Mid- to lower-crustal architecture of the northern Lachlan and southern Thomson orogens: evidence from O–Hf isotopes

Abstract

The nature of the substrate below the northern Lachlan Orogen and the southern Thomson Orogen is poorly understood. We investigate the nature of the mid- to lower-crust using O and Lu–Hf isotope analyses of zircons from magmatic rocks that intrude these regions, and focus on the 440–410 Ma time window to minimise temporal effects while focussing on spatial differences. Over the entire region, weighted mean δ¹⁸O values range from 5.5 to 9.8‰ (relative to VSMOW, Vienna Standard Mean Oceanic Water), and weighted mean ?Hf_t range from ?8.8 to +8.5. In the northern Lachlan Orogen and much of the southern Thomson Orogen, magmatic rocks with unradiogenic ?Hf_t (～?7 to ?4) and elevated δ¹⁸O values (～9 to 10‰) reflect a supracrustal source component that may be common to both orogens. Magmatic rocks intruding the Warratta Group in the western part of the Thomson Orogen also have unradiogenic ?Hf_t (～?9 to ?6) but more subdued δ¹⁸O values (～7‰), indicating a distinct supracrustal source component in this region. Some regions record radiogenic ?Hf and mantle-like δ¹⁸O values, indicative of either a contribution from arc-derived rocks or a direct mantle input. In the northeast Lachlan Orogen Hermidale Terrane, magmatic rocks record mixing of the supracrustal source component with input from a infracrustal or mantle source component (?Hf_t as high as +8.5, δ¹⁸O values as low as 5.5‰), possibly of Macquarie Arc affinity. Samples in the west-southwestern Thomson Orogen also record some evidence of radiogenic input (?Hf_t as high as ?0.5, δ¹⁸O values as low as 6.4‰), possibly from the Mount Wright Arc of the Koonenberry Belt. Overall, our results demonstrate a strong spatial control on isotopic compositions. We find no isotopic differences between the bulk of the Lachlan Orogen and the bulk of the Thomson Orogen, and some indication of similarities between the two.     

Age and tectonic significance of the Louth Volcanics: implications for the evolution of the Tasmanides of eastern Australia

Abstract

Re-evaluation of geochemical and geophysical datasets, and analysis of magmatic and detrital zircons from drill-core samples extracted from the Louth region of the southern Thomson Orogen (STO), augmented by limited field samples, has shown that two temporally and compositionally distinct igneous groups exist. The older Lower Devonian, calc-alkaline group corresponds to complexly folded, high-intensity curvilinear magnetic anomalies in the Louth region (Louth Volcanics) and are probable equivalents to Lower Devonian volcanics in the northern Lachlan Orogen. A younger Permo-Triassic alkaline assemblage forms part of an E–W corridor of diatremes that appears to relate to focussed lithospheric extension associated with the later stages of the Hunter–Bowen Orogeny in the New England Orogen. The alkaline group includes gabbros previously considered as Neoproterozoic, but all magmatic rocks, including alkaline basalts, contain an unusual number of xenocrystic zircons. The age spectra of the xenocrystic zircons mimic detrital zircons from Cobar Basin sedimentary rocks and/or underlying Ordovician turbidites, suggesting incorporation of upper crustal zircons into the alkaline basaltic magmas. A distinct difference of detrital zircon age spectra from central Thomson Orogen metasediments indicates the STO metasediments have greater affinities to the Lachlan Orogen, but both orogens probably began in the Early Ordovician during widespread backarc extension and deposition of turbidites in the Tasmanides. A surprising result is that Ordovician, Devonian and Permo-Triassic basaltic rocks from the STO and elsewhere in the Tasmanides, all yield the same Nd-model ages of ca 960–830 Ma, suggesting that Neoproterozoic subcontinental lithospheric mantle persisted throughout the evolution of the Tasmanide orogenic system.     

The age and tectonic significance of the Warraweena Volcanics and related rocks,southern Thomson Orogen

Abstract

Magmatic-textured zircon from medium- to high-K calc-alkaline Warraweena Volcanics (WV) in two drill holes have yielded concordant U–Pb dates of 417?±?3.5 and 414?±?4.0?Ma and are interpreted as maximum emplacement ages. The Warraweena volcanics were previously considered to be either Neoproterozoic or Macquarie arc equivalents. Whole-rock εNd_t values of these volcanics are +4.5 and +4.8. Along strike of the drill holes, Devonian zircon U–Pb ages (411?±?5.5?Ma) were obtained from coherent S-type rhyolite flows that have highly negative εNd_t values (–7.9 and –7.8). These are a component of the Oxley volcanics. The ages of the Warraweena and Oxley volcanics are identical within uncertainty.

The Oxley volcanics (OV) are interbedded with predominantly fine- to medium-grained metasedimentary and so imply a Lower Devonian deposition age for these host rocks. Based on their geophysical characteristics, the metasediments are widely distributed. These metasedimentary rocks yield a wide range of maximum depositional ages, from Early Devonian to earliest Ordovician–latest Cambrian, similar to the Cobar Basin. The absence of complex fabric development typical of Ordovician supracrustal rocks in the region, and conformity with the OV where observable suggest the widespread sedimentation was synchronous with rift-related volcanism in the Early Devonian.

Regionally, the WV is temporally, geochemically and isotopically (εNd values) similar to the calc-alkaline Louth Volcanics located over 100?km to the southwest of the WV. Louth Volcanics define a complexly folded belt in geophysical data. Other potentially correlative Early Devonian igneous rocks occur in the nearby Cobar Superbasin and elsewhere in the eastern Lachlan Orogen and are considered to represent the products of a post-orogenic, nascent continental back-arc rift system.     

Crustal and thermal structure of the Thomson Orogen: constraints from the geochemistry,zircon U–Pb age,and Hf and O isotopes of subsurface granitic rocks

Abstract

The origin of elevated geothermal gradients in the subsurface Thomson Orogen and the nature of the crustal basement beneath it, whether oceanic or continental, remain enigmatic. Previous studies have demonstrated that a higher crustal radiogenic input is required to explain these anomalous thermal gradients. In this study, we have investigated the nature and age of this crustal input by undertaking geochemical, geochronological and Hf and O isotope analyses of buried granitic rocks as well as evaluating the heat-producing potential of metasedimentary rocks. The mineralogy, composition and Neoproterozoic/Cambrian to Devonian age of the low to moderate heat-producing I- and S-type granitic rocks strongly contrast with the Carboniferous A-type high-heat-producing granites of the Big Lake Suite, which have been suggested to be an important contributor to the elevated geothermal gradients, near the southwest corner of the Thomson Orogen. These differences suggest the Big Lake Suite rocks do not extend into the Queensland part of the temperature anomaly. Heat production of the metasedimentary rocks is also low to moderate. Based on Hf isotope compositions of zircons characterised by mantle-like oxygen signature (?_Hf(t) = –12 to +2), we propose the temperature anomaly results from the occurrence of Mesoproterozoic and/or Paleoproterozoic high-heat-producing rocks beneath the Thomson Orogen. Precambrian crust, therefore, lies well east of the Tasman line. The results do not support a Neoproterozoic to Cambrian oceanic crust, as previously suggested, but instead point to a continental substrate for the Thomson Orogen. Hf isotopes indicate an overall trend towards more isotopically juvenile compositions with a progressive reduction in the contribution of older crustal sources to granitic magmas towards the present time. Different Hf isotopic signatures for the Lachlan (?_Hf(t) = –13 to +15), Thomson (?_Hf(t) = –14 to +5) and Delamerian (?_Hf(t) = –7 to +4) orogens highlight lateral variations in the age structures of crustal basement beneath these orogens.     

Constraints from zircon U–Pb ages, O and Hf isotopic compositions on the origin of Neoproterozoic peraluminous granitoids from the Jiangnan Fold Belt, South China

Zircon textures and micro-chemical compositions precisely record the origin and petrogenesis of granitoids, which are crucial for evaluating crustal growth and reworking, thermal and geodynamic evolution. Zircons in peraluminous granitoids from the three largest 820 Ma complexes (Guibei, Yueyang and Jiuling) in the Jiangnan Fold Belt in South China are used to constrain their sources and petrogenetic processes. Zircons in the Guibei granitoids have complex internal structures. Nearly all magmatic and inherited zircons have similar εHf (?6.8 to +5.6) and δ¹⁸O values (8.8–11.6 ‰) and dominantly lie between εHf evolution vectors for a crust created between 1.7 and 2.1 Ga, suggesting that the Guibei granitoids were produced by partial melting of recycled heterogeneous supracrustal material. However, the Yueyang granitoids contain zircons with high εHf (?0.5 to +9.7) and relatively low δ¹⁸O values (5.9–8.4 ‰) and two-stage model ages of 1.1–1.8 Ga, and thus may have been formed by melting of mafic rocks from the lower crust. The Jiuling granitoids and their enclaves contain more complex zircons with more variable εHf (?7.2 to +9.7) and δ¹⁸O values (7.0–10.6 ‰), and lie along the mixing trend between the above-proposed infracrustal and supracrustal granitoids. Therefore, the Neoproterozoic peraluminous granitoids in the Jiangnan Fold Belt were produced by melting and mixing of continental crust. Compared with extremely low (≤4 ‰) and negative δ¹⁸O values of Neoproterozoic igneous zircons formed in its northern active continental margin, the high δ¹⁸O peraluminous granitoids in the southeastern Yangtze Block are considered to have been formed by melting of hydrothermally unaltered continental crust triggered by asthenosphere upwelling in the Nanhua back-arc basin.     

Provenance and deformation history of the Eastern Thomson Orogen and implications for the Paleozoic evolution of the Tasmanides (Eastern Australia)

Abstract

Cambrian deformation associated with the Delamerian Orogeny is most evident in the Delamerian Orogen (southwestern Tasmanides) but has also been documented in the Thomson Orogen (northern Tasmanides). The tectonic evolution of the Thomson Orogen in the context of the Delamerian Orogeny is poorly understood. In particular, tectonostratigraphic relationships between the different parts of the Thomson Orogen (Anakie Inlier, Nebine Ridge, and southern Thomson Orogen) are still unclear. New detrital zircon data from the Nebine Ridge revealed an age spectrum that is consistent with published geochronological data from the Anakie Inlier. These results, in conjunction with petrographic observations and the interpretation of geophysical data, suggest that along the eastern part of the Thomson Orogen, the?～?NNE-trending Nebine Ridge represents the southward continuation of the?～?N–S-trending Anakie Inlier. New detrital zircon geochronological data are also presented for metasedimentary rocks from both sides of the Thomson–Lachlan boundary. The results constrain the maximum age of deposition (Ordovician–Devonian), and show that both sides of the Thomson–Lachlan boundary received detritus from a similar provenance. This might suggest that the Thomson–Lachlan boundary did not play a major role as a crustal-scale boundary prior to the Devonian. We speculate that transpressional deformation along this?～?E–W boundary, during the Early Devonian, was responsible for disrupting the original belt that connected the Delamerian Orogen (Koonenberry Belt) with the eastern Thomson Orogen (Nebine Ridge and Anakie Inlier).

1. Highlights

2. The Nebine Ridge is the southward continuation of the Anakie Inlier.

3. The Anakie Inlier and Nebine Ridge represent a northern segment of the Cambrian Delamerian–Thomson Belt.

4. ～E–W-trending crustal-scale structures at the southern Thomson Orogen were active during Devonian.

     

Geochronology and geochemistry of the Devonian Gumbardo Formation (Adavale Basin): evidence for cratonisation of the Central Thomson Orogen by the Early Devonian

Abstract

The Devonian subsurface Adavale Basin occupies a central position in the Paleozoic central Thomson Orogen of eastern Australia and records its tectonic setting during this time interval. Here, we have focussed on the basal volcanics of the Gumbardo Formation to clarify the tectonic setting of the basin. The approach has been to undertake stratigraphic logging, LA-ICP-MS U–Pb zircon geochronology and whole-rock geochemical analysis. The data indicate that basin initiation was rapid occurring at ca 401?Ma. The volcanic rocks are dominated by K-feldspar phyric rhyodacitic ignimbrites. The whole-rock geochemical data indicate little evidence for extensive fractional crystallisation, with the volcanic suite resembling the composition of the upper continental crust and exhibiting transitional I- to A-type tectonomagmatic affinities. One new U–Pb zircon age revealed an Early Ordovician emplacement age for a volcanic rock previously interpreted to be part of the Early Devonian Gumbardo Formation, and older basement age is consistent with seismic interpretations of uplifted basement in this region of the western Adavale Basin. Five ignimbrites dated from different stratigraphic levels within the formation yield similar emplacement ages with a pooled weighted age of 398.2?±?1.9?Ma (mean square weighted deviation?=?0.94, n?=?93). Significant zircon inheritance in the volcanic rocks records reworking of Ordovician and Silurian silicic igneous basement from the Thomson Orogen and provides insight into the crustal make-up of the Thomson Orogen. Collectively, the new data presented here suggest the Adavale Basin is a cover-type basin that developed on a stabilised Thomson Orogen after the major Bindian deformation event in the late Silurian.     

Zircon ages and Hf isotopic compositions of plutonic rocks from the Central Tianshan (Xinjiang,northwest China) and their significance for early to mid-Palaeozoic crustal evolution

We present new zircon ages and Hf-in-zircon isotopic data for plutonic rocks and review the crustal evolution of the Chinese Central Tianshan (Xinjiang, northwest China) in the early to mid-Palaeozoic. The Early Ordovician (ca. 475–473 Ma) granitoid rocks have zircon ε_Hf(t) values either positive (+0.3 to +9.5) or negative (?6.0 to ?12.9). This suggests significant addition of juvenile material to, and coeval crustal reworking of, the pre-existing continental crust that is fingerprinted by numerous Precambrian zircon xenocrysts. The Late Ordovician–Silurian (ca. 458–425 Ma) rocks can be assigned to two sub-episodes of magmatism: zircon from rocks of an earlier event (ca. 458–442 Ma) has negative zircon ε_Hf(t) values (?6.3 to ?13.1), indicating a predominantly crustal source; zircon from later events (ca. 434–425 Ma) has positive zircon ε_Hf(t) values (+2.6 to +8.9) that reveal a predominantly juvenile magma source. The Early Devonian (ca. 410–404 Ma) rocks have near-zero zircon ε_Hf(t) values, either slightly negative or positive (?1.4 to +3.5), whereas the Mid-Devonian rocks (ca. 393 Ma) have negative values (?11.2 to ?14.8). The Late Devonian (ca. 368–361 Ma) granites are undeformed and are chemically similar to adakite but have relatively low negative whole-rock ε_Nd(t)values (?2.4 to ?5.3). We interpret the Early Ordovician to Mid-Devonian magmatic event to reflect combined juvenile crustal growth and crustal reworking processes via episodic mafic underplating and mantle–crust interaction. The Late Devonian episode may signify delamination of the over-thickened Chinese Central Tianshan crust.     

New constraints on the origin and evolution of the Thomson Orogen and links with central Australia from isotopic studies of detrital zircons

Interpretation of the Thomson Orogen and its context within the Tasmanides of eastern Australia is hampered by vast areas of deep sedimentary cover which also mask potential relationships between central and eastern Australia. Within covered areas, basement drill cores offer the only direct geological information. This study presents new detrital zircon isotopic data from these drill cores and poorly understood outcropping units to provide new age and provenance information for sedimentary rocks from the Thomson Orogen. Two distinct detrital zircon signatures are revealed. One is dominated by Grenvillian-aged (1300–900 Ma) zircons with a significant peak at ~ 1180 Ma and lesser peak at ~ 1070 Ma. These age peaks, along with Lu–Hf isotopic compositions (median εHf(t) = + 1.5), dominantly mantle-like δ¹⁸O values (median = 5.53‰) and model ages of ~ 1.89 Ga, support a Musgrave Province (central Australia) source. The dominance of Grenvillian-aged material additionally points to deposition during the Petermann Orogeny (570–530 Ma) when the Musgrave Province was uplifted, shedding abundant material to the Centralian Superbasin. Comparable age spectra suggest that parts of the Thomson Orogen were connected to the Centralian Superbasin during this period. We use the term ‘Syn-Petermann’ to describe this signature which is observed in two drill cores adjacent to the North Australian Craton and scattered units in the outcropping Thomson Orogen. The second signature marks a significant provenance shift and is remarkably consistent throughout the Thomson Orogen. Age spectra exhibit dominant peaks at 600–560 Ma, lesser 1300–900 Ma populations and maximum depositional ages of ~ 495 Ma. This pattern is termed the ‘Pacific Gondwana’ detrital zircon signature and is recognised throughout eastern Australia, Antarctica and central Australia. Lu–Hf isotope data for Thomson Orogen rocks with this signature are highly variable with εHf(t) values between ‐ 49 and + 10 and dominantly supracrustal δ¹⁸O values suggesting input from different and more diverse source regions relative to those exhibiting the Syn-Petermann signature.     

Basement geology of the southern Thomson Orogen

Abstract

The diverse geological and geophysical data sets compiled, interrogated and interpreted for the largely undercover southern Thomson Orogen region reveal a Paleozoic terrane dominated by deformed metasedimentary rocks intruded by S- and I-type granites. An interpretive basement geology map and synthesis of geochronological constraints allow definition of several stratigraphic packages. The oldest and most widespread comprises upper Cambrian to Lower Ordovician metasedimentary rocks deposited during the vast extensional Larapinta Event with maximum depositional ages of ca 520 to ca 496 Ma. These units correlate with elements of the northern Thomson Orogen, Warburton Basin and Amadeus Basin. The degree of deformation and metamorphism of these rocks varies across the region. A second major package includes Lower to Middle Devonian volcanic and sedimentary units, some of which correlate with components of the Lachlan Orogen. The region also includes a Middle to Upper Ordovician package of metasedimentary rocks and a Devonian or younger package of intermediate volcaniclastic rocks of restricted extent. Intrusive units range from diatremes and relatively small layered mafic bodies to batholithic-scale suites of granite and granodiorite. S-type and I-type intrusions are both present, and ages range from Ordovician to Triassic, but late Silurian intrusions are the most abundant. Two broad belts of intrusions are recognised. In the east, the Scalby Belt comprises relatively young (Upper Devonian) intrusions, while in the west, the Ella Belt is dominated by intrusions of late Silurian age within a curvilinear, broadly east–west trend. The stratigraphic distributions, characteristics and constraints defined by this interpretive basement mapping provide a basic framework for ongoing research and mineral exploration.     

Stable isotope evidence for magmatic fluid input during large-scale Na–Ca alteration in the Cloncurry Fe oxide Cu–Au district, NW Queensland, Australia

Sodic–calcic alteration is common in mineralized hydrothermal systems, yet the relative importance of igneous vs. basinal fluid sources remains controversial. One of the most extensive volumes of sodic–calcic rocks occurs near Cloncurry, NW Queensland, and was formed by overlapping hydrothermal systems that were active synchronously with emplacement of mid‐crustal batholithic granitoids (c. 1.55–1.50 Ga). Altered rocks contain albite–oligoclase, actinolite, diopside, titanite and magnetite. Alteration was localized by: (A) composite veins and breccias containing crystallized magma intimately intergrown with hydrothermal precipitates; (B) intrusions that host setting A veins and breccias; and (C) extensive breccia and vein systems linked to regional fault systems. Isotope analyses of actinolites in settings A and B indicate calculated δ¹⁸O_H2O (+8.2 to +10.6‰) and variably depleted δD_H2O (?130 to ?54‰) compared with typical magmatic fluids, whereas those from setting C typically indicate calculated δ¹⁸O_H2O (+8.0 to +12.8‰) and δD_H2O (?29 to ?99‰). The lowest δD_H2O values are interpreted as representing residual fluids after significant (> 90%) open‐system magmatic degassing. Overall the stable isotope, field, geochronological and geobarometric data suggest that these sodic–calcic alteration systems were formed by the episodic incursion of magmatic fluids that underwent minor isotopic modification as a result of varying degrees of interaction with country rocks.     

Geochronology and Genesis of the Tiegelongnan Porphyry Cu(Au) Deposit in Tibet: Evidence from U–Pb,Re–Os Dating and Hf,S, and H–O Isotopes

The Tiegelongnan Cu (Au) deposit is the largest copper deposit newly discovered in the Bangong–Nujiang metallogenic belt. The deposit has a clear alteration zoning consisting of, from core to margin, potassic to propylitic, superimposed by phyllic and advanced argillic alteration. The shallow part of the deposit consists of a high sulphidation‐state overprint, mainly comprising disseminated pyrite and Cu–S minerals such as bornite, covellite, digenite, and enargite. At depth porphyry‐type mineralization mainly comprises disseminated chalcopyrite, bornite, pyrite, and a minor vein molybdenite. Mineralization is disseminated and associated with veins contained within the porphyry intrusions and their surrounding rocks. The zircon U–Pb ages of the mineralized diorite porphyry and granodiorite porphyry are 123.1 ± 1.7 Ma (2σ) and 121.5 ± 1.5 Ma (2σ), respectively. The molybdenite Re–Os age is 121.2 ± 1.2 Ma, suggesting that mineralization was closely associated with magmatism. Andesite lava (zircon U–Pb age of 111.7 ± 1.6 Ma, 2σ) overlies the ore‐bodies and is the product of post‐mineralization volcanic activity that played a critical role in preserving the ore‐bodies. Values of ?4.6 ‰ to + 0.8 ‰ δ³⁴S for the metal sulfides (mean ? 1.55 ‰) suggest that S mainly has a deep magmatic source. The H and O isotopic composition is (δD = ?87 ‰ to ?64 ‰; δ¹⁸O_H2O = 5.5 ‰ to 9.0 ‰), indicating that the ore‐forming fluids are mostly magmatic‐hydrothermal, possibly mixed with a small amount of meteoric water. The zircon ε_Hf(t) of the diorite porphyry is 3.7 to 8.3, and the granodiorite porphyry is 1.8 to 7.5. Molybdenite has a high Re from 382.2 × 10^?6 to 1600 × 10^?6. Re and Hf isotope composition show that Tiegelongnan has some mantle source, maybe the juvenile lower crust from crust–mantle mixed source. Metallogenesis of the Tiegelongnan giant porphyry system was associated with intermediate to acidic magma in the Early Cretaceous (~120 Ma). The magma provenance of the Tiegelongnan deposit has some mantle‐derived composition, possibly mixed with the crust‐derived materials.     

Ages,Sources and Tectonic Settings of the Triassic Igneous Rocks in the Easternmost Segment of the East Kunlun Orogen,Central China

U–Pb analysis of zircons from igneous rocks in the Elashan Mountain, easternmost segment of the East Kunlun Orogen yielded 252–232 Ma. Geochemically, these rocks are mainly high in SiO_2, K_2O and K_2O+Na_2O contents, low in P_2O_5 and TiO_2 contents, depleted in Ba, Sr, P, Ti and enriched in U, Hf, Zr, showing features of I–type granite. The zircon εHf(t) values of the Early Triassic Jiamuge'er rhyolite porphyry(252±3 Ma) are positive(+1.6 to +12.1), suggesting a juvenile crustal source mixing with little old crustal component, and the zircon εHf(t) values of the Middle Triassic Manzhang'gang granodiorite(244±3 Ma) and Dehailong diorite(237±3 Ma) are predominately negative(-8.4 to +1.0), indicating an older crustal source. In comparison, the zircon εHf(t) values of the Late Triassic syenogranites from Suigen'ergang(234±2Ma), Ge'ermugang(233±2 Ma) and Yue'ergen(232±3 Ma) plutons vary from-3.8 to +5.0, suggesting a crust-mantle mixing source. From Early–Middle Triassic(252–237 Ma) to Late Triassic(234–232 Ma), the geochemical characteristics of these rocks show the change from a subduction–collision setting to a post-collision or within-plate setting. By comparing of these new age data with 77 zircon U–Pb ages of igneous rocks of the eastern part of East Kunlun orogen from published literatures, we conclude that the igneous rocks of Elashan Mountain and these of the eastern part of East Kunlun Orogen belong to one magmatic belt. All these data indicate that the Triassic magmatic events of the eastern part of East Kunlun Orogen can be divided into three stages: 252–238 Ma, 238–226 Ma and 226–212 Ma. Statistically, the average εHf(t) values of the threestage igneous rocks show a tendency, from the old to young, from-0.75±0.25 to lower-2.65±0.52 and then to-1.22±0.25, respectively, which reveal the change of their sources. These characteristics can be explained as a crust-mantle mixing source generated in a subductional stage, mainly crust source in a syn–collisional stage and a crust-mantle mixing source(lower crust with mantle-derived underplating magma) in a post-collisional stage. The identification of these three magmatic events in the Elashan Mountain, including all the eastern part of East Kunlun Orogen, provides new evidence for better understanding of the tectonic evolution of the northward subduction and closure of the Paleo-Tethyan(252–238 Ma), the collision of the Songpan–Ganzi block with the southern margin of Qaidam block(238–226 Ma), and the post–collisional setting(226–212 Ma) during the Early Mesozoic period.     

Detrital zircon ages and Hf isotopic compositions of metasedimentary rocks in the Wuqia area of Southwest Tianshan,NW China: implications for the early Paleozoic tectonic evolution of the Tianshan orogenic belt

ABSTRACT

The Chinese Southwest Tianshan Orogenic Belt is located along the boundary between the Central Asian Orogenic Belt (CAOB) and the Tarim Block (TB), NW China. It records the convergence of the Tarim Block and the Middle Tianshan, and is, therefore, a crucial region for understanding the Eurasia continental growth and evolution. The Wulagen (geographical name) metasedimentary rocks of the Wuqia area (mainly metamorphic sandstones and mica schists) form one of the metamorphic terranes in the Southwestern Tianshan Orogenic Belt. The geochronology of these rocks is poorly known, which hampers our understanding of the tectonic evolution of the belt. We analyzed 517 zircon grains for detrital zircon U–Pb dating and 93 zircon grains for in situ Lu–Hf isotopic compositions from the Wulagen metasedimentary rocks. The analyzed zircon grains yield Neoarchean to late Paleozoic U–Pb ages with major age peaks at ~2543 Ma, 1814 Ma, 830 Ma, 460 Ma, and the youngest cluster of zircon (magmatogene) ages is 395 Ma. The zircon U–Pb data show that the late Paleozoic (Early Devonian) is the maximum depositional age of the Wulagen metasedimentary rocks, rather than the previously considered Precambrian period. The zircons with Paleozoic ages yield εHf(t) values of ?22.0 to +11.3 and two-stage model ages (TDM₂) of 3.95 to 1.30 Ga, suggesting that the parental magmas were formed from partial melting of pre-existing crustal rocks. Our zircon U–Pb geochronology and Hf isotopic data indicate the major source regions for the Wulagen metasedimentary rocks was the Kyrgyzstan North Tianshan. The zircon age population of 600–400 Ma (peak at ~460 Ma) has negative εHf(t) values (?15.0 to ?0.6) and Mesoproterozoic two-stage model ages, suggesting that the early Paleozoic magmatism resulted mainly from the melting of ancient crust, which played an important role in crustal evolution in the southern CAOB.     

Age and nature of 560–520 Ma calc-alkaline granitoids of Biarjmand,northeast Iran: insights into Cadomian arc magmatism in northern Gondwana

ABSTRACT

The Biarjmand granitoids and granitic gneisses in northeast Iran are part of the Torud–Biarjmand metamorphic complex, where previous zircon U–Pb geochronology show ages of ca. 554–530 Ma for orthogneissic rocks. Our new U–Pb zircon ages confirm a Cadomian age and show that the granitic gneiss is ~30 million years older (561.3 ± 4.7 Ma) than intruding granitoids (522.3 ± 4.2 Ma; 537.7 ± 4.7 Ma). Cadomian magmatism in Iran was part of an approximately 100-million-year-long episode of subduction-related arc and back-arc magmatism, which dominated the whole northern Gondwana margin, from Iberia to Turkey and Iran. Major REE and trace element data show that these granitoids have calc-alkaline signatures. Their zircon O (δ¹⁸O = 6.2–8.9‰) and Hf (–7.9 to +5.5; one point with εHf ~ –17.4) as well as bulk rock Nd isotopes (εNd(t) = –3 to –6.2) show that these magmas were generated via mixing of juvenile magmas with an older crust and/or melting of middle continental crust. Whole-rock Nd and zircon Hf model ages (1.3–1.6 Ga) suggest that this older continental crust was likely to have been Mesoproterozoic or even older. Our results, including variable zircon εHf(t) values, inheritance of old zircons and lack of evidence for juvenile Cadomian igneous rocks anywhere in Iran, suggest that the geotectonic setting during late Ediacaran and early Cambrian time was a continental magmatic arc rather than back-arc for the evolution of northeast Iran Cadomian igneous rocks.     

Origins and evolution of rhyolitic magmas in the central Snake River Plain: insights from coupled high-precision geochronology,oxygen isotope,and hafnium isotope analyses of zircon

We present new high-precision CA-ID-TIMS and in situ U–Pb ages together with Hf and O isotopic analyses (analyses performed all on the same grains) from four tuffs from the 15?10 Ma Bruneau–Jarbidge center of the Snake River Plain and from three rhyolitic units from the Kimberly borehole in the neighboring 10?6 Ma Twin Falls volcanic center. We find significant intrasample diversity in zircon ages (ranges of up to 3 Myr) and in δ¹⁸O (ranges of up to 6‰) and ε_Hf (ranges of up to 24 ε units) values. Zircon rims are also more homogeneous than the associated cores, and we show that zircon rim growth occurs faster than the resolution of in situ dating techniques. CA-ID-TIMS dating of a subset of zircon grains from the Twin Falls samples reveals complex crystallization histories spanning 10⁴–10⁶ years prior to some eruptions, suggesting that magma genesis was characterized by the cyclic remelting of buried volcanic rocks and intrusions associated with previous magmatic episodes. Age-dependent trends in zircon isotopic compositions show that rhyolite production in the Yellowstone hotspot track is driven by the mixing of mantle-derived melts (normal δ¹⁸O and ε_Hf) and a combination of Precambrian basement rock (normal δ¹⁸O and ε_Hf down to ??60) and shallow Mesozoic and Cenozoic age rocks, some of which are hydrothermally altered (to low δ¹⁸O values) by earlier stages of Snake River Plain magmatism. These crustal melts hybridize with juvenile basalts and rhyolites to produce the erupted rhyolites. We also observe that the Precambrian basement rock is only an important component in the erupted magmas in the first eruption at each caldera center, suggesting that the accumulation of new intrusions quickly builds an upper crustal intrusive body which is isolated from the Precambrian basement and evolves towards more isotopically juvenile and lower-δ¹⁸O compositions over time.     

Geochronology, Geochemistry, Whole Rock Sr-Nd and Zircon Hf-O Isotopes of the Early Neoproterozoic Volcanic Rocks in Jiangshan, Eastern Part of the Jiangnan Orogen: Constraints on Petrogenesis and Tectonic Implications

This paper presents the results of combined studies of geochronology, geochemistry, whole rock Sr-Nd and zircon Hf-O isotopes carried out upon the rhyodacite and ignimbrite of Shangshu village, Shangyu town and Shanghupeng village of Jiangshan City in Zhejiang Province, along the northwestern side of the Jiangshan–Shaoxing suture. SHRIMP zircon U-Pb dating of samples in the three areas yielded weighted mean 206 Pb/238 U ages of 842.8 ± 6.9 Ma and 850.0 ± 7.3 Ma, 839 ± 9 Ma and 832.2 ± 8.1 Ma, 828.3 ± 8.5 Ma and 836.9 ± 9.9 Ma, respectively. These ages are older than the volcanic rocks of the Shangshu Formation dated at around 780 Ma distributed in Fuyang City, Hangzhou City, Kaihua County, etc. The volcanic rocks generally have high SiO2(54.08–76.80 wt%) and Al2 O3(12.40–21.31 wt%), low Fe2 O3(0.68–8.92 wt%), MgO(0.29–2.49 wt%), CaO(0.12–2.86 wt%), TiO2(0.10–1.59 wt%) and P2 O5(0.01–0.39 wt%), with variable total alkalis(K2 O + Na2 O =5.42–8.29 wt%). There exists a clear negative correlation between SiO2 and P2 O5. The volcanic rocks have A/CNK ratios of 1.03–2.77 and thus are peraluminous. They are characterized by enrichment in LREE, Rb, Ba, Zr, Hf, K, Th, La, U and depletion in Nb, Sr, P, Ti, with distinct LREE and HREE fractionation of(La/Yb)N values of 5.68–11.67, and with a moderate negative Eu anomaly(Eu=0.58–0.89). Whole-rock geochemical data shows that the Jiangshan volcanic rocks are possibly I-type granitic rocks, even though they have some characteristics of AS-type granites due to the magma fractional crystallization and water-rock interaction. Zircon δ18 O values are 3.97‰–5.49‰(average 4.50‰), 2.90‰–5.21‰(average 4.32‰) for ignimbrite from Shangshu village section, and Shanghupeng village section, respectively. They are slightly lower than the average δ18 O values of igneous zircons in equilibrium with mantle magmas(5.3 ± 0.6‰(2σ)), the lower δ18 O value also demonstrating the presence of high temperature water-rock interactions. The ignimbrite rocks have positive εNd(t)(4.02, 3.37, 3.91, 4.74, 2.85, 4.39, totals from the three areas) and εNd(t)(in-situ zircon)(4.3–14.6, a weighted mean of 8.4; 6.6–12.7, a weighted mean of 9.0; 8.1–12.0, a weighted mean of 9.5, respectively, from the three areas). In conjunction with the trace element studies, they indicate that the source region of the Jiangshan volcanic rocks was mainly composed of juvenile lower crustal material, mixed with some mantle-sourced magma. Detailed elemental and isotopic data suggest that the Jiangshan volcanic rocks were formed in a continental arc setting. There is a series of ca. 860–830 Ma volcanic rocks formed in a back-arc extensional setting in the southern margin of the eastern Jiangnan Orogen, along the northwest side of the Jiangshan–Shaoxing suture. The first stage rift-related anorogenic magmatism may have occurred as early as ca. 860 Ma in the eastern Jiangnan Orogen.     

East Antarctic sources of extensive Lower–Middle Ordovician turbidites in the Lachlan Orogen,southern Tasmanides,eastern Australia

Lower to upper Middle Ordovician quartz-rich turbidites form the bedrock of the Lachlan Orogen in the southern Tasmanides of eastern Australia and occupy a present-day deformed volume of ～2–3 million km³. We have used U–Pb and Hf-isotope analyses of detrital zircons in biostratigraphically constrained turbiditic sandstones from three separate terranes of the Lachlan Orogen to investigate possible source regions and to compare similarities and differences in zircon populations. Comparison with shallow-water Lower Ordovician sandstones deposited on the subsiding margin of the Gondwana craton suggests different source regions, with Grenvillian zircons in shelf sandstones derived from the Musgrave Province in central Australia, and Panafrican sources in shelf sandstones possibly locally derived. All Ordovician turbiditic sandstone samples in the Lachlan Orogen are dominated by ca 490–620 Ma (late Panafrican) and ca 950–1120 Ma (late Grenvillian) zircons that are sourced mainly from East Antarctica. Subtle differences between samples point to different sources. In particular, the age consistency of late Panafrican zircon data from the most inboard of our terranes (Castlemaine Group, Bendigo Terrane) suggests they may have emanated directly from late Grenvillian East Antarctic belts, such as in Dronning Maud Land and subglacial extensions that were reworked in the late Panafrican. Changes in zircon data in the more outboard Hermidale and Albury-Bega terranes are more consistent with derivation from the youngest of four sedimentary sequences of the Ross Orogen of Antarctica (Cambrian–Ordovician upper Byrd Group, Liv Group and correlatives referred to here as sequence 4) and/or from the same mixture of sources that supplied that sequence. These sources include uncommon ca 650 Ma rift volcanics, late Panafrican Ross arc volcanics, now largely eroded, and some <545 Ma Granite Harbour Intrusives, representing the roots of the Ross Orogen continental-margin arc. Unlike farther north, Granite Harbour Intrusives between the Queen Maud and Pensacola mountains of the southern Ross Orogen contain late Grenvillian zircon xenocrysts (derived from underlying relatively juvenile basement), as well as late Panafrican magmatic zircons, and are thus able to supply sequence 4 and the Lachlan Ordovician turbidites with both these populations. Other zircons and detrital muscovites in the Lachlan Ordovician turbidites were derived from relatively juvenile inland Antarctic sources external to the orogen (e.g. Dronning Maud Land, Sør Rondane and a possible extension of the Pinjarra Orogen) either directly or recycled through older sedimentary sequences 2 (Beardmore and Skelton groups) and 3 (e.g. Hannah Ridge Formation) in the Ross Orogen. Shallow-water, forearc basin sequence 4 sediments (or their sources) fed turbidity currents into outboard, deeper-water parts of the forearc basin and led to deposition of the Ordovician turbidites ～2500–3400 km to the north in backarc-basin settings of the Lachlan Orogen.     

The Paleoarchean Northern Mundo Novo Greenstone Belt,São Francisco Craton: Geochemistry,U–Pb–Hf–O in zircon and pyrite δ34S-Δ33S-Δ36S signatures

Greenstone belts contain several clues about the evolutionary history of primitive Earth. Here, we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gavião Block, named the Northern Mundo Novo Greenstone Belt (N-MNGB), and present data collected with different techniques, including U–Pb–Hf–O isotopes of zircon and multiple sulfur isotopes (³²S, ³³S, ³⁴S, and ³⁶S) of pyrite from this supracrustal sequence. A pillowed metabasalt situated in the upper section of the N-MNGB is 3337 ± 25 Ma old and has zircon with ε_Hf(t) =  ?2.47 to ?1.40, Hf model ages between 3.75 Ga and 3.82 Ga, and δ¹⁸O = +3.6‰ to +7.3‰. These isotopic data, together with compiled whole-rock trace element data, suggest that the mafic metavolcanic rocks formed in a subduction-related setting, likely a back-arc basin juxtaposed to a continental arc. In this context, the magma interacted with older Eoarchean crustal components from the Gavião Block. Detrital zircons from the overlying quartzites of the Jacobina Group are sourced from Paleoarchean rocks, in accordance with previous studies, yielding a maximum depositional age of 3353 ± 22 Ma. These detrital zircons have ε_Hf(t) =  ?5.40 to ?0.84, Hf model ages between 3.66 Ga and 4.30 Ga, and δ¹⁸O = +4.8‰ to +6.4‰. The pyrite multiple sulfur isotope investigation of the 3.3 Ga supracrustal rocks from the N-MNGB enabled a further understanding of Paleoarchean sulfur cycling. The samples have diverse isotopic compositions that indicate sulfur sourced from distinct reservoirs. Significantly, they preserve the signal of the anoxic Archean atmosphere, expressed by MIF-S signatures (Δ³³S between ?1.3‰ to +1.4‰) and a Δ³⁶S/Δ³³S slope of ?0.81 that is indistinguishable from the so-called Archean array. A BIF sample has a magmatic origin of sulfur, as indicated by the limited δ³⁴S range (0 to +2‰), Δ³³S ~ 0‰, and Δ³⁶S ~ 0‰. A carbonaceous schist shows positive δ³⁴S (2.1‰–3.5‰) and elevated Δ³³S (1.2‰–1.4‰) values, with corresponding negative Δ³⁶S between ?1.2‰ to ?0.2‰, which resemble the isotopic composition of Archean black shales and suggest a source from the photolytic reduction of elemental sulfur. The pillowed metabasalt displays heterogeneous δ³⁴S, Δ³³S, and Δ³⁶S signatures that reflect assimilation of both magmatic sulfur and photolytic sulfate during hydrothermal seafloor alteration. Lastly, pyrite in a massive sulfide lens is isotopically similar to barite of several Paleoarchean deposits worldwide, which might indicate mass dependent sulfur processing from a global and well-mixed sulfate reservoir at this time.     

Geology and geochemistry of the Triassic Wenquan Mo deposit and Mo-mineralized granite in the Western Qinling Orogen,China

The recently-discovered Wenquan porphyry Mo deposit hosted in the Wenquan granite of the West Qinling Orogen has been recognized as a product of the Indosinian metallogenesis. Three generations of mineral assemblage for the deposit are identified as follows: (1) quartz–biotite–K-feldspar; (2) quartz–sulfide and (3) sulfide–calcite. Geochemical study shows that the mafic microgranular enclaves (MMEs) in the ore-bearing Wenquan granite have lower SiO₂, and higher Mg# and Nb/Ta ratios than the host granite itself. Different from the granite which have zircon ε_Hf(t) values of − 3.6–3.0 and T_DM2 of 1234–890 Ma, the MMEs are characterized by the ε_Hf(t) values of − 10.1–10.8 and T_DM1 of 865–441 Ma. This can be interpreted to indicate a mixture origin of the Meso- and Neoproterozoic crust-derived component and Neoproterozoic SCLM-derived materials for the formation of the Wenquan granite, which played an essential role in the Mo mineralization. Comparative Pb isotopic data between ores and K-feldspar suggest that the Wenquan granitic magma originated from the middle-lower crust of the South China Block and the ore-forming materials were incorporated by hydrothermal fluid differentiated from the Triassic magmatic system, with minor contribution of sedimentary rocks. The δ³⁴S values of 5.0–11.7‰ with a pronounced mode at 5.0 to 6.1‰ for the ores probably represent the sulfur incorporation of a typical magmatic hydrothermal fluid contaminated by heavy sulfur of Devonian sediments. The granite yielded the zircon U–Pb ages of 218 ± 2.4 Ma and 221 ± 1.3 Ma, as the same as the ages of 217 ± 2.0 Ma and 218 ± 2.5 Ma obtained for the MMEs. These ages are indistinguishable with the molybdenite Re–Os isochron age of 219 ± 5.2 Ma which is the timing for the Mo mineralization. Tectonically, the magmatic mixture processes of the Wenquan granite and the Mo mineralization to form the Wenquan Mo deposit contemporaneously occurred during the transition of tectonic regime from syn- to post-collision orogeny in the Qinling Orogen in the Late Triassic.