are located at contacts between the igneous and sedimentary rocks. Our exposure and denudation results inform isostatic-flexural modeling, which together support the conclusion that late Quaternary–Recent surface uplift driven by differential density and erosion focused in the igneous core is creating significant topography and relief in this intraplate setting.

Post-collisional magmatism in NE Australia during Mesoproterozoic supercontinent Nuna: Insights from new zircon U Pb and Lu Hf data

The geodynamic regimes that operated during the Mesoproterozoic amalgamation of the first supercontinent on Earth, Nuna, remain poorly understood. Palaeogeographic Nuna reconstructions indicate that NE Australia and NW Laurentia were adjacent at its core—the ca. 1600 Ma collision between the Australian upper-plate (i.e., the Mount Isa Inlier) and the western Laurentian lower-plate (i.e., the Georgetown Inlier) was followed by a post-collisional, extensional regime associated with widespread low-pressure ( LP ) and high-temperature ( HT ) metamorphism and the formation of voluminous granitic melts. To better understand the tectonic regimes and crustal processes that operated during this Proterozoic orogenic system, new coupled U Pb and Lu Hf analyses from Mesoproterozoic granites in NE Australia are used to examine the ca. 1560 to 1540 Ma magmatism that occurred during the waning stages of Nuna amalgamation. LT , hydrous, S-type granites from the Georgetown Inlier yield crystallisation ages of ca. 1560–1550 Ma, whereas 100 km west of it, a newly identified Proterozoic Cudgee Creek Granite yield a crystallisation age of ca. 1540 Ma. The Cudgee Creek Granite is a HT , hybrid I−/A-type granite, reflecting a transitional geochemical trend towards the A-type granites in the Mount Isa Inlier further to the west. Zircon Lu Hf isotopic composition supports this geochemical trend with more radiogenic εHf (t) values from east to west, reflecting an increase in mantle input westward. The younging and the geochemical trend from the Georgetown Inlier S-type (ca. 1560–1550 Ma) to the I−/A- Cudgee Creek granite (ca. 1545 Ma) and the A-type granites in the Mount Isa Inlier (ca. 1530–1490 Ma) indicate progressive lithospheric thinning of the upper plate due to slow rollback of the Laurentian lower plate. These results suggest ongoing subduction drove hot and slow orogenesis involving slab rollback that was active for at least 100 m.y after the collisional stage. • Mesoproterozoic 1560–1490 Ma S-I-A-type magmatism occurred during lithospheric thinning of the Australian upper plate. • Lu Hf in magmatic zircon indicates an increase in juvenile material during mantle wedge opening due to slab rollback. • Nuna final assembly is associated with the formation of hot, wide slow-soft orogenic systems.

Structural Setting and Heterogeneous Deformation of the Mesoproterozoic Granite–Gneisses of North Tianshan with the Example of the Karadjilga Massif (Kyrgyzstan)

Structural Setting and Heterogeneous Deformation of the Mesoproterozoic Granite–Gneisses of North Tianshan with the Example of the Karadjilga Massif (Kyrgyzstan)

Sources and oxidation state of the Permian arc magmatic rocks of SW Jilin Province in the eastern Central Asian Orogenic Belt: evidence from Li, Hf isotopes and oxygen fugacity

AbstractThe late Palaeozoic continental-arc magmatic rocks in the Gongzhuling area are located in the Liaoyuan Accretionary Belt. Here we present new zircon U–Pb ages, whole-rock major- and trace-element compositions, Li and zircon Hf isotopic compositions and oxygen fugacity of these rocks with an aim to constrain the lithium isotopic composition of the source region and origin of the magmas. These rocks were formed during 269–258 Ma in middle–late Permian time. The dioritic rocks were formed through mixing processes, with the mafic melts originating from a metasomatized mantle wedge and the felsic melts from the lower crust of a Neoproterozoic arc. The mantle wedge has been metasomatized by Li-rich fluids derived from subducted oceanic crust, as indicated by the δ7Li values of +0.4 ‰ to +3.5 ‰ and positive ϵHf(t) values (+0.7 to +13.1). Redox-sensitive Ce in the zircons indicates the fO2 of the magmas to be low to intermediate (FMQ−2.2 to FMQ+2.6; FMQ is the fayalite–magnetite–quartz redox buffer), precluding large-scale porphyry Cu–Mo mineralization. The middle–late Permian magmatic rocks represent the terminal magmatic record of the subduction of the Palaeo-Asian oceanic crust, meaning that the final closure of the Palaeo-Asian Ocean in the eastern Central Asian Orogenic Belt occurred at the end of the Permian Period. Recent identification of Mesoproterozoic (c. 1400 Ma) granites suggests some Palaeoproterozoic crustal fragments still exist in the Liaoyuan Accretionary Belt, but only in a small amount; therefore, it is concluded that the crustal growth of the Liaoyuan Accretionary Belt occurred mainly during the Neoproterozoic period.

Discovery of a Mesoproterozoic granite in the northern Alxa Block and its tectonic implication

A Mesoproterozoic granite covering about 5 km2 is firstly recognized in the Zhusileng area, which is located between the northern Alxa Block and the South Gobi microcontinent. New geochronological data from this area provide new insight for understanding the Precambrian crustal evolution of the middle segment of the southern Central Asian Orogenic Belt (CAOB). Based on the crystal shapes, internal textures, and high Th/U ratios, the zircons from this Mesoproterozoic granite are interpreted as magmatic origin. Meanwhile, LA‐ICP‐MS zircon U–Pb dating shows that it was emplaced at 1458 ± 7.5 Ma. Published geochronological data and Sr–Nd isotopic compositions of igneous rocks from the Zhusileng area and its vicinity are also compiled in this study. The Late Palaeozoic granitoids from the Zhusileng area have strongly negative εNd(t) values (−0.1 to −10.8), high (87Sr/86Sr)i (0.69657–0.73423) and old Nd model ages (0.9–1.8 Ga), suggesting the existence of Precambrian microcontinent in the Zhusileng area. Furthermore, the statistical analysis (or isotopic mapping) of Sr–Nd isotopic compositions of Palaeozoic granitoids from the middle segment of the southern CAOB also displays similar εNd(t) values and relatively old Nd model ages along the Hanshan Block, Zhusileng, Tsaggan Uul terrane, and Hutag Uul terrane. Therefore, we propose that those units may share a common basement during Mesoproterozoic to Early Neoproterozoic.

Geochemistry and new zircon U–Pb geochronology of Mesoproterozoic Punugodu granite pluton, SE India: implications for anorogenic magmatism along the western margin of Nellore Schist Belt, India

AbstractWe report a new zircon U–Pb age of 1257 ± 6 Ma for the Punugodu granite (PG) pluton in the Eastern Dharwar Craton (EDC), Southern India. The Mesoproterozoic PG is alkali feldspar hypersolvus granite emplaced at shallow crustal level, as evident from the presence of rhyodacite xenoliths and hornfelsic texture developed in the metavolcanic country rocks of the Neoarchaean Nellore Schist Belt (NSB). Geochemically, the PG is metaluminous, ferroan and alkali-calcic, and is characterized by high SiO2 and Na2O + K2O, Ga/Al ratios &gt;2.6, high-field-strength elements and rare earth element (REE) contents with low CaO, MgO and Sr, indicating its similarity to anorogenic, alkali (A-type) granite. The highly fractionated REE patterns with negative europium anomalies of PG reflect its evolved nature and feldspar fractionation. Mafic (MME) to hybrid (HME) microgranular enclaves represent distinct batches of mantle-derived magmas that interacted, mingled and undercooled within the partly crystalline PG host magma. Felsic microgranular enclaves (FME) having similar mineralogical and geochemical characteristics to the host PG most likely represent fragments of marginal rock facies of the PG pluton. The PG appears to be formed from an oceanic island basalt (OIB)-like source in an anorogenic, within-plate setting. The emplacement of PG (c. 1257 Ma) in the vicinity of Mesoproterozoic Kanigiri Ophiolite (c. 1334 Ma) shows an age gap of nearly 77 Ma, which probably suggests PG emplacement in an extensional environment along a terrain boundary at the western margin of the Neoarchaean NSB in the EDC.

Mesoproterozoic magmatism redefines the tectonics and paleogeography of the SW Yangtze Block, China

The geodynamic setting of the Yangtze Block and its paleogeography in the reconstructed Rodinia supercontinent during the late Mesoproterozoic is loosely constrained due to the limited geological information reported. In this contribution, we identify a new suite of amphibolites in the SW Yangtze with an 1175 ± 5 Ma emplacement age. Geochemical modeling demonstrates they were derived from low-degree (3.5%–6.5%) partial melting of spinel lherzolite mantle. Additionally, three suites of late Mesoproterozoic granites in the SW Yangtze Block are revisited by providing new zircon U-Pb-Hf-O isotopes and whole-rock geochemical data. The granites have been dated to emplace during 1060–1042 Ma and have Zr + Nb + Y + Ce and Tzr values mostly at 304–576 ppm and 831–886 ℃, respectively, similar to A-type granites. Zircon Hf-O isotopic data indicate two subgroups of granites, in which one group has suprachondritic εHf(t) values of + 2.4 to + 11.4 with δ18O values of 6.5‰ to 7.4‰ while the other group has εHf (t) values of −3.9 to +1.5 with δ18O values of 10.0‰–11.6‰. The first group of granites was mainly derived from partial melting of mafic tholeiitic rocks while the second group of granites was mainly formed by partial melting of metasedimentary rocks. Integrated with published work, we propose that an intra-continental rift basin was developed in the SW Yangtze Block during ∼ 1.2–1.04 Ga, arguing against a late Mesoproterozoic orogenic event in the SW Yangtze Block. Statistical analyses based on compiled zircon U-Pb-Hf isotopes from Laurentia, Yangzte, and Amazonia show that the Yangtze has an isotopic affinity with west Nuna. These continents likely remained united during the transition from Nuna/Columbia to Rodina along the western periphery of the Rodinia supercontinent.

Building the Proterozoic basement of the western Xing'an-Airgin Sum Block in the eastern Central Asian Orogenic Belt and its implications for the Nuna breakup and Rodinia assembly

The Xing’an-Airgin Sum Block (XAB) is located in central and northern Inner Mongolia, North China, and belongs to one of the oldest blocks in the Central Asian Orogenic Belt (CAOB). Previous studies have shown that there are Mesoproterozoic granites in the XAB; however, the formation process of the Precambrian basement, and also the tectonic evolution and origin of the block are unclear. In this work, the Mesoproterozoic and early Neoproterozoic stratigraphic sequences, ages and the related sedimentary basin types of the western XAB are reported for the first time. Our study reveals that the Mesoproterozoic to early Neoproterozoic stratigraphic sequence contains the Baoyintu Group and Airgin Sum Group with respective depositional ages of 1516–1392 Ma, and 1237–921 Ma. Sedimentation analysis suggests that the Baoyintu Group and the Airgin Sum Group formed in the coastal marine environment with clastic and carbonate provenances and the shallow marine clastic sedimentary environment, respectively. The age distribution patterns of detrital zircons indicate that there were two different types of sedimentary basins, including two convergent basins formed in ∼ 1400 and ∼ 930 Ma, and a collisional basin developed in ∼ 1237 Ma. However, the 1392–1379 Ma syenogranites and alkali-feldspar granites, which are geochemically similar to A-type granites, were generated by the partial melting of mixed Paleo- and Mesoproterozoic crustal materials in an extensional setting. The newly identified early Neoproterozoic I-type granite (921 Ma) was derived from reworking of the Paleo- to Mesoproterozoic crustal source in a hydrous, garnet-free and plagioclase residual condition. These geochemical signatures, coupled with the three sedimentary stages included on the block, suggest that the 1.4–1.3 Ga period recorded an extensional regime at a convergent continental margin (e.g., back-arc basin), whereas the early Neoproterozoic period was likely controlled by a continental arc setting. In summary, the tectonic evolution process of the XAB documented by three sedimentary and two magmatic events could be its response to the supercontinent cycle from the Nuna breakup to the Rodinia assembly.

Detrital U-Pb zircon and 40Ar/39Ar muscovite geochronology from Middle Pennsylvanian strata in the Anadarko Basin, Texas Panhandle, USA

The Late Mississippian-Permian Anadarko Basin formed in Texas and Oklahoma, USA as the result of inversion of Neoproterozic and Cambrian rift structures. Subsidence was driven by flexural loading of the Amarillo-Wichita Uplift, and this uplift may represent the easternmost element of the Ancestral Rocky Mountains system. The northwestern part of this basin has generally been interpreted to have been filled by sediment derived from the Ancestral Front Range Uplift, ~475 km to the northwest during the early stages of basin filling. We test this model using U-Pb detrital zircon and 40Ar/39Ar detrital muscovite results from three subsurface samples of the Morrow B sandstone in the northwestern part of the Anadarko Basin. We provide a new maximum depositional age of 310.9 ± 4.9 Ma that indicates the age of the Morrow B to be no older than late Atokan to early Desmoinesian Age (Moscovian), ~10 Myr younger than previously interpreted. In contrast to some previous interpretations, we propose that the most likely source for the sediment in the Morrow B is the Amarillo Uplift to the south. Detrital zircon and detrital muscovite data have age peaks at 900–1300 Ma, 1370 Ma and 1600–1800 Ma corresponding to derivation from Grenville, Granite-Rhyolite and Yavapai-Mazatzal basement provinces, respectively. A dominant detrital zircon peak at 1370 Ma suggests that Mesoproterozoic granites in the Amarillo Uplift were exposed by Middle Pennsylvanian time, and detritus eroded from the Amarillo Uplift dominated the lowstand sediment budget of the Texas Panhandle during this time; small volumes of sediment were likely sourced from the Ancestral Front Range. This study presents the first detrital geochronology data from the subsurface Anadarko Basin and the first detrital muscovite data from late Paleozoic southwestern Laurentia. The results presented here highlight the interpretive power of combined detrital zircon and muscovite datasets.

Neoarchaean and Proterozoic crustal growth and reworking in the Western Bastar Craton, Central India: Constraints from zircon, monazite geochronology and whole-rock geochemistry

The Bastar Craton is one of the oldest cratonic nuclei of the Indian shield, comprising Paleoarchean to Mesoproterozoic crust that preserves the record of protracted crustal evolution and metallogeny. This study describes whole-rock geochemistry, U-Th-Pb ages of zircon and monazite from Neoarchean TTGs, sanukitoids, and Paleo- to Mesoproterozoic granites, and Sm-Nd isotope data from mafic enclave within TTGs of the Western Bastar Craton (WBC). The TTGs and sanukitoids represent Neoarchean crust formed by collision-accretion processes. The TTGs are of two types: low-HREE and high-HREE with both groups derived from low-K mafic sources. The sanukitoids have moderate SiO2 (55.1–65.1 wt%, average = 61.7 wt%), Mg# (20–36, average = 24.7), Ni (10–40 ppm; average = 14 ppm), Sr (339–528; average = 418 ppm) and moderate to high concentrations of incompatible elements like Rb (26–112 ppm, average = 48), Ba (482–2300 ppm, average = 1542), Zr (69–593 ppm, average = 334 ppm), Nb (3.8–14.8 ppm, average = 8.7 ppm), Y (13.2–24.5 ppm, average = 19.5 ppm), and REE (91–301 ppm, average = 212)]. They post-date TTG emplacement and formed by mixing between metabasalt-derived and mantle wedge-derived melts in an arc environment. The Mul granite represents a younger Paleo- to Mesoproterozoic suite of granites that was derived by reworking of pre-existing crust. U–Pb ages of zircon constraints the TTG magmatism to 2544–2496 Ma, while Nd-model ages (3259–3142 Ma) of mafic enclaves within the TTG suggest the presence of Paleoarchean crust in the WBC.Zircon and monazite ages indicate that the emplacement of the Mul granite was synchronous with 1666–1547 Ma regional tectonothermal event. These granites were produced by reworking of older granitoid crust. The Neoarchean TTGs/sanukitoids were affected by 1666–1547 Ma tectonothermal event which constitutes a widely documented Paleo- to Mesoproterozoic orogeny in the Central Indian Tectonic Zone and Bhopalpatnam granulite belt to the south of the WBC. Copper and gold mineralization in the Thanewasna belt along the craton's western margin is linked to this tectono-magmatic event. In the global supercontinent outlook, the WBC preserves the imprints of Neoarchean events related to the Ur supercontinent as well as Paleo-Mesoproterozoic and Neoproterozoic (Grenville-age) events associated with the Columbia and Rodinia supercontinent

Petrogenesis of Mesoproterozoic Granites of the Swartoup Hills Region, Kakamas Domain, Namaqua Belt, South Africa

The Swartoup and Polisiehoek plutons in the Swartoup Hills (South Africa) formed during an episode of significant magma emplacement in the Mesoproterozoic Namaqua Sector of the Namaqua Metamorphic Province. They intruded into mid-crustal metasedimentary rocks of the metapelitic Koenap and mafic to carbonate-bearing Bysteek Formations during and shortly after the ∼1,200–1,220 Ma regional metamorphic peak that reached ultrahigh temperatures. Subsequent to pluton emplacement, the crust underwent regional high-temperature deformation during slow near-isobaric cooling. A further episode of pluton emplacement associated with fluid circulation truncated the first-order regional tectonic structures at ∼1,100 Ma. Based on their petrography, the Swartoup pluton is subdivided into leuco-granitoids with biotite as the sole mafic phase, pyroxene granitoids, and garnet-bearing granitoids, which may contain significant biotite. These subgroups display distinctive geochemical variations from one another, and from the Koenap Formation migmatites and the Polisiehoek granites, which are exposed nearby. Incompatible trace element distributions suggest that the Swartoup and Polisiehoek granitoids represent modified A-type granite magma, consistent with derivation from partial melting of quartzo-feldspathic crust. The magmas incorporated significant amounts of juvenile mantle-derived magma (εNd1200 of ∼−5, and LREE-depleted), but do not require older, early to late Paleoproterozoic crust. Particularly close to contacts to the calcic Bysteek Formation, localized contamination of the Swartoup granites by supracrustal carbonates is evident. A relatively pervasive alkali metasomatic effect is manifested strongly in the initial 87Sr/86Sr and LILE profiles of the Polisiehoek granites in particular, as well as in some of the Swartoup pyroxene granitoids, which could be either a symptom of CO2 metasomatism related to the Bysteek Formation carbonates, or to post-magmatic fluid metasomatism, perhaps linked to regional shearing. The comparison of our results with literature data suggests that similar sources, A-type granitic, Meso- to Paleoproterozoic crustal, and enriched mantle, have contributed, in locally differing proportions, to granites in most parts of the Namaqua Sector. Most likely, these plutons were generated during crustal and mantle melting in a long-lived hot continental back-arc environment.

Northern limit of Gondwana in northwestern Mexico from detrital zircon data

UPb geochronology along a north-south transect from central Sonora to northern Sinaloa in northwestern Mexico indicates several changes in provenance sources through time and space. Lower Cambrian arenites of the Proveedora Quartzite yield a single main age peak at 1075 Ma with minor Paleoproterozoic contribution. Arenites from the Sonobari Complex of southwestern Sonora-northwestern Sinaloa, purportedly assigned to the early Paleozoic, yield main peaks at 1424, 1662, and 1736 Ma, probably derived from the Yavapai and Mazatzal provinces, which are pervasively intruded by early Mesoproterozoic granites coeval to those forming the Granite-Rhyolite Province. Lower to Upper Ordovician units deposited on shelf, slope, and abyssal environments display very similar zircon age patterns, with main peaks ca. 2700 and 1850 Ma, indicating a large drainage system arising from the Archean cratons of Laurentia such as the Wyoming or Superior Provinces, and Paleoproterozoic sources similar to the Trans-Hudsonian orogen, which are older than the Yavapai Province. The Río Fuerte quartzites from southern Sonora-northern Sinaloa contain main peaks at 534, 542, and 637 Ma whose sources may be peri-Gondwanan blocks similar to Avalonia or Carolina terranes. A minor peak at 475 Ma suggests input from igneous suites reported from the Acatlán Complex in southern Mexico. UPb geochronology permit inference of a late Paleozoic collisional orogen in northwestern Mexico that was originated by the collision of Gondwanan blocks against southern Laurentia and outline the boundary between terranes related to continental blocks in the Pangaea supercontinent.

Enigmatic 1146 ± 4 Ma old granite in the southeastern rim of the West African craton, now part of the Dahomeyan orogenic belt in Ghana

A unique exposure of a strongly deformed late Mesoproterozoic augen-granite (U–Pb zircon age 1146 ± 4 Ma) was detected within rocks on the eastern rim of the West African Craton. These rocks, together with their sedimentary cover, were reworked during the Neoproterozoic Pan-African Dahomeyan orogeny, and are now incorporated in the Dahomeyan orogenic belt of southeast Ghana. To the authors’ knowledge no similar ages have been reported from elsewhere in the West African Craton or in the Pan-African orogenic belts farther north or their continuation in northeastern Brazil. Most other granitoid rocks in the Dahomeyan belt are of Palaeoproterozoic age. Two of them were dated for this study and yielded Concordia ages of 2067 ± 15 Ma and 2119 ± 13 Ma. Lower intercepts around 600 Ma in Wetherill discordia diagrams demonstrate disturbance of the U–Pb systems during the Pan-African orogenic cycle. Chemical data indicate that the Palaeoproterozoic granitoid rocks are arc-type granites (sensu lato), while the Mesoproterozoic augen-granite has the chemical composition of an alkaline within-plate granite. Chemical, whole rock Sm–Nd and zircon Lu–Hf data suggest that the magma that yielded the augen-granite was formed by melting within a mafic Palaeoproterozoic source at moderate crustal depth. The occurrence of a Mesoproterozoic granite in the (now) eastern margin of the West African Craton is evidence of a previously unknown igneous event within this terrain.

Perda de Pb por Difusão Contínua em Zircão Ígneo: Estudo da Espectroscopia BSE, ID-TIMS e Raman em Granito Mesoproterozoico do SW do Cráton Amazônico

This work presents a detailed investigation on zircon crystals of a Mesoproterozoic granite from the SW Amazonian Craton using a combination of back scattered electron (BSE) imaging, thermal ionisation mass spectrometry (TIMS) and Raman scattering spectroscopy. Six zircon grains were analysed. The results plotted in the Concordia diagram yielded an upper intercept age of 1423.0 ± 3.8 Ma, which is interpreted as the crystallization age of the rock. The scattering of the analyses in such diagram indicates that U-Pb isotopes are volume-dependent in the studied zircon grains. BSE reveals zonation in the zircon crystals, and Hf is the main element causing the variability in the BSE intensity and U has a secondary effect. Both elements have much higher atomic number than the principal constituents of zircon (Zr, Si and O), so the substitution of Zr by U and Hf results in increasing image brightness. BSE images from the zircon grains exhibit a euhedral external shape and fine-scale euhedral oscillatory zoning. The predominance of magmatic zoned crystals points out that the granite has not suffered metamorphic heating or any other processes that induce the formation of new phases. The results of the Raman scattering technique indicate that a SiO 4 tetrahedral internal vibrational structure is sufficiently sensitive to determine the increasing degree of metamictization. Furthermore, evidence for metamictization is given by the heterogeneity of the half-width values of the Raman scattering peaks, which exhibit a decreasing trend from the core to the rims of the analysed grains. We concluded that the combination of the three techniques provides information on the Pb loss and that the degree of U-Pb isotopic discordance correlates closely with the volume of the zircon grain. The data, in addition to the lateral degree of metamictization detected by the Raman scattering technique, indicate that the loss of radiogenic Pb may be linked to the continuous diffusion of this element. The correlation of the U-Pb discordance and metamictization emphasizes the importance of the Raman scattering spectroscopy analysis to performing zircon dating. Indeed, it is a helpful tool for geochronologists.

Petrography and geochemistry of granitoids from the Samphire Pluton, South Australia: Implications for uranium mineralisation in overlying sediments

The Blackbush uranium deposit (JORC Inferred Resource: 12,580tonnes U), located on the north-eastern Eyre Peninsula, is currently the only sediment-hosted U deposit investigated in detail in the Gawler Craton. Uranium is hosted within Eocene sandstone of the Kanaka Beds, overlying Mesoproterozoic granites of the Samphire pluton, affiliated with the Hiltaba Intrusive Suite (~1.6Ga). These are considered the most probable source rocks for uranium mineralisation. By constraining the petrography and mineralogy of the granites, insights into the post-emplacement evolution can be gained, which may provide an exploration indicator for other sediment-hosted uranium systems. Three geochemically distinct granite types were identified in the Samphire Pluton and correspond to domains interpreted from geophysical data. All granites show complex alteration overprints and textures with increasing intensity closer to the deposit, as well as crosscutting veining. Alkali feldspar has been replaced by porous K-feldspar and albite, and plagioclase is overprinted by an assemblage of porous albite+sericite±calc-silicates (prehnite, pumpellyite and epidote). This style of feldspar alteration is regionally widespread and known from Hiltaba-aged granites associated with iron-oxide copper‑gold mineralisation at Olympic Dam and in the Moonta-Wallaroo region. In two granite types biotite is replaced by calcic garnet. Calc-silicates are indicative of Ca-metasomatism, sourced from the anorthite component of altered plagioclase. Minor clay alteration of feldspars is present in all samples. Mineral assemblages in veins include quartz+hematite, hematite+coffinite, fluorite+quartz, and clay minerals. Minor chlorite and sericite are found in all vein types. All granite types are anomalously rich in U (concentrations between 10 and 81ppm). Highly variable Th/U ratios, as well as hydrothermal U minerals (mostly coffinite) in granites and veins, are clear evidence for U mobility. Uranium may have been preconcentrated in veins in the upper parts of the pluton, and was subsequently leached after deposition of the sediment.

Naturaliste Plateau: constraints on the timing and evolution of the Kerguelen Large Igneous Province and its role in Gondwana breakup

ABSTRACTVolcanism associated with the Kerguelen Large Igneous Province is found scattered in southwestern Australia (the ca 136 to ca 130 Ma Bunbury Basalts, and ca 124 Ma Wallaby Plateau), India (ca 118 Ma Rajmahal Traps and Cona Basalts), and Tibet (the ca 132 Ma Comei Basalts), but apart from the ∼70 000 km2 Wallaby Plateau, these examples are spatially and volumetrically minor. Here, we report dredge, geochronological and geochemical results from the ∼90 000 km2 Naturaliste Plateau, located ∼170 to ∼500 km southwest of Australia. Dredged lavas and intrusive rocks range from mafic to felsic compositions, and prior geophysical analyses indicate these units comprise much of the plateau substrate. 40Ar/39Ar plagioclase ages from mafic units and U–Pb zircon ages from silicic rocks indicate magmatic emplacement from 130.6 ± 1.2 to 129.4 ± 1.3 Ma for mafic rocks and 131.8 ± 3.9 to 128.2 ± 2.3 Ma for silicic rocks (2σ). These Cretaceous Naturaliste magmas incorporated a significant component of continental crust, with relatively high 87Sr/86Sr (up to 0.78), high 207Pb/204 Pb ratios (15.5–15.6), low 143Nd/144Nd (0.511–0.512) and primitive-mantle normalised Th/Nb of 11.3 and La/Nb of 3.97. These geochemical results are consistent with the plateau being underlain by continental basement, as indicated by prior interpretations of seismic and gravity data, corroborated by dredging of Mesoproterozoic granites and gneisses on the southern plateau flank. The Cretaceous Naturaliste Plateau igneous rocks have signatures indicative of extraction from a depleted mantle, with trace-element and isotopic values that overlap with Kerguelen Plateau lavas reflect crustal contamination. Our chemical and geochronological results therefore show the Naturaliste Plateau contains evidence of an extensive igneous event representing some of the earliest voluminous Kerguelen hotspot magmas. Prior work reports that contemporaneous correlative volcanic sequences underlie the nearby Mentelle Basin, and the Enderby Basin and Princess Elizabeth Trough in the Antarctic. When combined, the igneous rocks in the Naturaliste, Mentelle, Wallaby, Enderby, Princess Elizabeth, Bunbury and Comei-Cona areas form a 136–124 Ma Large Igneous Province covering >244 000 km2.

Depositional environment and tectono-provenance of Upper Kaimur Group sandstones, Son Valley, Central India

The Mesoproterozoic Upper Kaimur Group consists of Bijaigarh Shale, Scarp Sandstone, and Dhandraul Sandstone. Based on the lithofacies data set, two major facies associations were identified, namely—tidal sand flat/sand bar facies association (TSFA) and tidally influenced fluvial channel facies/tidal channel facies association (TIFCFA). The Dhandraul Sandstone has been interpreted as a product of TIFCFA and the underlying Scarp Sandstone in TSFA which endorses a tidal dominated estuarine setting. Detrital modes of the Dhandraul and Scarp Sandstones fall in the quartz arenite to sub-litharenite types. Petrographical data suggest that the deposition of the Upper Kaimur Group sandstones took place in humid climate and was derived from mixed provenances. The sandstone composition suggests detritus from igneous rocks, metamorphic rocks, and recycled sedimentary rocks. The sandstone tectonic discrimination diagrams suggest that the provenances of the Upper Kaimur Group sandstones were continental block, recycled orogen, rifted continental margin to quartzose recycled tectonic regimes. It is envisaged that the Paleo- and Mesoproterozoic granite, granodiorite, gneiss, and metasedimentary rocks of Mahakoshal Group and Chotanagpur granite–gneiss present in the western and northwestern direction are the possible source rocks for the Upper Kaimur Group in the Son Valley.

Sources of Mesoproterozoic igneous rocks and formation time of the continental crust of the Kokchetav Massif (Northern Kazakhstan)

Within the Kokchetav massif (Northern Kazakhstan), Mesoproterozoic granites and acid volcanics are widespread: these are the youngest Precambrian igneous rocks forming basement of the region. The Nd isotopic characteristics (eNd(t)–4.4 ÷–9.6, t Nd(DM) 2.1–2.6 Ga) obtained for these rocks indicate that the source of their melts was the Early Precambrian continental crust. Thus, the continental crust of the Kokchetav Massif had basically been formed by the beginning of the Mesoproterozoic and during the Late Precambrian: later it became a source for the granitoid melts.