Presence Of Zircon Xenocrysts Research Articles

Zircon xenocryst evidence for crustal recycling at the Mid-Atlantic Ridge

The presence of zircon xenocrysts in Mid Atlantic Ridge (MAR) rocks poses essential questions concerning the path they have followed from their sources, how they have survived transport through the mantle, and why they are much more abundant than in any other ocean. Sixteen out of twenty-five seafloor samples collected between S 21° to N 42° contained xenocrysts, here defined as those grains that coexist with a significantly younger population or are significantly older than the band of the Atlantic seafloor age from which they were collected. The xenocrysts are of two types, young and old. Hf and O isotopes and trace elements indicate that the younger crystals (3.1 Ma to 25.6 Ma) are oceanic, derived from previous MAR rocks, whereas the older crystals (190 Ma to 3.2 Ga) are continental, derived from crust slivers located at the MAR or immediate vicinity. The high ∂18O (~7‰ to 11‰) of many older xenocrysts indicates ultrafast recycling through the mantle; otherwise, the fast diffusivity of oxygen would have driven the xenocrysts ∂18O to that of their mantle hosts, ∂18O ≈ 5.3‰. The incorporation of xenocrysts into MAR magmas mainly occurs after ridge jumps, when hot mafic magma pervades a new crustal section scavenging xenoliths, or locally melts felsic wall-rocks and mixes with the resulting zircon-laden low-T magmas. In either case, the fusible component of the xenocrysts source contaminates the upwelling mafic magma. When the source was oceanic, the resulting chemical and isotopic signatures of the contaminated magma would remain MORB-like, albeit enriched in incompatible elements. When the source was continental, the resulting magma would reflect a perceptible continental influence. Crustal contamination at the ridge explains the abundance of zircon xenocrysts and the distinctive chemistry of the MAR magmas compared with other MORBs. These features are ultimately related to the Atlantic slow-spreading because it promotes ridge jumping and, hence, active crustal recycling.

Bimodal zircon ages from Natash volcanics (southeast Egypt) and the link between eruption mechanisms and Late Cretaceous tectonics

Trachyte plugs from the Natash volcanic field have been precisely dated using in situ U-Pb geochronology on zircon grains at ~93 Ma. Available zircon ages thus suggest that the overall volcanic history at Natash was probably shorter than previously estimated based on Rb-Sr and K-Ar ages, which are more sensitive to element mobility. Basaltic flows and trachyte plugs were produced by a largely synchronous, Upper Cretaceous volcanic activity that occurred in the Natash area at the onset of the extensional fracturing that preceded and accompanied the doming of the Afro-Arabian Shield. Extraction and eruption of volumetrically dominant basalts from deeper lithospheric levels were ruled by major NW-SE strike-slip faults, whereas both major and subordinate fault systems activated by successive tectonic pulses favoured the ascent of more evolved melts (i.e. trachytes and rhyolites) from shallow-level magma chambers. The study revealed also the presence of zircon xenocrysts in some trachyte plugs associated or not to magmatic zircon. These xenocrysts have pre-Cambrian ages (~681 Ma) matching those of zircons from basement rocks in the area (~700 Ma) and were most probably incorporated during the emplacement of trachyte plugs at shallow depths. The strictly bimodal ages recorded by zircons suggest a very large time gap (> 500 Ma) between the end of the Pan-African Orogeny and the onset of new tectono-magmatic activity in the Natash area.

Zircon U-Pb geochronology, Hf isotopes and geochemistry of intrusive rocks in the Simorgh prospecting area, Lut Block, eastern Iran: petrogenesis and geological implications

The Simorgh prospecting area is located in the central part of the Lut Block in eastern Iran. The Lut Block, the eastern part of the Central Iranian Microcontinent (CIM), has a complex tectonic evolution and is characterized by extensive magmatic activities with a range of geochemical signatures. The Simorgh intrusions have features typical of calc-alkaline to high-K calc-alkaline rocks, metaluminous to slightly peraluminous and formed in a volcanic arc setting. Mineralization in the area is believed to be related to these intrusions. To better understand the petrogenesis of these intrusions, we report zircon U-Pb dating and Hf isotopes and whole rock geochemistry of two granites and one diorite. Age dating using Zircon U-Pb method on pyroxene diorite porphyry stock and two granite porphyry dikes revealed 24.85 ± 0.51 Ma, 25.37 ± 0.56 Ma and 25.94 ± 0.76 Ma ages respectively (late Oligocene, Chattian). Field observations and U-Pb dating indicate that there are at least two stages of emplacement: pre-mineralization intrusions (diorite porphyry) and syn-mineralization intrusions (granite porphyry, granodiorite porphyry and pyroxene diorite porphyry). Mantle-normalized, trace-element spider diagrams display enrichment in large ion lithophile elements (LILE) and light rare earth elements ((La/Yb)N = 9.26–14.48), and depletion in high field strength elements (HFSE) and heavy rare earth elements, as well as negative Eu anomalies (Eu/Eu* = 0.49–0.91). Zircon Hf isotope data from the Simorgh intrusive rocks show largely positive eHf(t) (average = 3.6 ± 2.0). Hf model ages are ranging from 720 to 1320 Ma, indicating its derivation from a relatively juvenile source. The presence of zircon xenocrysts may prove its contamination by older continental crust (Siderian).

Petrogenesis of volcanic rocks that host the world-class Ag[sbnd]Pb Navidad District, North Patagonian Massif: Comparison with the Jurassic Chon Aike Volcanic Province of Patagonia, Argentina

We present the first study of the volcanic rocks of the Cañadón Asfalto Formation that host the Navidad world-class Ag+Pb epithermal district located in the North Patagonian Massif, Patagonia, Argentina. These volcanic and sedimentary rocks were deposited in a lacustrine environment during an extensional tectonic regime associated with the breakup of Gondwana and represent the mafic to intermediate counterparts of the mainly silicic Jurassic Chon Aike Volcanic Province. Lava flows surrounded by autobrecciated carapace were extruded in subaerial conditions, whereas hyaloclastite and peperite facies suggest contemporaneous subaqueous volcanism and sedimentation. LA-ICPMS UPb ages of zircon crystals from the volcanic units yielded Middle Jurassic ages of 173.9±1.9Ma and 170.8±3Ma. In the Navidad district, volcanic rocks of the Cañadón Asfalto Formation show arc-like signatures including high-K basaltic-andesite to high-K dacite compositions, Rb, Ba and Th enrichment relative to the less mobile HFS elements (Nb, Ta), enrichment in light rare earth elements (LREE), YTi depletion, and high Zr contents. These characteristics could be explained by assimilation of crustal rocks in the Jurassic magmas, which is also supported by the presence of zircon xenocrysts with Permian and Middle-Upper Triassic ages (281.3Ma, 246.5, 218.1, and 201.3Ma) and quartz xenocrysts recognized in these volcanic units. Furthermore, Sr and Nd isotope compositions suggest a contribution of crustal components in these Middle Jurassic magmas. High-K basaltic andesite has initial 87Sr/86Sr ratios of 0.70416–0.70658 and ξNd(t) values of -5.3 and -4. High-K dacite and andesite have initial 87Sr/86Sr compositions of 0.70584–0.70601 and ξNd(t) values of -4,1 and -3,2. The range of Pb isotope values (206Pb/204Pb=18.28–18.37, 207Pb/204Pb=15.61–15.62, and 208Pb/204Pb=38.26–38.43) of Navidad volcanic rocks and ore minerals suggest mixing Pb sources with contributions of mantle and crust. 206Pb/204Pb isotopic ratios of Jurassic volcanic rocks of the Chon Aike Volcanic Province and sulfides of associated epithermal deposits increase with time from the volcanic event V1 (188–178Ma) to volcanic events V2 (172–162Ma) and V3 (157–153Ma), reflecting variations in the radiogenic Pb source as volcanism was migrating towards the Proto Pacific margin of Gondwana.

Zircon U–Pb geochronology, Hf isotopes and geochemistry of intrusive rocks in the Gazu copper deposit, Iran: Petrogenesis and geological implications

The Gazu Cu deposit is located at the boundary between the Lut and Tabas Blocks in Central Iranian Microcontinent. The intrusions in the Gazu area are characterized by SiO2 contents ranging from 52.6 to 66.4wt.%, K2O from 1.98 to 3.64wt.%, and Na2O from 2.5 to 5.2wt.%. The intrusions have features typical of high-K calc-alkaline rocks, are metaluminous to weakly peraluminous, and formed in a volcanic arc setting. Zircon U–Pb age data for hornblende monzonite porphyry, hornblende monzodiorite porphyry, pyroxene monzonite, hornblende granite, hornblende quartz monzonite porphyry, and hornblende diorite porphyry are 68.1±1.7Ma, 67.74±0.72Ma, 67.41±0.94Ma, 65.9±3.7Ma, 65.1±1.2Ma, and 60.2±1.2Ma, respectively. Copper mineralization is related to the hornblende granite and hornblende quartz monzonite porphyry. Field observations and the U–Pb dating indicated three stages for intrusive rock emplacement: pre-mineralization intrusions (hornblende monzonite porphyry, hornblende monzodiorite porphyry, and pyroxene monzonite), syn-mineralization intrusions (hornblende granite and hornblende quartz monzonite porphyry), and post-mineralization intrusions (hornblende diorite porphyry). Mantle-normalized, trace-element spider diagrams display enrichment in large ion lithophile elements, such as Rb, Ba, K, and Cs, and depletion in high field strength elements, e.g., Nb, Ti, Zr, Y, and heavy rare earth elements, with light rare earth element enrichment ((La/Yb)N=6.4–13.4) and a weak negative Eu anomaly (Eu/Eu*=0.87–1.01). Zircon Hf isotope data indicate that the Gazu intrusive rocks are characterized by low positive εHf(t) values, with an average of 4.23, and show young Hf model ages (789–188Ma), indicating derivation from juvenile crust. Relatively low positive εHf(t) values, the wide range for εHf(t), and the presence of zircon xenocrysts can be explained by involvement of a mantle component that mixed with older continental crust (Cadomian).

Post-Collisional Potassic and Ultrapotassic Magmatism in SW Tibet: Geochemical and Sr-Nd-Pb-O Isotopic Constraints for Mantle Source Characteristics and Petrogenesis

Major and trace element, Sr–Nd–Pb–O isotope and mineral INTRODUCTION chemical data are presented for post-collisional ultrapotassic, silicic The high plateau of Tibet, the Himalaya and the Kapotassic and high-K calc-alkaline volcanic rocks from SW Tibet, rakoram Ranges are the product of the continuing colwith Ar/Ar ages in the range 17–25 Ma. The ultrapotassic lision of India with the southern margin of Eurasia during lavas contain mantle xenocrysts (olivine ± rutile/armalcolite). the past 50 my (e.g. Klootwijk & Radhakrishnamurty, Their initial Sr/Sr (0·7172–0·7220) and Nd/Nd 1981). Despite numerous studies of the region, the pro(0·51190–0·51200) ratios suggest that they originated from cesses responsible for the formation of the plateau and lithospheric sources enriched in Rb with low Sm/Nd ratios. Initial the surrounding mountain ranges are still controversial. Pb isotopic compositions ( Pb/Pb = 18·41–18·51; Pb/ Three hypotheses have been suggested to explain the Pb = 15·68–15·72; Pb/Pb = 39·42–39·60) and crustal thickening and altitude of Tibet. In the first geochemical features such as high Th/Ta, low Sr/Nd, low Ce/ (e.g. Argand, 1924; Powell & Conaghan, 1975; Ni & Pb and negative Eu anomalies are consistent with a recycled crustal Baranzagi, 1983), virtually the entire Tibetan plateau is component. Nd depleted mantle model ages range from 1·3 to 1·9 underthrust by Indian lithosphere. In the second (e.g. Ga, whereas Pb model ages record an Archaean event, suggesting Zhao & Morgan, 1985, 1987), thickening is by inflow of that the source had a complex multi-stage evolution. In contrast, material from India. In the third, the crust of Tibet is the high-K calc-alkaline dacites and rhyolites have less enriched interpreted as having thickened by shortening (e.g. Dewey initial Sr (0·7091–0·7097) and Nd (0·51213–0·51225) iso& Burke, 1973). Quantitative analysis of the third model, topic compositions. The presence of zircon xenocrysts with a Pbassuming a vertically average lithosphere rheology, preevaporation age of 471 ± 33 Ma documents the importance of dicts lithospheric thickening and additional uplift of the crustal anatexis in their genesis. Processes responsible for the partial entire plateau by convective thinning of the lower conmelting of metasomatized lithospheric mantle and post-collisional tinental lithosphere (e.g. England & Houseman, 1988, magmatism in the Lhasa block could be a consequence of (1) 1989). Molnar et al. (1993) argued that a sudden increase convective removal of the lower lithosphere or (2) of slab breakoff. in uplift occurred at ~8 Ma and that the start of post-

Age and Petrogenesis of Two Late Archean Magmatic Suites, Northwestern Superior Province, Canada: Zircon U-Pb and Lu-Hf Isotopic Relations

The paper presents U–Pb ages for zircon, titanite, and monazite, and Hf isotopic data for zircon, from the rocks of two magmatic suites occurring mostly in the Archean Uchi Subprovince and partly in the neighbouring Berens River and English River subprovinces of the northwestern Superior Province, Ontario. These data, together with observations on the morphologies and, where evident, the inheritance of the zircon crystals, constrain the nature of the sources of the magmas and provide a record of various crustal processes in their evolution. The older of the two magmatic suites formed at 2744–2740 Ma along segments of a common arc system. The suite consists of (1) several trondhjemitic to granodioritic plutons, with εHf values of 6⋅1, intruded into older crust and possibly formed from magma produced by partial melting of subducted, juvenile oceanic crust; (2) an assemblage of dacitic pyroclastic volcanic rocks, with εHf values of 3⋅2–4⋅0, associated with tholeiitic basalts and probably derived from magma melted from arc mantle; and (3) a bimodal assemblage of tholeiitic basalts, rhyolites, and porphyries, also with εHf values of 6⋅1, associated with a volcanogenic massive sulphide deposit and apparently formed by differentiation of mantle-derived basaltic melts at shallow levels in an extensional back-arc setting. The second magmatic suite, formed between 2702 and 2693 Ma, comprises late orogenic plutons and batholiths of dioritic to dominantly granodioritic composition. The characteristics of these intrusions are consistent with a process combining melting of a metasomatized mantle source and subsequent fractional crystallization of the derived magmas at shallow depths. However, most of the studied occurrences show evidence of crustal contamination through various combinations of assimilation of lower-crustal material, assimilation of underthrust sedimentary rocks, and contamination by wall rock materials during the latest stages in the emplacement of the plutons. The involvement of crustal material is indicated by the presence of zircon xenocrysts and by εHf values ranging from 1⋅4 to 4⋅4. Only one intrusion, with an εHf value of 5⋅0 and no xenocrystic zircon, appears to have escaped widespread contamination, perhaps because the ascent of its magma was facilitated by a crustal-scale fracture system.

A comparative study of the geochemical and isotopic systematics of late archaean flood basalts from the pilbara and kaapvaal cratons

Geochemical and SmNd isotopic data are reported for igneous rocks of the 2.76 to 2.69 Ga Fortescue Group of the Pilbara Craton, Western Australia, and the ∼ 2.70 Ga Ventersdorp Supergroup of the Kaapvaal Craton, southern Africa. The metamorphic history of the Fortescue Group has also been investigated using whole-rock RbSr and PbPb isochron techniques. Igneous of both sequences have mixed tholeiitic and calc-alkaline affinities and show immobile element correlations indicating derivation from chemically heterogeneous, LREE-enriched sources. Both sequences have similar negative ϵ Nd values (between −1.5 and −4.4 for the Fortescue Group and 0 and −3.4 for the Ventersdorp Supergroup) which are not correlated with Mg/(Mg + Fe 2+). The negative ϵ Nd values of the Fortescue Group rocks probably do not result from crustal assimilation within crustal-level magma chambers during or following differentiation as the Fortescue Group felsic differentiates have ϵ Nd values within the ranges of values found in associated mafic rocks and zircon xenocrysts are absent in the felsic differentiates of the Fortescue Group. Instead, these results are consistent with either extensive crustal contamination of primitive (komatiitic or picritic) Fortescue Group parent magmas prior to their differentiation or derivation from negative ϵ Nd (or enriched) mantle sources. The presence of zircon xenocrysts in some Ventersdorp samples provides compelling evidence of some crustal contamination, although correlations between immobile element ratios (such as La/Yb, Ti/Zr, Ti/Sc and V/Zr) and ϵ Nd indicate derivation from chemically heterogeneous, enriched mantle sources. A whole-rock SmNd isochron date of 3308 ± 138 Ma, ∼ 600 Ma older than the time of eruption indicated by UPb zircon data and within, ∼ 300 Ma of the formation of the Kaapvaal granite-greenstone terrane, may date an SmNd fractionation event within the subcontinental lithospheric mantle sources of the Ventersdorp rocks. RbSr and PbPb isochrons obtained for several flows from the Fortescue Group register hydrothermal events occurring between ∼ 2.45 and 2.0 Ga and probably associated with burial metamorphism. Similarities in the field geology, geochemistry and isotopic characteristics of the Fortescue and Ventersdorp sequences and Phanerozoic examples of continental flood basalt volcanism suggest a common mode of origin, possibly involving the interaction of asthenospheric mantle plumes with subcontinental lithospheric mantle sources which have been modified by subduction processes.