This study reports zircon LA-ICPMS U–Pb ages of 50 igneous rock samples from the Urumieh–Dokhtar magmatic arc (UDMA) and Sanandaj–Sirjan structural zone (SSZ) in Iran. These results, together with literatures and our unpublished age data, better delineate the magmatic evolution related to the Neotethyan subduction and subsequent Zagros orogeny that resulted from the Arabia–Eurasia collision. Subduction-related magmatism was active during Jurassic time, as evidenced by the presence of widespread I-type granitoids from the Middle to Late Jurassic (176–144 Ma) in the SSZ. After a protracted magmatic quiescence in the Early Cretaceous, igneous activity renewed inland in the UDMA from which we identify Late Cretaceous granitoids (81–72 Ma) in Jiroft and Bazman areas, the southeastern segment of the UDMA. The UDMA volcanism was most active and widespread during the Eocene and Oligocene (55–25 Ma), much longer lasting than previously thought as just an Eocene pulse. Such a prolonged igneous “flare-up” event in the UDMA can be correlated to Armenia where coeval calc-alkaline rocks are common. The UDMA magmatism ceased progressively from northwest to southeast, with magmatic activities ending the Early Miocene (ca. 22 Ma) in Meghri, the Middle Miocene (ca. 16 Ma) in Kashan and the Late Miocene (ca. 10–6 Ma) in Anar, respectively. The southeastward magmatic cessation is consistent with the notion of oblique and diachronous collision between Arabia and Eurasia. Post-collisional volcanism started ca. 11 Ma in Saray, east off the Urumieh Lake, which, along with later eruptions in Sahand (6.5–4.2 Ma) and Sabalan (≤ 0.4 Ma) volcanoes, forms a compositionally unique component of the vast volcanic field covering much of the Lesser Caucasus, NW Iran and eastern Anatolia regions. ► Reporting new zircon U–Pb and Ar–Ar ages of two major magmatic belts across Iran ► Arc magmatism migrated inland and renewed in the UDMA during the Late Cretaceous ► Magmatic flare-ups in the UDMA lasted from the Eocene and Oligocene (55–25 Ma). ► The UDMA magmatism terminated progressively from northwest to southeast.
To understand the connection between continental cratonization and global tectonothermal event is essential for recognizing the formation and evolution of continental crust. Paleoproterozoic is an important era with occurrence of megascale tectonomagmatism in the world, but it has been intriguing whether they also influenced the oldest continent in South China. In order to decipher the nature of Paleoproterozoic event in South China, a combined study of zircon U-Pb dating, Hf and O isotope analyses was carried out for metasediments and amphibolite from the Kongling terrane, the only Archean microcontinent outcropped in South China. U-Pb ages of 1.97 ± 0.03 Ga were obtained with low Th/U ratios of 0.01–0.14, indicating that the ages are a record of Paleoproterozoic metamorphic event. δ O values of ∼11‰ and ∼8‰ were measured for quartz from the metasediments and garnet from the amphibolite, respectively, suggesting that their sources experienced supracrustal recycling. ( ) values of about −6.5 and model Hf ages of about 3.0 Ga were acquired for zircons from the metapelites, suggesting an Archean source. Thus a response to the Paleoproterozoic global tectonothermal event in South China is reworking of Archean continental nucleus. Compared with Archean rocks at Kongling, abrupt changes in K O/Na O, REE and other trace elements are observed in the Paleoproterozoic metasedimentary rocks. This is interpreted to reflect a change in upper crustal composition at the Archean–Proterozoic boundary. A survey of Paleoproterozoic ages throughout the Yangtze Block suggests that metamorphic event and subsequent magmatic activity occurred in the north, but only magmatic activity in the south. Both metamorphic and magmatic activities are associated with formation of a unified basement responsible for cratonization of the Yangtze Block. This provides a geodynamic connection between the formation of this craton and the global tectonomagmatism in the Paleoproterozoic, marking continental accretion by arc-continent collision orogeny during assembly of the supercontinent Columbia.
The Neoproterozoic Quruqtagh Group in the Tarim Block, NW China, contains multiple diamictites in the Bayisi, Altungol, Tereeken, and Hankalchough formations. These diamictites may represent three or possibly four discrete glaciations, although evidence for a glacial origin of the Bayisi and Altungol diamictite is ambiguous. To constrain their age and duration, we dated three volcanic beds (V1, V2, and V3) in the Quruqtagh Group using the SHRIMP (sensitive high-resolution ion microprobe) zircon U–Pb method. Volcanic bed V1 near the base of the Bayisi diamictite yields a 740 ± 7 Ma age, volcanic bed V2 near the top of the Bayisi Formation gives a 725 ± 10 Ma age, and volcanic bed V3 between the Tereeken and Hankalchough diamictites yields a 615 ± 6 Ma age. V1 and V2 have overlapping ages, and together these dates suggest that the Bayisi diamictite was deposited at around 730 Ma. The Tereeken and Altungol diamictites were deposited between 725 ± 10 Ma and 615 ± 6 Ma, and the Hankalchough diamictite between 615 ± 6 Ma and ∼542 Ma (i.e., the Neoproterozoic–Cambrian transition). These dates and previously published chemostratigraphic data are consistent with (but doe not require) the correlation of the Tereeken and Hankalchough diamictites with the 635 Ma Nantuo and 582 Ma Gaskiers glaciations, respectively. However, the new dates are inconsistent with a single and globally synchronous Sturtian glaciation that occurred in the pre-Nantuo Neoproterozoic Era. Instead, currently available data necessitate that either multiple glaciations occurred, or a globally diachronous glacial event developed during a protracted period between ∼750 Ma and ∼650 Ma.
Zircon U-Pb dating, Hf and O isotope analyses were carried out for ultrahigh-pressure eclogite and felsic gneiss from the Dabie orogen in China. The results provide constraints on their protolith origin, with significance for continental growth by rift magmatism during splitting of the Yangtze Block from the supercontinent Rodinia. Mafic and felsic protoliths formed as a bimodal volcanic suite at about 750 Ma, with incorporation of ca. 2.15 Ga crust into the felsic protolith. delta O-18 values of -4.1 parts per thousand to 4.4 parts per thousand suggest differential involvement of meteoric water in protolith magmas. Initial Hf isotope ratios are subdivided into two groups, with positive epsilon(Hf)(t) values of 12.9 +/- 0.7 to 5.9 +/- 0.9 and neutral epsilon(Hf)(t) values of 2.3 +/- 0.3 to -2.7 +/- 0.6, respectively. The positive epsilon(Hf)(t) values correspond to depleted mantle Hf model ages of 0.82 to 1.24 Ga, whereas the negative epsilon(Hf)(t) values correspond to crust Hf model ages of 1.82 +/- 0.07 Ga. A few zircons from the gneiss have strongly negative epsilon(Hf)(t) values of -22.6 +/- 0.6, which are associated not only with consistent depleted mantle and crust Hf model ages of 2.2 Ga but also with the old U-Pb ages falling on a 2147 +/- 22 Ma discordia line. Thus the felsic protolith contains Paleoproterozoic, crustal relicts. some of which were originally derived from coeval depleted mantle. On the other hand, strongly positive values are associated with young U-Pb ages falling on the similar to 750 Ma discordia lines. This suggests that both eclogite and gneiss protoliths principally formed by immediate reworking of mid-Neoproterozoic juvenile crust, with contrasting features only in petrochemistry. The highest epsilon(Ht)(t) values of 12.7 to 14.4 for the eclogite correspond to the youngest Hf model ages of 0.75 to 0.82 Ga relative to the depleted mantle reservoir. These Hf model ages are not only close to the timing of zircon growth from the mafic magma, but also similar to ages for bimodal magmatism in the periphery of the Yangtze Block. This demonstrates new addition of the depleted mantle material to the continental crust by rift magmatism in the northern margin of the Yangtze Block, with coeval crust-mantle interaction in the mid-Neoproterozoic rifting tectonic zone. Therefore, the bimodal magmatism at about 750 Ma transports both heat and material from the mantle to the crust, resulting in reworking of both the Paleoproterozoic old crust and the meteoric-hydrothermally altered juvenile crust along the active rifting zone. (c) 2006 Elsevier B.V. All rights reserved.
The southern Qiangtang magmatic belt was formed by the north-dipping subduction of the Bangong–Nujiang Tethyan Ocean during Mesozoic. To better understand the petrogenesis, time–space distribution along the length of this belt, 21 samples of several granitoid bodies, from west to east, in the Bangong Co, Gaize, Dongqiao and Amdo areas were selected for in-situ zircon U–Pb dating, Hf isotopic and whole-rock chemical analyses. The results suggest a prolonged period of magmatic activity (185–84 Ma) with two major stages during the Jurassic (185–150 Ma) and the Early Cretaceous (126–100 Ma). Both the Jurassic and Cretaceous granitoids are high-K calc-alkaline I-type rocks, except the Cretaceous two-mica granite from Amdo in the east, which belongs to S-type. The granitoids are generated from different source materials as indicated by zircon Hf isotopic compositions. The Bangong Co and Dongqiao granitoids show high zircon ε (t) values of − 1.3–13.6 with younger T ages of 293–1263 Ma, suggesting a relatively juvenile source; whereas the Gaize and Amdo granitoids have low ε (t) values of − 16.1–2.9 with older T ages of 999–2024 Ma, indicating an old crustal contribution. These source rocks melt at different P–T conditions as suggested by Sr/Y ratio and T . The Sr/Y ratio of both stage granitoids increases with decreasing age. However, the T of the Jurassic granitoids decreases, whereas the T of the Cretaceous granitoids increases with decreasing age. The contrasting geochemical signatures of these granitoids may be controlled by the varying contribution of slab-derived fluids involved in the generation of the Jurassic and Cretaceous granitic magmas; i.e. increasing amount of fluids in the Jurassic, whereas decreasing amount of fluids in the Cretaceous. Therefore, it is proposed that the Jurassic and Cretaceous magmatism may be related to subduction and closure of the Bangong–Nujiang Tethyan Ocean, respectively. The age pattern of the Jurassic and Cretaceous granitoids suggests an oblique subduction of the Bangong–Nujiang Tethyan Ocean and a diachronous collision between the Lhasa and Qiangtang blocks.
The South China Block, consisting of the Yangtze and the Cathaysia blocks, is one of the largest Precambrian blocks in eastern Asia. However, the early history of the Cathaysia Block is poorly understood due largely to intensive and extensive reworking by Phanerozoic polyphase orogenesis and magmatism which strongly overprinted and obscured much of the Precambrian geological record. In this paper, we use the detrital zircon U–Pb age and Hf isotope datasets as an alternative approach to delineate the early history of the Cathaysia Block. Compilation of published 4041 Precambrian detrital zircon ages from a number of (meta)sedimentary samples and river sands exhibits a broad age spectrum, with three major peaks at ~ 2485 Ma, ~ 1853 Ma and ~ 970 Ma (counting for ~ 10%, ~ 16% and ~ 24% of all analyses, respectively), and four subordinate peaks at ~ 1426 Ma, ~ 1074 Ma, ~ 780 Ma and ~ 588 Ma. Five of seven detrital zircon age peaks are broadly coincident with the crystallisation ages of ~ 1.89–1.83 Ga, ~ 1.43 Ga, ~ 1.0–0.98 Ga and ~ 0.82–0.72 Ga for known igneous rocks exposed in Cathaysia, whereas, igneous rocks with ages of ~ 2.49 Ga and ~ 0.59 Ga have not yet been found. The Hf isotopic data from 1085 detrital zircons yield Hf model ages (T ) between ~ 4.19 Ga and ~ 0.81 Ga, and the calculated εHf(t) values between − 40.2 and 14.4. The Archean detrital zircons are exclusively oval in shape with complicated internal textures, indicating that they were sourced by long distance transportations and strong abrasion from an exotic Archean continent. In contrast, the majority of detrital zircons in age between ~ 1.9 and ~ 0.8 Ga are euhedral to subhedral crystals, indicative of local derivation by short distance transportations from their sources. The oldest crustal basement rocks in Cathaysia were most likely formed by generation of juvenile crust and reworking of recycled Archean components in Late Paleoproterozoic at ~ 1.9–1.8 Ga, rather than in the Archean as previously speculated. Reworking and recycling of the continental crust are likely the dominant processes for the crustal evolution of Cathaysia during the Mesoproterozoic to Neoproterozoic time, with an intervenient period of significant generation of juvenile crust at ~ 1.0 Ga. Precambrian crustal evolutions of the Cathaysia Block are genetically related to the supercontinent cycles. By comparing detrital zircon data from Cathaysia with those for other continents, and integrating multiple lines of geological evidence, we interpret the Cathaysia Block as an orogenic belt located between East Antarctica, Laurentia and Australia during the assembly of supercontinent Columbia/Nuna at ~ 1.9–1.8 Ga. The Cathaysia Block amalgamated with the Yangtze Block to form the united South China Block during the Sibao Orogeny at ~ 1.0–0.89 Ga. The Laurentia–Cathaysia–Yangtze–Australia–East Antarctica connection gives the best solution to the paleo-position of Cathaysia in supercontinent Rodinia. The significant amount of ~ 0.6–0.55 Ga detrital zircons in Cathaysia and West Yangtze have exclusively high of > 300 Ma, indicating crystallisation from magmas generated dominantly by crustal reworking. This detrital zircon population compares well with the similar-aged zircon populations from a number of Gondwana-derived terranes including Tethyan Himalaya, High Himalaya, Qiangtang and Indochina. The united South China–Indochina continent was likely once an integral part of Gondwanaland, connected to northern India by a “Pan-African” collisional orogen.
Recently compiled global databases of igneous and detrital zircon U-Pb ages have been integrated with other types of isotope data (e.g., neodymium, hafnium, and oxygen) to characterize the episodic growth of the continental crust and the development of supercontinents. However, to what extent do the age peaks reflect the rate of continent crust growth or the differential preservation due to supercontinent settings is a long-standing question. Instead of analyzing amplitudes and periodicity of the age series, here, the analysis focuses on shapes of the individual age peaks described by a power law model for measuring the scale invariant fractality and singularity of time-series records. The results indicate that zircon age distributions around peaks follow power law distribution, regardless of the bin-size used to measure the age distribution. Based on the commonly accepted mechanisms (phase transition, self-organized criticality and multiplicative cascade processes) for generation of power law distributions one can relate the nonlinearity of the age peaks to short spurts of accelerated magmatic activities due to “avalanches” (superplumes, slab breakoff etc.) occurred during episodic convection of the mantle. The exponents of the power law age distributions estimated from the age peaks can be used as an index to quantify the intensity of a singularity. The values of exponents calculated at all major age peaks from five global databases exhibit an episodic nature, with a mean duration of approximately 600–800 Myr. Both intensity of the zircon episodes and their duration for the interval from 3 Ga to 0.5 Ga depict a descending trend, which may signify mantle cooling.
LA-ICPMS zircon U-Pb dating has been greatly advanced and widely applied in the past decade because it is a cheap and fast technique. The internal error of LA-ICPMS zircon U-Pb dating can be better than 1%, but reproducibility （accuracy） is relatively poor. In order to quantitatively assess the accuracy of this technique, zircons from two dioritic rocks, a Mesozoic dioritic microgranular enclave （FS06） and a Neoproterozoic diorite （WC09-32）, were dated independently in eight laboratories using SIMS and LA-ICPMS. Results of three SIMS analyses on FS06 and WC09-2 are indistinguishable within error and give a best estimate of the crystallization age of 132.2 and 760.5 Ma （reproducibility is -1%, 2RSD）, respectively. Zircon U-Pb ages determined by LA-ICPMS in six laboratories vary from 128.3±1.0 to 135.0±0.9 Ma （2SE） for FS06 and from 742.9±3.1 to 777.8±4.7 Ma （2SE） for WC09-32, suggesting a reproducibility of -4% （2RSD）. Uncertainty produced during LA-ICPMS zircon U-Pb analyses comes from multiple sources, including uncertainty in the isotopic ratio measurements, uncertainty in the fractionation factor calculation using an external standard, uncertainty in the age determination as a result of common lead correction, age uncertainty of the external standards and uncertainty in the data reduction. Result of our study suggests that the uncertainty of LA-ICPMS zircon U-Pb dating is approximately 4% （2RSD）. The uncertainty in age determination must be considered in order to interpret LA-ICPMS zircon U-Pb data rationally.
Four populations of zircons are recognized by U–Pb analyses of the Neoproterozoic Liantuo Formation in the northern part of the Yangtze Block, South China. They are grains of detrital zircon older than 3.0 Ga, ca. 2.95 Ga, ca. 1.95 Ga and 820–750 Ma, respectively. The oldest zircon has a U–Pb age of 3802 ± 8 Ma with a ( ) value of − 0.8 and model Hf ages of 3.96 Ga ( ) and 4.00 Ga ( ). This demonstrates the existence of 3.8 Ga old crustal remnants in South China, with possible crustal growth as early as 4.0 Ga. A series of 3.3 Ga zircons have positive ( ) values as high as 4.2, providing compelling evidence for growth of juvenile crust from a depleted mantle reservoir at Paleoarchean. All the zircons have Archean Hf model ages, with prominent peaks at 3.2 to 3.6 Ga, indicating an important period of crustal growth in this period. The other three zircon populations at ∼ 2.95 Ga, ∼ 1.95 Ga and 820–750 Ma have negative ( ) values and consistent Archean Hf model ages, suggesting multi-stage episodic reworking of Archean crustal materials. The youngest zircons have U–Pb ages of ∼ 750 Ma, very close to the deposition time of the Liantuo Formation. This indicates rapid recycling of supracrustal materials in a rift basin, possibly in association with breakup of the supercontinent Rodinia at that time.
A central prediction of the Snowball Earth hypothesis is that glacial onset should be synchronous at low latitudes, and its termination should be rapid and synchronous globally. High precision U/Pb zircon ages provide supporting evidence for the synchronous onset (within error) of the Sturtian glaciation (ca. 716 Ma) on multiple continents. Successful application of Re-Os techniques on organic rich shales and carbonates allow for the possibility of a globally synchronous Sturtian deglaciation (ca. 660 Ma), but the sparse isotopic age constraints leave this open to debate. Here we report the first high precision U-Pb zircon age of 663.03 ± 0.11 Ma (2σ) for the end Sturtian recorded in the Wilyerpa Formation of South Australia. This age supports previously published ages and is permissive with a globally synchronous deglaciation. In conjunction with the timing of glacial onset, this age reinforces the ca. 58 Myr duration of the Sturtian Snowball.