Zircon Xenocrysts Research Articles

Solid-phase transfer into the forearc mantle wedge: Rutile and zircon xenocrysts fingerprint subducting sources

Voluminous flux of a hydrous component from subducting oceanic lithosphere into the forearc mantle and its contribution to arc magmas have long been recognised but the transfer of solid-phase materials has received less consideration. Although xenocrystic zircons are known from some arc magmas and ophiolitic chromitites their origins remain enigmatic. How and when such materials are transferred into the overlying wedge of mantle lithosphere and the length of their residency therein are unclear. Using zircons and rutiles recovered from ophiolitic rocks in New Caledonia, we demonstrate unambiguous association with subducting sediments and hence evidence for inter-plate transfer. The ‘fingerprints’ of age spectra for both minerals can be matched to lithospheric slab components subducted during Eocene time. Their occurrence in forearc harzburgite suggests that they were incorporated into their mantle wedge host at subduction depths beyond the onset of serpentinisation at ca. 300°C but before the ca. 600°C destabilisation temperature of the rutile U-Pb system. Relocation from slab to forearc mantle is therefore likely to have occurred at ca. 50-60 km depth within the subduction channel. Fluid-assisted conveyance of high field strength elements within solid-state accessory mineral phases into supra-subduction mantle wedges may represent a little-recognised but significant phenomenon that is both predictable and pervasive at convergent margins globally.

Formation of ultrapotassic magma via crustal contamination and hybridization of mafic magma: an example from the Stomanovo monzonite, Central Rhodope Massif, Bulgaria

AbstractGenerally all orogenic ultrapotassic rocks are formed after melting of metasomatized sub-continental lithospheric mantle via subducted crustal mica-bearing lithologies. Here we present another possible model, based on the study of the small Stomanovo ultrapotassic monzonite porphyry intrusion in the Central Rhodope Massif, Bulgaria. The monzonite dated at 30.50 ± 0.46 Ma is intruded into the voluminous Oligocene (31.63 ± 0.40 Ma) Bratsigovo–Dospat ignimbrite. The monzonite hosts both normally and reversely zoned clinopyroxene phenocrysts. The normally zoned clinopyroxene is characterized by gradually diminishing core-to-rim Mg no. (89–74), whereas the reversely zoned clinopyroxene has green Fe-rich cores (Mg no. 71–55) mantled by normally zoned clinopyroxene (Mg no. 87–74). Neither the core of the normally zoned clinopyroxene nor the Fe-rich green cores are in equilibrium with the host monzonite. This ultrapotassic monzonite shows more radiogenic Sr isotopes ((87Sr/86Sr)i= 0.71066) and ϵNd(t) = −7.8 to −8.0 that are distinct from the host ignimbrites with (87Sr/86Sr)i= 0.70917–0.70927 and ϵNd(t) = −4.6 to −6.5. The Sr–Nd isotopic data and the presence of copious zircon xenocrysts from the underlying metamorphic basement suggest extensive crustal assimilation. Our observations indicate that the Stomanovo ultrapotassic monzonite formed after extensive lower or middle crustal fractional crystallization from an evolved magma producing cumulates. The process was followed by hybridization with primitive mantle-derived magma and subsequent continuous crustal contamination. We suggest that instead of inheriting their high K2O and large-ion lithophile element enrichments from slab-derived/metasomatic fluids, the Stomanovo ultrapotassic monzonite may owe some of its unusually high alkalinity to the assimilation of potassium-rich phases from the Rhodope Massif basement rocks.

An ancient continental crustal source for Mo mineralisation in the eastern Central Asian Orogen: A case study of the Bilugangan Mo deposit

The Bilugangan porphyry Mo deposit is located in the southern Great Xing'an Range, and the Bilugangan area near the Hegenshan-Heihe Fault in the eastern part of the Central Asian Orogen (CAO). The deposit is hosted by a NE-trending, partly concealed, composite granite consisting of highly altered porphyritic granite (G1), porphyritic monzogranite (G2), fine- to medium-grained monzogranite (G3), medium -grained monzogranite (G4), and monzogranitic porphyry dykes (G5). The Mo mineralisation forms hydrothermal stockworks or disseminated sulphides in the G2 and G3 phases and the adjoining hornfels. Magmatic zircons from the intrusions yield 206Pb/238U weighted mean ages of 295 ± 2 Ma for G1, 240 ± 1 Ma for G2, 235 ± 2 Ma for G3, 230 ± 1 Ma for G4, and 100 ± 2 Ma for G5,indicating that multiple magmatic events have been active during the Early Permian to Early Cretaceous emplacement of the Bilugangan Granite. The age of the Mo is constrained between the Middle and Late Triassic and the magma-related hydrothermal activity proceeded in at least two pulses over a period of around 5 Ma. The first pulse has a molybdenite Re-Os weighted mean age of 238 ± 2 Ma, and the second dates at 236 ± 1 Ma. These dates are coeval with those of G2 and G3, indicating a genetic association between them and the mineralisation.The Sr-Nd-Hf isotopic systematics shows that the Bilugangan Granite was probably formed by partial melting of at least two sources. The first source is a dominant Late Mesoproterozoic to Early Carboniferous juvenile lower crust indicated by the whole rock εNd(t) values of −1.9 to 5.1 and zircon εHf(t) values of 3.5 to 14.3 with two-stage depleted mantle Nd (TDM2) and Hf (tDM2) model ages of ca. 1086 to 350 Ma, which is possibly related to the evolution of the Palaeo-Asian Ocean. The second source includes Neoarchaean to Palaeoproterozoic crustal components and a subordinate Mesoproterozoic crustal component. The Neoarchaean to Palaeoproterozoic component is indicated by the presence of ca. 2700 to 2600 Ma xenocrystic zircons, negative εHf(t) values of −16.4 to −10.1 with tDM2 ages of ca. 2300 to 1900 Ma. The Mesoproterozoic component yields εHf(t) values of −4.5 to 1.1 and tDM2 ages of ca. 1600 to 1200 Ma. These data show that pre- Mesoproterozoic substrate is present beneath the southern Great Xing'an Range. Although the post-mineralisation G4 and G5 phases have comparable positive εHf(t) values with the earlier granitic phases, the G2 and G3 phases have a more complex source with distinctly negative εHf(t) values of −16.4 to –1.3, indicating that the older crustal components in the magma’s source are also probable sources for the Mo mineralisation.

Reconstructing the Lancang Terrane (SW Yunnan) and implications for early Paleozoic Proto-Tethys evolution at the northern margin of Gondwana

The Lancang Group in SW Yunnan constitutes a pivotal component of the continental margin bounding the Proto- and Paleo-Tethys oceans, yet its formation timing, composition and tectonic affinity remain ill-defined. In this contribution, we present new zircon U-Pb age and whole-rock geochemistry data from volcanic and sedimentary rock units, which are compiled with published data, to refine the depositional age, provenance and nature of the group and its host terrane. The Lancang Group consists of five formations, two of which (Manlai and Huimin) contain abundant metavolcanic rocks. The metavolcanic rocks show a secular change in geochemistry, from EMORB-like calc-alkaline series of the ca. 495 Ma Manlai Formation, to arc-like high-K series of the Huimin Formation. Five metavolcanic samples from the Huimin Formation yielded zircon U-Pb ages between 478 ± 5 Ma and 442 ± 5 Ma. The late Cambrian-Ordovician arc magmatism is related to an active continental margin setting associated with subduction of the Proto-Tethys Ocean. The time-integrated geochemical variation shown by the volcanic rocks likely reflects evolving maturity of the arc system. The youngest detrital zircon ages of the five formations, in conjunction with age constraints from the interbedded volcanic rocks, suggest that the Lancang Group was largely accumulated during the early Paleozoic. The Lancang Terrane is reconstructed as a continental block, which formed part of the magmatically-active margin of the Proto-Tethys during the early Paleozoic. Provenance analysis based on detrital zircon age spectra and Hf isotope data indicates Gondwana affinities of the terrane. Furthermore, xenocrystic zircon ages and zircon Hf and whole-rock Nd model ages from igneous rocks support presence of Neoarchean-Mesoproterozoic basement underlying the terrane. The reconstructed Lancang Terrane bears important implications for understanding the configuration of the northern margin of Eastern Gondwana and the relationship between the Proto- and Paleo-Tethys oceans.

The timing of rifting events in the easternmost Mediterranean: U-Pb dating of zircons from volcanic rocks in the Levant margin

ABSTRACT The Levant basin, at the easternmost Mediterranean, formed during the opening of the Neo-Tethys at Permian to Jurassic times, as recorded by subsidence and magmatism along its SE passive margin. While the subsurface extensional structure was reconstructed in detail, the spatial and temporal distribution of igneous activity is not well known. Here we present new SIMS U-Pb dating of zircons from rifting related volcanic sequences recovered from four deep boreholes in the Levant margin of coastal Israel. The Asher volcanics, a deeply buried 2.5 km thick basaltic sequence, which accumulated within fault-bounded basin, was previously dated by K-Ar and Ar-Ar to early Jurassic times. Zircons (n=46) separated from the bottom and top of the volcanic sequence yielded 206±2 Ma and 205 to 202 Ma, respectively, indicating that Asher volcanics are of Late Triassic (Rhaetian) age. Zircon xenocrysts indicate Permian (289±2 Ma) felsic magmatism. Rifting activity in the Levant margin included several Permian-Triassic pulses of faulting and subsidence, with a prolonged magmatism-bearing rifting event at Late Carnian to Rhaetian times (~230-201 Ma). Pulsed rifting is also indicated in the conjugate, northern, margin of the southern Neo-Tethys, and the timing of subsidence and magmatic pulses well correlate to the Levant margin.

First identification of a Cathaysian continental fragment beneath the Gagua Ridge, Philippine Sea, and its tectonic implications

AbstractThe tectonic history of the Philippine Sea plate is an essential piece in understanding the tectonic evolution of Southeast Asia, but it is still unclear and controversial. We present the first geochemical data obtained from lavas from the Gagua Ridge (GR) within the Philippine Sea. The GR lavas exhibit geochemical signatures typical of subduction-related arc magmatism. Plagioclase Ar-Ar ages of ca. 124–123 Ma and subduction-related geochemical signatures support the formation of GR lavas in the vicinity of an arc during the Early Cretaceous induced by subduction of the oceanic plate along East Asia. The ages of trapped zircon xenocrysts within the GR lavas cluster at 250 Ma, 0.75 Ga, and 2.45 Ga and match well the ages of zircons recovered from the Cathaysian block, southern China. Our results imply that the GR basement is partially composed of continental material that rifted away from the Eurasian margin during opening and spreading of the Huatung Basin. The depleted mantle wedge-derived magmas evolved and picked up the continental zircons during ascent. The youngest zircon ages and the GR lava Ar-Ar ages (ca. 124–123 Ma) presented in this study newly constrain an Early Cretaceous age for the Huatung Basin. Our study provides further evidence that the Huatung Basin is a remnant of a Mesozoic-aged ocean basin that dispersed from southern China during the Cretaceous. Transport of continental slivers by growth and closure of marginal seas along the East Asia margin may have been more prevalent than previously recognized.

Geochronological Constraint on the Evolution of the Aktyuz Terrane, Kyrgyz North Tianshan, and the Fate of the Taldybulak Levoberezhny Gold Deposit

The Aktyuz Terrane in Kyrgyz North Tianshan is of particular interest due to the occurrence of high and ultrahigh pressure (HP–UHP) rocks and it containing the third largest gold deposit in Kyrgyz North Tianshan, i.e., Taldybulak Levoberezhny (abbreviated to Taldybulak Lev.). To constrain the ages of the host Kemin Complex and its auriferous monzogranite porphyry, detailed zircon U–Pb dating [by laser ablation inductively coupled plasma-mass spectrometry (LA-ICPMS) and secondary ion mass spectroscopy (SIMS)] and Lu–Hf isotopic analyses were carried out. The intensively altered auriferous monzogranite porphyry yielded two weighted mean ages of 444 ± 3 Ma (n = 14, mean squared weighted deviation (MSWD) = 0.49, by LA-ICPMS) and 440 ± 5 Ma (n = 8, MSWD = 0.82, by SIMS) that are indistinguishable within error ranges. Such ages are consistent with a previously reported sulfide Re–Os isochron age of 434 ± 18 Ma, supporting a Silurian porphyry gold mineralization. The granitic gneiss yielded a protolith age of 773 ± 7 Ma (n = 7, MSWD = 0.04) and two metamorphic ages of 514 ± 4 Ma (n = 8, MSWD = 0.09) and 483 ± 3 Ma (n = 11, MSWD = 0.04). Detrital zircons from one fuchsite schist sample yielded highly variable ages from 729 ± 13 Ma to 2,463 ± 30 Ma, with 12 data points weighted at 740 ± 5 Ma (MSWD = 0.95). The metamorphic overgrowth yielded a weighted mean age of 460 ± 4 Ma (n = 4, MSWD = 0.15). Detrital zircons in the migmatitic amphibolite are aged from 788 ± 7 Ma to 3,447 ± 32 Ma, with two major concentrations at 941 ± 7 Ma (n = 13, MSWD = 0.95) and 794 ± 5 Ma (n = 8, MSWD = 0.19). The metamorphic overgrowth yielded an average age of 555 ± 4 Ma (n = 8, MSWD = 0.65). The detrital and xenocryst zircons, and evolved εHf(t) values (−20.9 to −7.8) and old two-stage Hf model ages (1,367–3,159 Ma), revealed the presence of a Precambrian basement that may be dated back to the Archean Eon. The two metamorphic ages may correlate with oceanic subduction and continental collision, respectively.

U-Pb-Hf isotopic systematics of zircons from granites and metasediments of southern Ouaddaï (Chad), implications for crustal evolution and provenance in the Central Africa Orogenic Belt

We present the first combined U-Pb ages and Lu-Hf isotope data of zircon grains from the southern Ouaddaï area in Eastern Chad in the Central Africa Orogenic Belt. Zircon xenocrysts from granites and detrital zircons from quartzites of the Ouaddaï document crustal accretion from depleted mantle sources at 3.0 ± 0.5 Ga, rarely up to 3.8 Ga, and for subsequent reworking during four magmatic events, during the Neoarchean at ca. 2.6 Ga (εHft = −7.0 to 4.6), Paleoproterozoic at ca. 1.9–2.1 Ga (εHft = −17.7 to 5.3), Mesoproterozoic at ca. 1.1 Ga (εHft = −15.3 to 5.9), and Neoproterozoic/early Cambrian at 635–540 Ma (εHft = −31.1 to 1.4). Superchondritic εHft values point to new crust addition during all four events. Metamorphic zircon rims yield a Concordia age of 600 ± 2 Ma, and provide evidence that greenschist to amphibolite-facies metamorphism affecting the quartzites occurred between syn-tectonic (635 Ma) and post-kinematic (540 Ma) granite intrusions. Our new set of zircon data, combined with field relationships, also indicate that the quartzite precursor rocks were deposited between 1045 and 635 Ma, possibly after 670 Ma in a basin that subsequently became the Central Africa Orogenic Belt. The detritus was supplied from surrounding cratons, although the sources of the Mesoproterozoic zircons remain ambiguous. Comparison of age-εHft data from worldwide sources suggests sediment supply from the Irumide, Katanga and Mozambique belts, located to the south-east of the Congo Craton. Paleogeographic reconstructions further require sediment transport prior to final closure of several oceanic basins surrounding Central and Northern Africa during the Neoproterozoic, comprising the Khomas and Mozambique oceans

Evidence for discrete Archean microcontinents in the Yangtze Craton

It is unclear whether most Archean continents have always existed in their present-day size, or were initially microcontinents or supercratons. The Archean evolution of the Yangtze Craton in South China provides an ideal opportunity to reveal Archean continental architecture. Archean rocks in the Yangtze Craton have been documented in seven separate areas prior to this study. In this paper, we report the first discovery of Archean rocks in a new area in the southern Dabie orogenic belt, which we term the Xishui gneiss. The protolith of the granitic gneiss is strongly peraluminous with high A/CNK ratios of 1.10–1.23 and K2O/Na2O ratios of 0.70–1.72, belonging to the high-K calc-alkaline series. Three samples of the gneiss from different localities yielded protolith ages of 2.51, 2.47, and 2.47 Ga (zircon U–Pb weighted mean ages). Xenocrystic zircons with magmatic texture from the gneiss yielded U–Pb ages of 3.57–2.41 Ga, and records four phases of magmatism and crustal reworking at 3.56, 3.34, 3.18, and 2.51–2.47 Ga. The history of crustal growth recorded by the gneiss can be traced back to ~3.80 Ga, with peaks at 3.8–3.7 and 3.5–3.4 Ga. A comparison of the crustal evolution in the eight exposed Archean terranes of the Yangtze Craton suggests they were independent microcontinents during the Archean and that the present-day Yangtze Craton was amalgamated at 2.0–1.9 Ga during assembly of the Columbia supercontinent.

Crustal growth during Western Gondwana amalgamation and onset of the Brasiliano orogeny: Insights from geochemistry and Pb–Sr–Nd–O isotopes from granites in northeastern Brazil

The lithological units of the eastern Pernambuco–Alagoas superterrane of the Borborema Province, northeastern Brazil, includes the Santana do Ipanema (SI) batholith, whose framework is defined by a large multi-intrusive plutonic complex. The results from field, petrologic, geochemical, whole-rock Sr–Nd–O isotopes, and zircon U-Pb geochronological studies on a suite of three porphyritic granites from this batholith are reported here. The studied granites carry biotite, amphibole, titanite, and ± magmatic epidote as major mafic phases. These rocks have high Mg# (up to 52), and are metaluminous to slightly peraluminous, high-K calc-alkaline, and magnesian akin to Cordilleran granitic batholiths. All of the studied granitic rocks are enriched in LREE- and LILE-enriched (e.g., Rb, Ba, K, and Th), and HFSE-depleted (e.g., Nb, Ta, and Ti), typical of subduction-related volcanic arc rocks. Estimated intensive parameters suggest crystallization pressure from 3 to 4 kbar, temperature > 1000 °C (near-liquidus) to 640 ± 35 °C (near-solidus), and magmatic oxidizing conditions between ΔNNO+0 to ΔNNO+1. SHRIMP UPb zircon data point to crystallization ages from 643 to 625 Ma, while zircon xenocrysts are 800 to 1230 Ma old. Nd isotopes reveal depleted mantle model ages from 0.96 to 1.32 Ga, and εNd(t), initial 87Sr/86Sr, and δ18O[zircon] values from −3.35 to +0.73, 0.7054 to 0.7086, and 6.36 to 8.62‰, respectively. The geochemical and isotopic signatures of the studied granites, coupled with those from the Santana do Ipanema batholith available in the literature, suggest magma derivation from mixed juvenile Neoproterozoic (late Cryogenian to Early Ediacaran) high-K basalts and reworked Mesoproterozoic to late-Tonian components modified (metasomatized) by subduction-related processes during the amalgamation of the São Francisco Craton and Borborema Province. These data evidence crustal growth during Western Gondwana amalgamation, associated with the onset of the Brasiliano/Pan African orogeny.

Multiple tectonomagmatic reactivation of the unexposed basement in the northern Siberian craton: from Paleoproterozoic orogeny to Phanerozoic kimberlite magmatism

ABSTRACT Zircon xenocrysts from two diamond-barren kimberlite pipes (Leningrad and Ruslovaya) in the West Ukukit kimberlite field opened a ‘window’ to the buried crustal basement in the northern Siberian craton. Zircon U-Pb ages reveal a close affinity of the basement of the Khapchan belt to the Archaean Anabar province and a significant tectonomagmatic reworking in the Paleoproterozoic (~2.1–1.8 Ga) due to collision between the Anabar province and the Olenek province. The West Ukukit kimberlite field experienced multiple tectonomagmatic reactivation from ~670 to 144 Ma, which can be attributed to interaction of the deep crust with mantle-derived melts. Hf isotope composition of zircon xenocrysts reveals significant addition of juvenile material into the crust during the Paleoproterozoic orogeny in diamond-barren kimberlite fields, which is different from the reworking crust in the southern Yakutia diamondiferous kimberlite fields. Eruption of the Leningrad and Ruslovaya pipes were constrained as the Late Jurassic, much later than the well-known Late Silurian-Earth Devonian kimberlites in the West Ukukit kimberlite field. A NE-trending, >2000 km long kimberlite corridor is proposed to account for a prolonged lithospheric channel for episodic eruption of kimberlites in the Siberian craton. The diamond storage in the lithosphere beneath the West Ukukit kimberlite field may have been largely reduced by the Paleoproterozoic orogeny and Phanerozoic reworking.

Petrogenesis of the Cretaceous Intraplate Mafic Intrusions in the Eastern Tianshan Orogen, NW China

In this study, we conducted zircon U-Pb dating, and whole-rock geochemical and Sr-Nd isotope analyses on the Late Mesozoic dolerite dykes in the Bailingshan Fe deposit (Eastern Tianshan Orogen, NW China) to unravel their petrogenesis and regional tectonic significance. Zircon U-Pb dating on the dolerite yielded an Early Cretaceous age of 129.7 ± 1.4 Ma. The dolerite is calc-alkaline sodic (Na2O/K2O = 4.71 to 6.80), and enriched in LILEs (Rb, K, Sr, and Pb) but depleted in HFSEs (Nb, Ta, and Ti). The intermediate Nb/U (16.7 to 18.5) and Ce/Pb (6.33 to 6.90) values, and the presence of xenocrystic zircons in these dolerite dykes suggest crustal assimilation during the magma evolution. Petrological modeling suggests fractionation of olivine, pyroxene, garnet, and spinel. All the dolerite samples have low initial 87Sr/86Sr (0.7041 to 0.7043) and positive εNd(t) (+ 4.6 to + 5.1) values, indicative of a depleted asthenospheric mantle source. Partial melting modeling suggests that the melting has occurred in the spinel-garnet stability field. Integrating the data from ore deposit geology, geochronology, geochemistry and Sr-Nd isotopes, we proposed that the Late Cretaceous Eastern Tianshan mafic magmatism was developed in an intraplate extension setting.

Archean to Proterozoic (3535–900 Ma) crustal evolution of the central Aravalli Banded Gneissic Complex, NW India: New constraints from zircon U-Pb-Hf isotopes and geochemistry

The central Aravalli Banded Gneissic Complex, a Proterozoic high-grade polymetamorphic terrane with Archean components in northwest India, is mainly composed of granulites, granitoids, TTG gneisses and metasedimentary rocks. Combined sets of zircon U-Pb-Hf isotopes and whole-rock geochemical data reveal that the gneissic complex is made up of Neoarchean TTGs, Paleoproterozoic A-type granitoids, and Mesoproterozoic nepheline syenite, which intruded at ca. 2550, 1720–1705 and 1440 Ma, respectively. Zircon xenocrysts additionally point to Paleoarchean-Neoarchean magmatic episodes at ca. 3535 and 2665 Ma. The discovery of a ca. 3535 Ma inherited zircon with superchondritic εHft provides the first direct evidence for the existence of an early Paleoarchean crust in the Aravalli orogen, which was formed by the partial melting of a depleted mantle reservoir. Ages from zircon rims suggest Grenville-age metamorphic overprints at ca. 970 Ma and ~ 900 Ma. High HREE contents and subchondritic εHf2.55Ga = -7.3 to 0.7 of the Neoarchean TTG suggest partial melting of a heterogenous mafic crust derived from depleted mantle sources during the Paleo- to Mesoarchean. The Paleoproterozoic A-type granitoids with subchondritic εHft between −6.1 and −12.5 originated mainly from internal reworking of a heterogenous, predominantly Neoarchean felsic crust, between 1720 and 1705 Ma. Near chondritic Hf isotope composition of the ca. 1440 Ma Kishangarh nepheline syenite reflects derivation of the parental magma from enriched mantle sources.

REE Minerals as Geochemical Proxies of Late-Tertiary Alkalic Silicate ± Carbonatite Intrusions Beneath Carpathian Back-Arc Basin

The accessory mineral assemblage (AMA) of igneous cumulate xenoliths in volcanoclastic deposits and lava flows in the Carpathian back-arc basin testifies to the composition of intrusive complexes sampled by Upper Miocene-Pliocene basalt volcanoes. The magmatic reservoir beneath Pinciná maar is composed of gabbro, moderately alkalic to alkali-calcic syenite, and calcic orthopyroxene granite (pincinite). The intrusive complex beneath the wider area around Fiľakovo and Hajnáčka maars contains mafic cumulates, alkalic syenite, carbonatite, and calc-alkalic granite. Both reservoirs originated during the basaltic magma underplating, differentiation, and interaction with the surrounding mantle and crust. The AMA of syenites is characterized by yttrialite-Y, britholite-Y, britholite-Ce, chevkinite-Ce, monazite-Ce, and rhabdophane(?). Baddeleyite and REE-zirconolite are typical of alkalic syenite associated with carbonatite. Pyrochlore, columbite-Mn, and Ca-niobates occur in calc-alkalic granites with strong peralkalic affinity. Nb-rutile, niobian ilmenite, and fergusonite-Y are crystallized from mildly alkalic syenite and calc-alkalic granite. Zircons with increased Hf/Zr and Th/U ratios occur in all felsic-to-intermediate rock-types. If rock fragments are absent in the volcanic ejecta, the composition of the sub-volcanic reservoir can be reconstructed from the specific AMA and zircon xenocrysts–xenolith relics disintegrated during the basaltic magma fragmentation and explosion.

Biotite 40Ar/39Ar dating and chemical composition inform metallogenesis of Xiaoxi'nancha porphyry Au-Cu deposit, NE China

• Zircon U-Pb dating of ore-bearing rocks show age of ~255 Ma. • Hydrothermal biotite 40 Ar/ 39 Ar age show ~110 Ma. • The same age (biotite and molybdenite) indicate coeval Au-Cu-Mo mineralization . • Halogen fugacity ratios imply a relatively Cl-rich hydrothermal system . The Xiaoxi'nancha Au-Cu porphyry deposit, located in the eastern Jilin Province of Northeast China, is characterized by widespread biotite alteration, however, the age of Au-Cu mineralization is not well established. Hydrothermal biotite Ar/Ar dating and chemical composition are widely used to constrain the age and physico-chemical characteristics of hydrothermal fluids. In this paper, microscopy observation, electron microprobe analysis, and 40 Ar/ 39 Ar dating of hydrothermal biotite from the Xiaoxi'nancha porphyry Au-Cu deposit and zircon U-Pb dating on the ore-bearing granitoids have been carried out to constrain fluid characteristics and the age of mineralization. The zircon crystals from the ore-bearing granitoids display obvious inherited cores and oscillatory rims. A range of 206 Pb/ 238 U age populations were obtained on the cores (e.g., ~967 Ma, ~782 Ma, ~495 Ma, ~430 Ma, ~385 Ma, ~354 Ma, ~311 Ma) but a young population with a uniform concordia age of 254.4 Ma was determined on the rims. These features indicate the occurrence of xenocrystic zircon in the ore-bearing granitoids, and imply that these rocks were probably derived from partial melting of continental crust with significant crustal contamination. Au-Cu and Mo mineralization are found together and associated with potassic and chlorite alteration. The 40 Ar/ 39 Ar age of hydrothermal biotite (~110 Ma) is younger than the zircon U-Pb age (~255 Ma), but similar to the previously published molybdenite Re-Os age (~110 Ma). Thus, Au-Cu mineralization is coeval with Mo mineralization and formed at ~110 Ma, and has no genetic relationship with ore-bearing granitoids. The calculated log( f H 2 O/ f HF) and log( f H 2 O/ f HCl) ratios of hydrothermal fluids are similar to those determined at other major porphyry deposits. However, the log( f HF/ f HCl) ratio is lower than that of other porphyry deposits, indicating a relatively Cl-rich hydrothermal system at Xiaoxi'nancha. The Xiaoxi'nancha porphyry Au-Cu deposit is associated with early Cretaceous granitoids, generated during subduction of the Paleo-Pacific Ocean.

Petrogenesis of the 2.3 Ga Lengkou metavolcanic rocks in the North China Craton: Implications for tectonic settings during the magmatic quiescence

Paleoproterozoic (~2.3) Ga metavolcanic rocks occur in the Lengkou area, northern Zhongtiao Mountains of the North China Craton (NCC). The area is dominated by metabasic volcanic rocks, intercalated with metamorphosed intermediate to acid volcanic rocks. The Lengkou metavolcanic rocks have a magmatic zircon U-Pb age of 2317 ± 13 Ma, but also contain some 2508 Ma inherited zircons. The Lengkou metabasic and intermediate volcanic rocks are calc-alkaline with high SiO2 and K2O contents. They display fractionation of the HREE relative to the LREE, relative enrichments in large ion lithophile elements and depletion in high field strength elements resulting in negative Nb-Ta-Ti and Zr-Hf anomalies. Therefore, they are inferred to have formed in a volcanic arc setting. The meta-acid volcanic rocks of the Lengkou series have geochemical features similar to the surrounding Sushui Complex (Neoarchaean tonalitic gneiss), resulting from partial melting of ancient crust. The Lengkou metabasic volcanic rocks have a low 143Nd/144Nd ratio and εNd(t) value of 0.8–1.5, with TDM of 2.57–2.63 Ga, indicating the mantle-derived mafic rocks were variably contaminated by older continental material. The integrated data from the Lengkou metavolcanic rocks indicate a convergent subduction-related regime in the early Paleoproterozoic. The isotopic signatures and the xenocrystic zircons demonstrate that subduction occurred at a margin of continental crust, rather than within intra-ocean, exploring an Andean-like setting in the NCC at that time.

Palaeoarchaean TTGs of the Pilbara and Kaapvaal cratons compared; an early Vaalbara supercraton evaluated

AbstractThe continental crust that dominates Earth’s oldest cratons comprises Eoarchaean to Palaeoarchaean (4.0 to 3.2 Ga) felsic intrusive rocks of the tonalite-trondhjemite-granodiorite (TTG) series. These are found either within high-grade gneiss terranes, which represent Archaean mid-continental crust, or low-grade granite-greenstone belts, which represent relic Archaean upper continental crust. The Palaeoarchaean East Pilbara Terrane (EPT), Pilbara Craton, Western Australia, and the Barberton Granite-Greenstone Belt (BGGB), Kaapvaal Craton, southern Africa, are two of the best exposed granite-greenstone belts. Their striking geological similarities has led to the postulated existence of Vaalbara, a Neoarchaean-Palaeoproterozoic supercraton. Although their respective TTG domes have been compared in terms of a common petrogenetic origin reflecting a volcanic plateau setting, there are important differences in their age, geochemistry, and isotopic profiles. We present new zircon Hf isotope data from five granite domes of the EPT and compare the geochemical and isotopic record of the Palaeoarchaean TTGs from both cratons. Rare >3.5 Ga EPT evolved rocks have juvenile εHf(t) requiring a chondritic source. In contrast, younger TTG domes developed via 3.5 to 3.4 and 3.3 to 3.2 Ga magmatic supersuites with a greater range of εHf(t) towards more depleted and enriched values, trace element signatures requiring an enriched source, and xenocrystic zircons that reflects a mixed source to the TTGs, which variously assimilates packages of older felsic crust and a more juvenile mafic source. EPT TTG domes are composite and record multiple pulses of magmatism. In comparison, BGGB TTGs are less geochemically enriched than those of the EPT and have different age profiles, hosting coeval magmatic units. Hafnium isotopes suggest a predominantly juvenile source to 3.2 Ga northern Barberton TTGs, limited assimilation of older evolved crust in 3.4 Ga southern Barberton TTGs, but significant assimilation of older (Hadean-Eoarchaean) crust in the ca. 3.6 Ga TTGs of the Ancient Gneiss Complex. The foundation of the EPT is younger than that for the oldest components of the Eastern Kaapvaal. Although the broader prevailing Palaeoarchaean geologic framework in which these two cratons formed may reflect similar a geodynamic regime, the superficial similarities in dome structures and stratigraphy of both cratonic terranes is not reflected in their geochemical and age profiles. Both the similarities and the differences between the crustal histories of the two cratons highlights that they are formed from distinct terranes with different ages and individual evolutionary histories. Vaalbara sensu lato represents typical Palaeoarchaean cratonic crust, not in the sense of a single homogeneous craton, but one as diverse as the continents are today.

Characterisation and geodynamic setting of the 1 Ga granitoids of the Karagwe-Ankole belt (KAB), Rwanda

The Karagwe-Ankole belt (KAB) is a poorly understood deformation belt in the Great Lakes area of Central Africa, with several hypotheses put forward to explain the emplacement of the various generations of granites. The youngest generation of granites (G4; 1000 Ma) are spatially associated with Sn-W-Ta ore deposits, and are considered as the parental granites. The paragenesis of these mineral resources have been extensively scrutinised, but the geodynamic context and characterisation of these parental granites has been neglected. Tectonic models for the formation of this plutonic suite range from an intra-cratonic to full-scale continent–continent collision setting.In this study, the G4 intrusive suite of the KAB granites, are characterised based on whole-rock geochemical analysis and U-Pb zircon dating.Two main populations of U-Pb zircon ages are identified in the G4 granites at 1370–1420 Ma and 950–1020 Ma. The older population is attributed to inheritance of the bimodal magmatic Kibaran event, while the younger population is ascribed to the new crystallisation. G4 granites contain xenocrystic zircon, indicating inheritance of older granites or basement rocks. G4 granites are, however, much more differentiated compared to older Kibaran granites, containing higher K and Rb and lower Ca, Sr and Ba concentrations, which implicates recycling of older crustal material. The variable geochemistry, as well as the different proportions of xenocrystic zircons in the different granite bodies, suggest melting was highly localised and derived from a highly heterogeneous source region, for which an intracontinental setting is favoured. The Th/U ratios of the parental granites are lower than 0.1, which would suggest a metamorphic origin of the granites. Together with the low luminescence signal of these zircons an anatectic origin of the melt is considered, implying that the parental granites could be considered more like migmatites rather than actual granites formed by fractionation out of large magma chambers.

Tectonic juxtaposition of plume and subduction derived magmatic sequences in the Bababudan greenstone terrane, western Dharwar Craton, India: Constraining crustal accretion processes in a Neoarchean subduction-collision orogeny

Archean greenstone belts in the Western Dharwar Craton preserve important clues on Early Earth geodynamics. Here we investigate the ultramafic-mafic metavolcanic rock sequences from the Santaveri Formation of Bababudan greenstone terrane to address the tectonic evolution, mantle dynamics and crustal growth processes of western Dharwar Craton. The ultramafic volcanic rocks are geochemically classified as komatiite-picrite-basanites, whereas the mafic volcanic rocks correspond to magnesian andesite -Niobium-enriched basaltic andesite association. The pristine mineralogy for these ultramafic volcanic rocks is overprinted by tremolite-actinolite-biotite assemblage marking greenschist facies of metamorphism. The komatiites include Al-undepleted, Al-depleted and Ti-enriched types and share overlapping geochemical characteristics with the picrites. The basanites are distinguished as alkaline ultramafic rocks with total alkali content >3 wt% and MgO > 18 wt%. The coherent association of komatiite-picrite-basanite suggests pervasive plume-lithosphere interaction and complies with polybaric melting of a chemically heterogeneous mantle plume through a subduction-modified sub-continental lithospheric mantle at a rifted cratonic margin. Our results substantiate the role of an Archean mantle refertilized by subduction-driven recycling of Mesoarchean supracrustals concurrent with the onset of subduction event at ≥3.2 Ga. The mafic rocks were derived through melting of a hot oceanic slab and mantle hybridization. The xenocrystic zircon grains from komatiite suggest lower crustal inheritance and reworking of basement gneisses by ascending ultramafic melts. The 3177 Ma magmatic age obtained from these zircon grains indicate a Mesoarchean magmatic event in the genesis of the basement gneisses. The emplacement of komatiite-picrite-basanite association and magnesian andesite -Niobium-enriched basaltic andesite lithologies envisages a post 3.1 Ga Neoarchean crustal growth over Mesoarchean protocontinent of WDC. This crust generation episode can be translated in terms of plume-driven bottom-up and subduction-driven top-down tectono-magmatic processes at continental rift and ocean-continent convergence set-up respectively. Geochemical characteristics and petrogenetic aspects suggest that the studied volcanic supracrustals of Bababudan greenstone terrane encapsulate a Neoarchean subduction-collision orogenic system comprising plume-derived komatiite-picrite-basanite and subduction-derived magnesian andesite -niobium-enriched basaltic andesite magmatic sequences tectonically juxtaposed during Neoarchaen (~2.7 Ga) subduction-accretion-collision event.

Protracted Storage for Calc-Alkaline Andesitic Magma in Magma Chambers: Perspective from the Nageng Andesite, East Kunlun Orogen, NW China

Calc-alkaline andesitic rocks are a major product of subduction-related magmatism at convergent margins. Where these melts are originated, how long they are stored in the magma chambers, and how they evolved is still a matter of debate. In this study, we present new data of whole-rock elemental and Sr-Nd-Pb isotope compositions, and zircon U-Pb-Th isotopes and trace element contents of Nageng (basaltic-)andesites in the East Kunlun Orogen (NW China). The similar age and whole-rock elemental and Sr-Nd-Pb isotope contents suggest that the Nageng andesite and basaltic andesite are co-magmatic. Their low initial 87Sr/86Sr (0.7084–0.7086) but negative εNd(t) values (−10.61 to −9.49) are consistent with a magma source from the juvenile mafic lower crust, possibly related to the mantle wedge with recycled sediment input. The U-Pb age gap between the zircon core (ca. 248 Ma) and rim (ca. 240 Ma) reveals a protracted magma storage (~8 Myr) prior to the volcanic eruption. When compared to the zircon rims, the zircon cores have higher Ti content and Zr/Hf and Nb/Ta ratios, but lower Hf content and light/heavy rare earth element ratios, which suggests that the parental magma was hotter and less evolved than the basaltic andesite. The plagioclase accumulation likely resulted in Al2O3-enrichment and Fe-depletion, forming the calc-alkaline signature of the Nageng (basaltic-)andesites. The magma temperature, as indicated by the zircon saturation and Ti-in-zircon thermometry, remained low (725–828 °C), and allowed for the magma chamber to survive over ~8 Myr. The decreasing εHf(t) values from zircon core (avg. 0.21, range: −1.28 to 1.32) to rim (avg. −3.68, range: −7.30 to −1.13), together with the presence of some very old xenocrystic zircons (268–856 Ma), suggest that the magma chamber had undergone extensive crustal contamination.