Amphibolite Samples Research Articles

A case study of crushed stone for road construction application: amphibolite and basalt varieties from Croatia

Crushed stone is one of the most exploited and important mineral raw material. Emphasis is given to determination of mineralogical and petrographic properties, of two genetically different rocks of silicate composition, commonly used as crushed stone for road constructions, as their physical properties depend on them. First is amphibolite from Vetovo, and second is basalt from Vratnik locality, both sourced from Croatia. Tests conducted according to European and Croatian norms evaluated the real and apparent density, open and total porosity, and water absorption of stones. The relationship between mineralogical and physical properties led to the following conclusions: the open porosity values of amphibolite samples are low, which is a consequence of the medium to high degree of metamorphism, (plagioclase and hornblenda), sosiritization, and undisturbed rock features. The total water saturation was achieved in a brief time, confirming this statement. The basalts have different porosity and water absorption values, which are consequences of differences their textural-structural properties (glassy, intersertal or intergranular groundmass), amount of amygdules, presence or absence of chlorite/volcanic glass. Furthermore, the percentage of plagioclase in the modal composition differs, and the plagioclase itself has been altered, and the amount of clay, sericite and calcite vary in two different varieties of basalt. Compared to amphibolites, basalts have notably higher water absorption values as well. Accordingly, the studied amphibolite has water absorption less than 0.5% and is suitable for use in a road construction application for wearing course in the asphalt pavement, and studied basalt is not.

The tectonic implications of a rare metamorphic event in eastern China during the Earth's “middle age”

The Mesoproterozoic Era has long been considered a relatively stable, silent, and even "boring" period in Earth's history, during which the lithosphere was tectonically inactive. However, an increasing amount of evidence suggests that metamorphism and magmatism were much more widespread and intense than previously thought, implying that this period was dynamic. Here we report metamorphism around ca. 1.4 Ga along the southeastern margin of the North China Craton, eastern China. We conduct TIMA, EMPA, and zircon SHRIMP and LA-ICP-MS U-Pb dating on garnet amphibolites and their country rocks. The detailed mineral composition of a representive garnet amphibolite sample is identified and determined by TIMA. Phase equilibria modelling using pseudosections combined with EMPA data indicates the peak metamorphism stage (M 2 ) of the garnet amphibolites at a P-T condition of 10.6-11.2 kbar/820 ℃. SHRIMP Zircon U-Pb analysis yields an age of 1366 ± 17 Ma for the garnet amphibolites, which is slightly younger than that of its country rock schist (1376 ± 22 Ma). Integrating the metamorphic event with geochemical signatures, magmatic records, and sedimentation patterns in analogous geological settings on other continents, our findings collectively demonstrate a protracted and pervasive orogenic process along the periphery of the Columbia supercontinent.

Zircon U‐Pb provenance analysis of impact melt and target rocks from the Rochechouart impact structure, France

AbstractThe Rochechouart impact structure in the northwestern part of the French Massif Central (FMC) has a great diversity of impactites, including monomict impact breccias, suevite, and impact melt rocks (IMRs). The structure is strongly eroded, which allows the study of impactites of the crater fill and the transition into the crater floor. The FMC has had a multistage geological evolution from the late Neoproterozoic to the Ordovician (600–450 Ma) until the later stages of the Variscan orogeny (~300 Ma). Previous geochronological work on Rochechouart has been focused mainly on the impactites and constraining the impact age, and scarce work has been done on the FMC‐related target rocks. Here, U‐Pb isotope analysis by LA‐MC‐ICP‐MS has been conducted on zircon from two IMRs from the Recoudert and Montoume localities, and from a monzodiorite, a paragneiss, and two amphibolite samples of the basement to the impact structure. Zircon from the target rocks yielded mainly Neoproterozoic to Carboniferous ages (~924 to ~301 Ma) that can mostly be correlated to different stages of the geological evolution of the FMC. The monzodiorite also yielded a Permian age of 272 ± 12 Ma. Zircon from the IMRs, and especially from the Montoume sample, gave a comparatively higher diversity of Neoproterozoic to Jurassic ages (~552 to ~195 Ma). Provenance analysis for the zircon age populations of the impactites compared to those of the basement rocks shows overall poor correlation between the two age groups. This suggests that other target lithologies were involved in the formation of these impact melts as well. Post‐Variscan and preimpact ages (281–226 Ma) obtained for both melt rocks probably reflect a previously unconstrained event in the evolution of the regional geological history. Ages similar to the currently most widely accepted impact age of ~204–206 Ma were obtained from both IMR samples. In addition, the Montoume melt rock yielded several post‐204 Ma ages, which might reflect a to date unconstrained, about 194 Ma postimpact thermal/hydrothermal event.

Metamorphic Ages and PT Conditions of Amphibolites in the Diebusige and Bayanwulashan Complexes of the Alxa Block, North China Craton

The metamorphism and geological significance of amphibolites in the Diebusige and Bayanwulashan Complexes of the eastern Alxa Block, North China Craton, were poorly understood until now. This study presents the results of petrology, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb analysis, phase equilibrium modeling and geothermobarometry for these rocks. The peak mineral assemblage of clinopyroxene + hornblende + plagioclase + K-feldspar + ilmenite + quartz + melt is inferred for amphibolite sample ALS2164 in the Diebusige Complex. Correspondingly, the peak mineral assemblage of clinopyroxene + hornblende + plagioclase ± K-feldspar + ilmenite + quartz + melt is identified for amphibolite sample ALS2191 in the Bayanwulshan Complex. Phase equilibrium modelling constrained the peak metamorphic condition of amphibolite sample ALS2164 in the Diebusige Complex to be 825–910 °C/7.2–10.8 kbar, which is similar to that (800–870 °C/7.0–10.7 kbar) of amphibolite sample ALS2191 in the Bayanwulashan Complex. Hbl–pl–qz thermobarometry yielded the metamorphic PT conditions of 732–810 °C/3.0–6.7 kbar for these amphibolites, which are consistent with the average temperatures of 763 °C, 768 °C and 780 °C calculated by Ti-zircon thermometry. As a result, phase equilibrium modelling yielded wide PT condition ranges of 800–910 °C/7.0–10.8 kbar, the lower limit of which is consistent with the upper limit of estimates by the hbl–pl–qz thermobarometer. In addition, LA-ICP-MS U–Pb analysis on metamorphic zircons yielded weighted mean 207Pb/206Pb ages of 1901 ± 22–1817 ± 21 Ma, which represent the timing of amphibolite-facies metamorphism. As a whole, the PT estimates display a high geothermal gradient, which is consistent with coeval ultrahigh-temperature metamorphism and associated mantle-derived mafic-ultramafic rocks in the Diebusige Complex. Combing this information with the previously published data from the Diebusige Complex, an extensional setting after continental collision is inferred for the eastern Alxa Block during the late Paleoproterozoic. The HREE enrichment patterns of metamorphic zircons from the amphibolites in this study are in agreement with that these amphibolites formed at relatively shallower crust than the garnet-bearing mafic granulites in the Diebusige Complex.

Subduction initiation recorded in the Dadeville Complex of Alabama and Georgia, southeastern United States

Abstract The Dadeville Complex of Alabama and Georgia (southeastern United States) represents the largest suite of exposed mafic-ultramafic rocks in the southern Appalachians. Due to poor preservation, chemical alteration, and tectonic reworking, a specific tectonic origin for the Dadeville Complex has been difficult to deduce. We obtained new whole-rock and mineral geochemistry coupled with zircon U-Pb geochronology to investigate the magmatic and metamorphic processes recorded by the Dadeville Complex, as well as the timing of these processes. Our data reveal an up-stratigraphic evolution in the geochemistry of the volcanic rocks, from forearc basalts to boninites. Our new U-Pb zircon crystallization data—obtained from three amphibolite samples—place the timing of forearc/protoarc volcanism no later than ca. 467 Ma. New thermobarometry suggests that the Dadeville Complex rocks subsequently experienced deep, high-grade metamorphism, at pressure-temperature conditions of >7 kbar and >760 °C. The data presented here support a model for formation of the Dadeville Complex in the forearc region of a subduction zone during subduction initiation and protoarc development, followed by deep burial/underthrusting of the complex during orogenesis.

Constraints on the Palaeoproterozoic tectono‐metamorphic evolution of the Lewisian Gneiss Complex, NW Scotland: Implications for Nuna assembly

AbstractDespite extensive investigation, the tectono‐thermal evolution of the Archean crust in the Lewisian Gneiss Complex in NW Scotland (LGC) is debated. Most U–Pb zircon geochronological and metamorphic studies have focused on rocks from the central region of the mainland LGC, where granulite facies assemblages associated with the oldest (Badcallian) tectono‐metamorphic event at c. 2.75 Ga are overprinted by younger amphibolite facies assemblages related to the Inverian (c. 2.5 Ga) and subsequent Laxfordian (c. 1.9–1.65 Ga) tectono‐thermal events. In the southern and northern regions of the mainland LGC, deformation and metamorphism associated with the Laxfordian event are pervasive, although the timing and conditions are poorly constrained. Here, we present new field, petrographic and structural data, U–Pb zircon and titanite geochronology and phase equilibrium modelling of amphibolite samples from the northern and southern regions. Our field observations show that in both regions, pre‐Laxfordian structures are significantly reworked by steep NW‐striking fabrics that are themselves pervasively overprinted by co‐axial deformation and amphibolite facies metamorphism related to the Laxfordian event. In situ U–Pb titanite geochronology yields Laxfordian ages of 1853 ± 20 Ma in the southern region (P = 6–8 kbar and T = 640–690°C) and 1750 ± 20 Ma and 1776 ± 10 Ma in the northern region (P = 6–7.5 kbar and T = 740–760°C). While U–Pb dating of zircon rims from felsic gneisses in the central region shows a dominant Inverian metamorphic overprint at c. 2500 Ma, zircon rims in felsic gneisses from the northern and southern regions commonly yield Laxfordian dates as young as c. 1800 Ma. Combined, the results support the idea that, during the Palaeoproterozoic, the central region of the LGC acted as low‐strain domain, in which intense deformation and metamorphism were restricted to crustal‐scale shear zones. By contrast, in the southern and northern regions, early (c. 1.85 Ga) and late (c. 1.75 Ga) Laxfordian deformation and fluid‐mediated metamorphism were much more pervasive and at higher P–T conditions than previously proposed. The diachronous Laxfordian evolution of the southern and northern regions indicate that they reflect early and late snapshots of collisional to transpressional tectonics in the mainland LGC. The long‐lasting Laxfordian evolution documents the collision of the Rae and North Atlantic cratons during the Palaeoproterozoic amalgamation of the supercontinent Nuna, with implications for the palaeogeographic configuration of NW Scotland during Palaeoproterozoic Nuna.

Synchronous formation of the ‘forearc’ Bay of Islands ophiolite and its basal high-temperature metamorphic sole constrained by U–Pb zircon ages

The welded metamorphic sole at the base of the Bay of Islands Ophiolite Complex (BOIC) in the Northern Appalachians of Newfoundland shows a typical inverted pressure-temperature (P-T) metamorphic gradient from HT-MP granulite to LT-LP greenschist facies. It incorporates mafic volcanic/plutonic protoliths mixed with pelagic, hemi-pelagic and coarser epiclastic sedimentary protoliths. New LA-ICP-MS U–Pb concordia ages, trace elements, and Ti-in-zircon geothermometry for ∼250 zircon analyses from three metabasites of the upper HT sole amphibolites with N-MORB-like protoliths are reported. Two samples collected within meters of the ophiolite peridotite-sole contact of the Blow Me Down Mountain and North Arm Mountain massifs yielded the oldest comparable concordia ages of 487.7±2.6 Ma and 489.1±3.1 Ma, respectively, that are both within error of the igneous age of 488.3±1.5 Ma of the directly overlying BOIC ophiolite, which formed at a supra-subduction zone (SSZ) forearc spreading center. A third slightly younger age of 484.2±2.4 Ma was obtained for an upper HT amphibolite sample with similar phase assemblages but collected ∼30 m below the peridotite contact of the Table Mountain massif. Zircon crystals analyzed have similar size and morphologies, subparallel rare earth element (REE) variation patterns, and steep heavy REE-enrichments ((Lu/Gd)cn >20), significant positive Ce anomalies (dominantly >5) and slight positive to dominantly negative Eu anomalies (1.2 – 0.4). Zircon shows Th/U mean values of 0.37 – 0.48 with little to no rim to core variation. Minimum Ti-in-zircon mean crystallization temperatures range from ∼764 – 787 °C. These neocrystallized zircon crystals appear to be derived from thin leucosomes within the three amphibolites. Two other samples also from the upper HT sole show evidence of inherited detrital zircon with core dates spanning the Cambrian Notre Dame Arc through older Laurentian-like basement and rift age ranges.Subcretion of the sole took place below a hot forearc asthenospheric wedge, that is, a consequence of the newly-formed BOIC forearc spreading center extending from the back arc to a triple junction along the westward- (or paleo-northward) verging trench of the Notre Dame arc. The early HT sole formation age at ca. 489–488 Ma is long prior to initiation of obduction at ca. 470 Ma and long after initiation of subduction beneath the paleo-northward verging Notre Dame peri-Laurentian arc at ca. 514 Ma. This indicates Newfoundland sole ages of the BOIC and St. Anthony Complex are correlated with the age of SSZ spreading, but not necessarily subduction initiation because previously existing and self-sustaining subduction was ongoing. Sole ages are then not correlated with the younger age of obduction-related orogenic events (e.g., proposed Taconic I and II) in the Newfoundland Appalachians.

Geochemical data, Nd and Hf isotopes and U–Pb geochronology of meta-mafic rocks from western Gondwana suture zone

The West Gondwana orogen, represented in NE Brazil by Santa Quitéria continental magmatic arc and ultrahigh-pressure metamafic rocks of a suture zone, have amphibolite and coesite-bearing retro-eclogite in both margins of the magmatic arc. This research reports and discuss the geochemical and geochronological data (U–Pb and Lu–Hf in zircon and Sm–Nd whole-rock) of amphibolite and retro-eclogite from the eastern margin of a continental magmatic arc. The Santa Quitéria Complex and metasedimentary Ceará Complex hosted these rocks. The Santa Quitéria amphibolites shows evident fractionation of REE. Nb–Ta–Ce anomalies evidence magma generated in a subduction zone with contribution and recycling of crustal sedimentary material, corroborate for slightly negative εNd (−1.56) and Paleoproterozoic NdTDM value (1.80 Ga). Isotopic Lu–Hf in zircon data indicates positive εHf (+5.00 to +7.78) and Neoproterozoic model ages (HfTDM from 0.97 to 0.86 Ga) (t = 637 ± 34 Ma - sample TJF5-258) indicate their extraction from the mantle. For the amphibolite samples from the Ceará Complex, the elemental distribution suggest that these rocks place them among oceanic basalts of enriched segments of mid-ocean ridges (E-MORB) and oceanic island basalts (OIB) – intra-oceanic arc basalts (IOAB). U–Pb zircon data of Ceará Complex amphibolite points to crystallization at ca. 670 Ma (674 ± 9 Ma, sample TJJV-07) and two metamorphic events around 640 Ma (644 ± 9 Ma) and 600 Ma (591 ± 5 Ma) related to the continental collision. Positive εNd values (+1.66 to +4.16) and model ages (NdTDM from 1.26 to 1.00 Ga) suggest a protolith crystallized from juvenile material of mantle source. Our data support that the eastern edge of the Santa Quitéria magmatic arc constitutes an active margin with subduction of the Cryogenian oceanic crust.

The temporal evolution of subduction initiation in the Samail ophiolite: High‐precision U–Pb zircon petrochronology of the metamorphic sole

AbstractHigh‐precision dating of the metamorphic sole of ophiolites can provide insight into the tectonic evolution of ophiolites and subduction zone processes. To understand subduction initiation beneath a young, well‐preserved and well‐characterized ophiolite, we performed coupled zircon laser‐ablation inductively coupled mass spectrometry trace element analyses and high‐precision isotope dilution‐thermal ionization mass spectrometry U–Pb dating on 25 samples from the metamorphic sole of the Samail ophiolite (Oman‐United Arab Emirates). Zircon grains from amphibolite‐ to granulite‐facies (0.8–1.3 GPa, ~700–900°C), garnet‐ and clinopyroxene‐bearing amphibolite samples (n = 18) show systematic trends of decreasing heavy rare earth element slope (HREE; Yb/Dy) with decreasing Yb concentration, reflecting progressive depletion of the HREE during prograde garnet growth. For half of the garnet‐clinopyroxene amphibolite samples, Ti‐in‐zircon temperatures increase, and U–Pb dates young with decreasing HREE slope, consistent with coupled zircon and garnet growth during prograde metamorphism. In the remaining samples, there is no apparent variation in Ti‐in‐zircon temperature with decreasing HREE slope, and the combined U–Pb and geochemical data suggest zircon crystallization along either the prograde to peak or prograde to initial retrograde portions of the metamorphic P–T–t path. The new data bracket the timing of prograde garnet and zircon growth in the highest grade rocks of the metamorphic sole between 96.698 ± 0.094 and 95.161 ± 0.064 Ma, in contrast with previously published geochronology suggesting prograde metamorphism at ~104 Ma. Garnet‐free amphibolites and leucocratic pods from lower grade (but still upper amphibolite facies) portions of the sole are uniformly HREE enriched (Yb/Dy > 5) and are ~0.5–1.3 Myr younger than the higher grade rocks from the same localities, constraining the temporal offset between the metamorphism and juxtaposition of the higher and lower grade units. Positive zircon εHf (+6.5 to +14.6) for all but one of the dated amphibolites are consistent with an oceanic basalt protolith for the sole. Our new data indicate that prograde sole metamorphism (96.7–95.2 Ma) immediately predated and overlapped growth of the overlying ophiolite crust (96.1–95.2 Ma). The ~600 ky offset between the onset of sole metamorphism in the northern portion of the ophiolite versus the start of ophiolite magmatism is an order of magnitude shorter than previously proposed (~8 Ma) and is consistent with either spontaneous subduction initiation or an abbreviated period of initial thrusting during induced subduction initiation. Taken together, the sole and ophiolite crust preserve a record of the first ~1.5 Myr of subduction. A gradient in the initiation of high‐grade metamorphism from the northwest (96.7 Ma) to southeast (96.0–95.7 Ma) may record propagation of the nascent subduction zone and/or variations in subduction rate along the length of the ophiolite.

Geochemistry and geochronology of the Bay of Islands metamorphic sole, Newfoundland, Canada: Protoliths and implications for subduction initiation

The Bay of Islands complex, Newfoundland, Canada, represents a fossil subduction system that was obducted onto the Laurentian margin during the Taconic Orogeny. We present whole-rock geochemistry and geochronology for the Bay of Islands metamorphic sole from the Table Mountain and North Arm Mountain massifs. The Bay of Islands metamorphic sole can be subdivided into three units: a high metamorphic-grade Grt-Cpx amphibolite and Grt-Cpx granulite unit, a medium metamorphic-grade common amphibolite unit, and a medium metamorphic-grade metasedimentary-dominated unit. Based on their major and trace element contents, two types of protoliths can be inferred for the Bay of Islands sole metabasites: cumulate gabbroic protoliths with high Mg numbers and depleted trace element contents, and basaltic protoliths with trace element contents defining a normal mid-oceanic-ridge basalt (N-MORB) affinity. Titanite in situ U-Pb analyses of two common amphibolite samples yield dates of 486 ± 7 Ma and 484 ± 4 Ma, which are interpreted as a metamorphic age (ca. 485 Ma). One sample from the metasedimentary unit yields detrital zircon dates that range from Archean to Ordovician, with the youngest dates defining the most dominant peak at ca. 490 Ma. This youngest peak is consistent with an arc-related source that may be linked with arc magmatism of the upper plate during the Taconic Orogeny. These observations suggest that (1) the subduction responsible for the formation of the Bay of Islands complex nucleated in an oceanic domain before ca. 485 Ma; and (2) the sediments that form the metasedimentary unit were partly sourced from the upper plate and were deposited on the lower plate while subduction was active.

Juxtaposition of different-grade metamorphic rocks in an ancient orogen: Evidence from the Chengde Complex of the Trans-North China Orogen, North China Craton

Juxtaposition of different-grade metamorphic slices is a typical feature of Phanerozoic orogens but is relatively scarce in Precambrian orogens. Here, we focus on the Chengde Complex at the northern segment of the late Paleoproterozoic Trans-North China Orogen, the North China Craton as a case, to explore how different-grade orogeny-related metamorphism was juxtaposed in Precambrian orogens. The Trans-North China Orogen is a typical Precambrian collisional orogen and records abundant information about the Paleoproterozoic orogeny. High-pressure (HP) mafic granulite and two types of amphibolite samples were collected from the Chengde Complex, and we conducted a combined study involving litho-structural assemblage investigation, metamorphic petrology, and geochronology to decipher their metamorphic P-T−t history. HP mafic granulite and amphibolite occur as enclaves or dikes within felsic gneisses. HP mafic granulite records clockwise P-T paths with isothermal decompression (ITD) segments, and the peak metamorphic P-T conditions are 13.0−14.9 kbar/790−830 °C. Peak metamorphic P-T conditions retrieved from garnet-bearing amphibolite are 8.3 kbar/675 °C, whereas peak metamorphic P-T conditions retrieved from garnet-free amphibolite are 4.0−5.5 kbar/500−510 °C. Zircon U-Pb dating of HP granulite and amphibolite yield different prograde (1914−1871 Ma), peak (1869−1816 Ma), and retrograde (ca. 1830 Ma) metamorphic ages, and all of these data indicate that these metamorphic rocks were diachronously transferred to different depths in the subduction channel and subsequently exhumed at shallower crustal levels. Therefore, we conclude that the Chengde Complex is composed of imbricate rocks with different metamorphic grades and ages, and such an unordered juxtaposition of diverse metamorphism could also be a typical feature of Precambrian orogens.

Zircon ages, mineralogy, and geochemistry of ophiolitic mafic and island-arc rocks from central Cuba: Implications for Cretaceous tectonics in the Caribbean region

Ophiolite-related rocks and volcanic-arc rocks are the main geological components in Cuba; deciphering their origin and evolution is fundamental in interpreting and reconstructing the tectonic processes for the Caribbean region. In this paper, we report new zircon U Pb chronology, mineralogy, and geochemistry of ophiolitic metadiabase dikes and island-arc rocks in central Cuba, and discuss their origin and petrogenesis. The metadiabase dikes have a zircon U Pb age of 135.0 ± 1.7 Ma; they are low-K tholeiites , and have geochemical characteristics intermediate between those of typical normal mid-ocean ridge basalt (N-MORB) and island-arc tholeiites (IAT), suggesting that they were formed in a back-arc basin. The investigated island-arc rocks, including amphibolite from the Mabujina amphibolite complex (MAC), plutonic diorite and their mafic enclaves, and volcanic dacite, have zircon U Pb age of 93.53 Ma, 92.31 Ma, 94.35 Ma and 83.60 Ma, respectively, recording different phases of island-arc magmatism. The amphibolite samples have geochemical compositions suggesting that the protolith of the amphibolite was basaltic igneous rocks that originated from a depleted mantle source mildly modified by fluids from a subducting slab, and that the mantle source might be deep and garnet-bearing. Microstructures and elemental geochemical data reveal that the mafic enclaves and diorites represent distinct island-arc magma pulses that originated from partial melting of depleted mantle which had been previously modified by subduction-related mobile elements. The dacites are evolved island-arc volcanic rocks that formed through fractional crystallization of hydrous basaltic magmas that originated from a strongly depleted mantle source. Above data support such a tectonic model, that is, during the Early Cretaceous (from ~135 Ma or even earlier), a typical trench–arc–basin system developed in the Caribbean region as a result of southwest-directed subduction of the Proto-Caribbean plate beneath the Caribbean plate; after that time, uninterrupted Proto-Caribbean plate subduction occurred, generating quasi-continuous arc magmatism from before ~135 to 81 Ma, and gradually consuming the Proto-Caribbean Ocean. • A diabase dike in the ophiolite in central Cuba has a zircon U Pb age of 135.0 ± 1.7 Ma. • A typical trench–arc–basin system developed in the Caribbean region during the Early Cretaceous. • The arc rocks have highly diverse major and trace elemental and isotopic compositions.

Precambrian Tectonic Affinity of the Southern Langshan Area, Northeastern Margin of the Alxa Block: Evidence from Zircon U‐Pb Dating and Lu‐Hf Isotopes

AbstractThe Alxa Block is the westernmost part of the North China Craton (NCC), and is regarded as one of the basement components of the NCC. Its geological evolution is of great significance for the understanding of the NCC. However, the Precambrian basement of the Alxa Block is still poorly studied. In this study, we present new in situ LA‐ICP‐MS zircon U‐Pb and Lu‐Hf isotope data from the Diebusige Metamorphic Complex (DMC) which located in the eastern Alxa Block. Field and petrological studies show that the DMC consists mainly of metamorphic supracrustal rocks and minor metamorphic plutonic rocks and has experienced amphibolite‐granulite facies metamorphism. Zircon U‐Pb dating results suggested that the amphibolite sample yields a crystallization age of 2636 ± 14 Ma and metamorphic ages of 2517–2454 Ma and 1988–1952 Ma, proving the existence of exposed Archean rocks in the Langshan area and indicating that late Neoarchean to Paleoproterozoic metamorphic events existed in the Alxa Block. Two paragneiss samples show that the magmatic detrital zircons from the DMC yield 207Pb/206Pb ages ranging from 2.48 Ga to 2.10 Ga with two youngest peaks at 2.13 Ga and 2.16 Ga, respectively, and they were also overprinted by metamorphic events at 1.97–1.90 Ga and 1.89–1.79 Ga. Compilation of U‐Pb ages of magmatic detrital and metamorphic zircons suggested that the main part of the DMC may have been formed at 2.1–2.0 Ga. Zircon Lu‐Hf isotope data show that the source materials of the main part of the DMC were originated from the reworking of ancient Archean crust (3.45–2.78 Ga). The Hf isotope characteristics and the tectonothermal event records exhibit different evolution history with the Khondalite Belt and the Yinshan Block and the other basements of the Alxa Block, indicating that the Langshan was likely an independent terrain before the middle Paleoproterozoic and was subjected to the middle to late Paleoproterozoic tectonothermal events with the Khondalite Belt as a whole.

Petrology and P-T-t Path of Huangyuan Group and Maxianshan Group in the Central Qilian Block, NW China: Implications for Tectonic Evolution of the Proto-Tethys Ocean

AbstractThe Qilian orogen marks the northern boundary of the Proto-Tethys Ocean and consists of the North Qilian belt, the Central Qilian block and the South Qilian belt. The metamorphic P–T–t path of the Central Qilian block and the tectonothermal processes that affected it are crucial to understanding the closure of the ocean. This paper presents a systematic investigation of the metamorphic P–T–t path of amphibolites from the Precambrian Huangyuan Group and Maxianshan Group in the Central Qilian block. An amphibole 40Ar/39Ar age of 456.7 ± 5.2 Ma from a garnet amphibolite and a 427.8 ± 3.8 Ma muscovite 40Ar/39Ar age from an orthogneiss in the Huangyuan Group, and amphibole and biotite 40Ar/39Ar ages of 408.7 ± 2.2 Ma and 349.9 ± 2.0 Ma from an amphibolite and a 361.4 ± 2.0 Ma biotite 40Ar/39Ar age from an orthogneiss in the Maxianshan Group record cooling stages after peak metamorphism. Phase equilibrium modeling of all amphibolite samples indicates clockwise P–T paths. A garnet amphibolite from the sillimanite zone in the Huangyuan Group recorded peak metamorphic conditions of 5.6–6.6 kbar and 730–770°C, reflecting granulite-facies metamorphism. Peak P–T conditions for a garnet amphibolite from the garnet zone are constrained to 580–600°C and 6–7 kbar. The amphibolite sample from the Maxianshan Group indicates peak P–T conditions of ~5.2 kbar and ~630°C. The Central Qilian block was affected by both southward subduction of the North Qilian Ocean and northward subduction of the South Qilian Ocean, which resulted in paired metamorphic belts along the northern margin of the Central Qilian block and accretion of the Hualong block to its southern margin. The subducted North Qilian oceanic slab detached and the remainder was exhumed at 453–445 Ma, accompanied by the extension and retrograde metamorphism of the overlying crust of Huangyuan Group at 457–428 Ma. Subsequent cooling of the Maxianshan Group indicates that the thermal regime relaxed during post-collisional extension at c. 409–350 Ma between the Hualong block and the Central Qilian block.

Trondhjemites from the Qianxi Complex, North China Craton: Implications for Neoarchean crustal growth

The TTG (tonalite‐trondhjemite‐granodiorite) rock suites are major building blocks of the Archean continental crust and their timing of formation and petrogenetic evolution are important keys to understanding the growth of continents in the Early Earth. Here we investigate trondhjemites from the Qiangxi Complex in the North China Craton and present petrological, geochemical and zircon U–Pb, REE and Lu–Hf data to understand the timing of their formation and petrogenetic history. The trondhjemites in our study belong to typical Archean Na‐rich TTG rocks and their geochemical features are similar to high‐Al TTG rocks. The geochemical characteristics of these rocks and the associated amphibolite indicate derivation from a subduction‐related hydrous basalt. The magmatic zircon grains from the trondhjemite samples show a peak 207Pb/206Pb age around 2,515 Ma. Magmatic zircon in the associated amphibolite also shows a similar age representing the timing of the protolith formation. Both the trondhjemites and amphibolite also contain metamorphic zircon grains which constrain the timing of metamorphism as 2,451–2,490 Ma, suggesting high‐grade metamorphism closely following the magmatism, associated with the assembly of the microblocks in the NCC. The geochemical characteristics and zircon age data presented in this study, in conjunction with those in previous studies from the Qiangxi Complex, suggest subduction‐related arc building occurred dominantly during 2.6–2.5 Ga. The Lu‐Hf isotope data from zircon grains in the trondhjemite and amphibolite samples of this study show dominantly positive εHf(t) values, indicating that the protolith magma was derived from juvenile components or depleted mantle. Our new results in conjunction with the data reported in earlier studies indicate a major crust‐building event through multiple magmatism during Neoarchean. The depleted mantle ages (TDM) of zircon in our samples range from 2,857 Ma to 2,605 Ma indicating that active subduction might have initiated in the late Mesoarchean, with the peak of magmatism during 2.6–2.5 Ga, followed by metamorphism at ca. 2.45–2.49 Ga.

Petrology and Detail Geological Mapping of the Precambrian Basement Rocks of the Sn-Ta-Nb Numbi Deposit, Democratic Republic of the Congo

The Numbi district of eastern Democratic Republic of the Congo (DRC) has high mineral interest; it has been exploited for decades by small scale artisanal miners for rare metals such as Sn, W, Ta and Nb. This area was investigated to elucidate the petrology of the basement rocks and to make their detailed geological map. From the petrographical and geochemical analyses, six lithological units were characterized. Granite and schist were the two main lithologies with some associated smaller rock units such as amphibolite, diorite, quartzite and pegmatite. Biotite is the common mineral found in all lithologies. The granite displays the mineralogical assemblages with the abundance of muscovite, biotite, feldspar, plagioclase and quartz. Diorite and amphibolite display less amount of quartz. Hornblende is very abundant in the amphibolite; less is the schist and a very small amount in the granite. Feldspar is mostly found in the granite, quite found in the quartzite and less in the schist rocks. In adding to these mineral assemblages, accessory and opaque minerals (iron oxide) are in a very small amount. Pegmatite is mostly found as altered veins. Schist, quartzite and granite samples share the fields of granodiorite while diorite and amphibolite samples share the fields of diorite and gabbroic diorite on the total alkali-silicate classification diagram and ranged under intermediate and acidic rocks. The area is abundantly rich in rare metals (pegmatite with Nb-Ta-Sn mineralization) and is fairly suitable for quarry activities.

P-T conditions and timing of metamorphism of the Yuanmou area, southern Neoproterozoic Kang-Dian Orogenic Belt, southwest China

Geological evolution of the Neoproterozoic Kang-Dian Orogenic Belt (KDOB, southwestern China) attracts great debate in the past decades, due to lack of systematic research on metamorphism and petrochronology. In this study, representative samples of metapelite and garnet amphibolite of the Yuanmou area, southern KDOB, were investigated to constrain their metamorphic P-T paths and timing. Metapelitic samples mainly consist of garnet + biotite + muscovite + quartz + zircon ± kyanite ± staurolite without plagioclase, while the amphibolite mainly contains garnet + amphibole + plagioclase + quartz + titanite. Two and three stages of metamorphic mineral assemblages were recognized in the metapelite and amphibolite, respectively. Clockwise metamorphic P-T paths were retrieved from the representative samples by geothermobarometry and phase equilibria modelling. The peak metamorphic P-T conditions of these samples are diverse, ranging from 7.2 to 13.6 kbar and 590–650 ℃, respectively. SHRIMP U-Pb dating of metamorphic zircon of the metapelite yielded different mean 206Pb/238U ages of ∼ 882–767 Ma, while LA-ICP-MS U-Pb dating of titanite of a garnet amphibolite sample yielded age of 834 ± 15 Ma. Combined with previous data, it is suggested that the Yuanmou area records subduction (as early as ∼ 885 Ma) to collision (before ∼ 682 Ma) processes of the southern KDOB, which experienced a prolonged orogenic process in the Neoproterozoic.

Paleo- and Mesoarchean TTG-sanukitoid to high-K granite cycles in the southern São Francisco craton, SE Brazil

The generation of the continental crust is widely accepted to have taken place predominantly in the Archean, when TTG magmatism associated with greenstone-belt supracrustal succession development was typically followed by emplacement of high-K granites before crustal stabilization. This study focuses on the Campos Gerais complex (CGC), which is an Archean granite-greenstone belt lithological association in a tectonic window located in the southwesternmost portion of the São Francisco craton (SFC). The CGC is an important segment of Paleo- to Mesoarchean continental crust to be integrated into paleogeographic reconstructions prior to the transition into the Paleoproterozoic. This investigation reports field relationships, 28 major and trace element compositions, U–Pb (zircon) geochronological results, and Hf and Sm–Nd isotope data for orthogneiss and amphibolite samples. The results indicate that the CGC records a complex Archean crustal evolution, where voluminous 2.97 Ga TTG tonalites and trondhjemites (εNd(t) = − 4.7; TDM = 3.24 Ga) were followed by 2.89 Ga sanukitoid tonalite production (εNd(t) = − 1.9; TDM = 3.02 Ga), broadly coeval with the development of the Fortaleza de Minas and Pitangui greenstone-belts. These events are interpreted to represent the initial stage of an important subduction-accretion tectonic cycle, which ended with the emplacement of 2.82–2.81 Ga high-K leucogranites and migmatization of the TTG-sanukitoid crust, with hybrid and two-mica, peraluminous compositions (εNd(t) = − 8.0 to − 8.6; TDM = 3.57 – 3.34 Ga). The presence of inherited zircons with 207Pb/206Pb ages of 3.08 Ga, 3.29 Ga, 3.55 Ga and 3.62 Ga indicates that the Mesoarchean tectonic processes involved reworking of Meso- to Eo-archean crust. Renewed TTG magmatism took place at ca. 2.77 Ga represented by juvenile tonalite stocks (εNd(t) = +1.0 to − 1.5; TDM = 2.80 – 2.88 Ga) which intrude the TTG-greenstone belt association. Crustal stabilization was attained by 2.67 Ga, allowing for the emplacement of within-plate tholeiitic amphibolites (εNd(t) = − 3.1; TDM = 2.87 Ga). The CGC shows important tectonic diachronism with respect to other Archean terrains in the southern São Francisco craton, including an independent Meso- to Neoarchean crustal evolution.

Paleoproterozoic medium to high pressure metamorphism in the Wanzi supracrustal association, Trans-North China Orogen: New insights from the gedrite-bearing gneiss, gedrite-free gneiss, and amphibolite

The Wanzi supracrustal association locates in the Fuping Complex, middle Trans-North China Orogen, and it was generally considered before as a lithological unit that underwent medium pressure metamorphism. The newly discovered gedrite-bearing gneiss, gedrite-free gneiss, and amphibolite of the Wanzi supracrustal association show abundant mineral assemblages with complex reaction textures, which are critical clues to retrieve the metamorphic evolution of this area. Gedrite-bearing/free gneiss samples mainly consist of garnet porphyroblast plus matrix minerals including biotite, kyanite, sillimanite, staurolite, ±gedrite, plagioclase, quartz, as well as accessory minerals such as ilmenite, magnetite, apatite, monazite, and zircon. Amphibolite samples are generally composed of garnet porphyroblast plus matrix minerals amphibole, ±biotite, plagioclase, quartz, as well as accessory minerals including ilmenite, magnetite, and zircon. Two to four generations of metamorphic mineral assemblages formed at the prograde (M1), peak (M2), and retrograde (M3-M4) stages have been identified from these samples, recording clockwise P-T paths including sequential near-isothermal decompression (ITD) and near-isobaric cooling (IBC) segments, indicative of convergence tectonic settings. The peak P-T conditions of these samples are different, ranging from 8.2 to 13 kbar and 695 to 822 °C. SIMS/SHRIMP U-Pb dating of metamorphic zircon yields 207Pb/206Pb age of 1838–1821 Ma, representing the metamorphic event. Combined with the previous data, the Wanzi supracrustal association is inferred to have undergone middle to high pressure metamorphism during 1.84–1.82 Ga, which was involved in the subduction and/or collision event between the Western and Eastern Blocks and led to the final assembly of the North China Craton.

Neoproterozoic (740-680 Ma) arc-back-arc magmatism in the Sergipano Belt, southern Borborema Province, Brazil

The Novo Gosto amphibolites represent the oldest unit of the Canindé domain, a Neoproterozoic sequence located in the NW Sergipano Belt, southern Borborema Province, Brazil. The domain comprises an association of metavolcano-sedimentary rocks, gabbroic intrusions, bimodal magmatic rocks, and syn-to post-tectonic granitoids. Due to the lithostratigraphic and geochemical complexity of this domain, several controversial interpretations of the tectonic setting have been proposed over the years. The protholits of the Novo Gosto amphibolites are here classified as high Fe tholeiitic basalt type rocks. Three geochemical groups were identified: Group 1 is more depleted in incompatible elements and has a pattern similar to island arc basalts with HFSE depletion (Nb–Ta, Zr–Hf, weak Ti); Group 2 displays a pattern similar to Group 1, but with more evident anomalies (Nb–Ta, Ti) and varied enrichment in incompatible elements; Group 3 is more enriched in incompatible elements than Group 1 rocks, with the pattern being flatter with milder HFSE depletion (Nb–Ta, Zr–Hf, weak Ti, Y) and resembling a geochemical pattern slightly more enriched than E-MORB. Our results suggest an environment similar to the Ryukyu-Okinawa (Japan) and Kamchatka arc-back-arc (Russia) setting. LA-ICP-MS U–Pb zircon dating of one amphibolite sample resulted in a date of 743 ± 3 Ma and confirms the geological field relationships that indicate this unit to be the oldest in the domain. The new petrographic, geochemical, and isotopic data for the Novo Gosto amphibolites lead to the recognition of an ancient Neoproterozoic arc-back-arc system, a rare setting in the earlier phases of the Brasiliano Orogeny within the Borborema Province, which ended with the closure of the restricted Canindé sea and the associated Sergipano ocean at around 630 Ma.