Sensitive High-resolution Ion Microprobe Zircon Research Articles

U-Pb zircon SHRIMP ages, geochemical and Sr-Nd isotopic compositions of the Early Cretaceous I-type Sarıosman pluton, Eastern Pontides, NE Turkey

The petrogenesis and U-Pb SHRIMP zircon ages of the Late Cretaceous Sarıosman pluton in the Eastern Pontides is investigated by means of whole-rock Sr-Nd isotope data with field, petrographic and whole-rock geochemical studies. The bulk of the I-type Sarıosman pluton consists of biotite-hornblende monzogranite, with minor quantities of porphyritic hornblende-biotite monzogranite. The biotite-hornblende monzogranite contains a number of mafic microgranular enclaves (MMEs) of quartz monzodiorite composition. U-Pb zircon sensitive high-resolution ion microprobe dating (SHRIMP) dates the magma emplacement age of the biotite-hornblende monzogranite at 82.7±1.5 Ma. The rocks of the pluton show high-K calc-alkaline, metaluminous to slightly peraluminous characteristics, and are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE), thus displaying features of arc-related granitoids. Chondrite-normalised rare earth-element (REE) patterns have concave upward shapes (La/_{cn}/Lu_{cn}= 10.1-17.4) with pronounced negative Eu anomalies (Eu/Eu*= 0.61-0.80). Initial \epsilon_{Nd} values vary between -3.0 and -4.1 and initial ^{87}Sr/^{86}Sr values between 0.7062 and 0.707. The MMEs are characterised by higher Mg-numbers (27-29) and lower values of both SiO_2 (56-58 wt percent) and aluminium saturation index (0.9-1.0), compared to the monzogranites. Fractionation of plagioclase, hornblende and Fe-Ti oxides played an important role in the evolution of the Sarıosman pluton. The crystallisation temperatures of the melts ranged from 700 to 800 °C and a relatively shallow intrusion depth (~2 to 7 km) is estimated from the Al-in-hornblende geobarometry. The geochemical and isotopic compositions of the Sarıosman pluton suggest an origin through dehydration melting of mafic lower crustal source rocks.

Multi-method chronometry of the Teletskoye graben and its basement, Siberian Altai Mountains: new insights on its thermo-tectonic evolution

Abstract The Altai Mountains form an intracontinental, transpressive deformation belt in the NW Central Asian orogenic system. Using a multi-method chronometric approach, the thermo-tectonic history of the basement underlying the Teletskoye graben area is constrained in more detail. The results provide new insights into the Siberian Altai basement evolution from the Early Palaeozoic to the present. Zircon SHRIMP (sensitive high-resolution ion microprobe) U–Pb ages (Late Ordovician–Early Silurian, 460–420 Ma) indicate an earlier crystallization age for the basement granitoids than previously thought (Late Devonian–Early Carboniferous, 370–350 Ma), while new multi-mineral 40 Ar/ 39 Ar age spectra suggest continuous basement cooling throughout the Devonian–Carboniferous. Reactivation of long-lived Palaeozoic structures controls the Teletskoye graben formation since the Plio-Pleistocene as a distant effect of India–Eurasian convergence. Deformation is propagated through Central Asia and Siberia along an inherited structural network closely associated with its basement fabric. A similar reactivation affected the Altai during the Mesozoic. Modelled apatite fission-track data suggest Late Jurassic–Cretaceous (150–80 Ma) cooling, interpreted to be related to denudation and the tectonic reactivation that we link to the coeval Mongol–Okhotsk orogeny. From the Late Cretaceous until the Pliocene, the thermal history models indicate a period of stability. Roughly around 5 Ma ago renewed cooling is observed that possibly represents the denudation and growth of the present-day Altai, and provides the context for the Teletskoye graben formation. A modelled Late Cenozoic cooling can be a result of, or overemphasized by, a modelling artefact. Some caution should be taken not to overinterpret this cooling phase.

SHRIMP zircon U–Pb age constraints on Neoproterozoic Quruqtagh diamictites in NW China

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.

Ages of Zircons from Basalt of Erdaogou Formation and Diabase Dyke Swarms in Northern Liaoning

The drainage area of Fanhe River around the Tieling City in the northern part of Liaodong Peninsula, is located in the north of the Tancheng-Lujiang fault zone, and belongs to the northeastern end of North China Block in the Sino-Korean plate. In this area, there is huge-thick basalt (Erdaogou Formation) and diabase dyke swarms, which are thought to be Mesoproterozoic basic magmatism. By means of zircon dating with sensitive high-resolution ion microprobe (SHRIMP), the eruption time of the Erdaogou Formation basalt is considered to be 36.4 ± 1.8 Ma, the Eocene Epoch. The emplacement time of the diabase swarms is considered to be 93.5 ± 1.4 Ma, the Late Cretaceous. The basic dyke swarms and basalts in the northern Liaoning were determined as Mesozoic and Cenozoic, respectively. This shows that the northern boundary that divides North China Block and Jiao-Liao-Korean Block within Sino-Korean Plate (i.e. the northern part of the Tancheng-Lujiang fault zone), was in a stretch-tensile tectonic environment in EW direction in Late Cretaceous. In addition, this tectonic environment resulted in the intrusion of the diabase swarms. In Paleogene Period, the stretching direction was changed to near SN, and the formation of the stretching tectonics led to the eruption of the huge-thick Erdaogou Formation basalt. The new SHRIMP zircon U-Pb ages in this article does not support the traditional opinion that the Erdaogou Formation basalt was formed in Mesoproterozoic.

Geochronology and geodynamics of Scottish granitoids from the late Neoproterozoic break-up of Rodinia to Palaeozoic collision

Thirty-seven granitoids from Scotland have been dated using the sensitive high-resolution ion microprobe zircon method. Granitoids were intruded during: (1) crustal stretching at c . 600 Ma after Rodinia broke up (A-types); (2) the Grampian event of crustal thickening when the Midland Valley Arc terrane collided with Laurentia at c . 470 Ma (S-types); (3) erosion and decompression of the over-thickened Laurentian margin at c . 455 Ma (S-types); (4) subduction of Iapetus Ocean lithosphere under Laurentia starting at 430 Ma (I-types); (5) roll-back beginning at 420 Ma (I-types); (6) bilateral slab break-off and lithospheric delamination at 410 Ma (I- and S-type granites) when Baltica hard-docked against the Northern Highland terrane and Avalonia soft-docked against the Grampian Highland terrane. Far-field Acadian events at 390 Ma were recorded by I-type granites intruded along active sinistrally transpressive faults. I-types formed in lower crustal hot zones above subduction zones, whereas S-types formed in lower crustal hot zones above lithospheric windows through which hot asthenosphere had risen.

SHRIMP zircon geochronology, and geochemical characteristics of metaplutonic rocks from the south-western Gyeonggi Block, Korea: Implications for Paleoproterozoic to Mesozoic tectonic links between the Korean Peninsula and eastern China

The Hongseong area in the south-western Gyeonggi Block of the Korean Peninsula consists of several Precambrian complexes that were regionally metamorphosed during the Triassic collision between the North and South China Blocks. Paleoproterozoic and Neoproterozoic magmatism and Triassic regional metamorphism in the Hongseong area suggest a possible tectonic link with the Yangtze Craton in the South China Block. The early magmatism in the western Hongseong area was Neoproterozoic, while that in the eastern Hongseong area was Paleoproterozoic. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb dating of zircon indicates that the Neoproterozoic igneous activity occurred at ca. 850–830 Ma. U–Pb ages of zircon cores and first metamorphic overgrowths in the Paleoproterozoic gneisses are indistinguishable (1.88–1.86 Ga), indicating that the igneous activity and early metamorphism occurred in relatively rapid succession. The geochemical signatures indicate an arc tectonic setting for both the Paleoproterozoic and Neoproterozoic rocks. The age and geochemical signatures of the Neoproterozoic magmatism can be matched with the Neoproterozoic magmatism at the margin of the Yangtze Craton before amalgamation of the Rodinian supercontitnent. The Paleoproterozoic igneous and metamorphic events may be related to the assembly of the Columbian supercontinent. Both the Neoproterozoic igneous rocks and the Paleoproterozoic orthogneisses in the Hongseong area were regionally metamorphosed in the Triassic, at ca. 235 Ma. This regional metamorphism is well correlated with metamorphism at the northern margin of the Yangtze Craton where the Qinling-Dabie-Sulu UHP metamorphic belt formed due to the Triassic collision between the Yangtze Craton and North China Blocks. Similarities in the overall age distribution and the geographic resemblance between the Hongseong area and the northern margin of the Yangtze Craton show that the Hongseong area in the south-western Gyeonggi Block was tectonically linked with the Yangtze Craton, at least since the time of Rodinia.

Origin of the Tongshankou porphyry–skarn Cu–Mo deposit, eastern Yangtze craton, Eastern China: geochronological, geochemical, and Sr–Nd–Hf isotopic constraints

The Tongshankou Cu–Mo deposit, located in the westernmost Daye district of the Late Mesozoic Metallogenic Belt along the Middle-Lower reaches of the Yangtze River, eastern China, consists mainly of porphyry and skarn ores hosted in the Tongshankou granodiorite and along the contact with the Lower Triassic marine carbonates, respectively. Sensitive high-resolution ion microprobe zircon U–Pb dating constrains the crystallization of the granodiorite at 140.6 ± 2.4 Ma (1σ). Six molybdenite samples from the porphyry ores yield Re–Os isochron age of 143.8 ± 2.6 Ma (2σ), while a phlogopite sample from the skarn ores yields an 40Ar/39Ar plateau age of 143.0 ± 0.3 Ma and an isochron age of 143.8 ± 0.8 Ma (2σ), indicating an earliest Cretaceous mineralization event. The Tongshankou granodiorite has geochemical features resembling slab-derived adakites, such as high Sr (740–1,300 ppm) and enrichment in light rare earth elements (REE), low Sc (<10 ppm), Y (<13.3 ppm), and depletion in heavy REE (<1.2 ppm Yb), and resultant high Sr/Y (60–92) and La/Yb (26–75) ratios. However, they differ from typical subduction-related adakites by high K, low MgO and Mg#, and radiogenic Sr–Nd–Hf isotopic compositions, with (87Sr/86Sr)t = 0.7062–0.7067, ɛNd(t) = −4.37 to −4.63, (176Hf/177Hf)t = 0.282469–0.282590, and ɛHf(t) = −3.3 to −7.6. The geochemical and isotopic data, coupled with geological analysis, indicate that the Tongshankou granodiorite was most likely generated by partial melting of enriched lithospheric mantle that was previously metasomitized by slab melts related to an ancient subduction system. Magmas derived from such a source could have acquired a high oxidation state, as indicated by the assemblage of quartz–magnetite–titanite–amphibole–Mg-rich biotite in the Tongshankou granodiorite and the compositions of magmatic biotite that fall in the field between the NiNiO and magnetite–hematite buffers in the Fe3+–Fe2+–Mg diagram. Sulfur would have been present as sulfates in such highly oxidized magmas, so that chalcophile elements Cu and Mo were retained as incompatible elements in the melt, contributing to subsequent mineralization. A compilation of existing data reveals that porphyry and porphyry-related Cu–Fe–Au–Mo mineralization from Daye and other districts of the Metallogenic Belt along the Middle-Lower reaches of the Yangtze River took place coevally in the Early Cretaceous and was related to an intracontinental extensional environment, distinctly different from the arc-compressive setting of the Cenozoic age that has been responsible for the emplacement of most porphyry Cu deposits of the Pacific Rim.

Formation of Distinct Granitic Magma Batches by Partial Melting of Hybrid Lower Crust in the Izu Arc Collision Zone, Central Japan

The Miocene Kofu Granitic Complex (KGC) occurs in the Izu Collision Zone where the Izu^Bonin^Mariana (IBM) arc has been colliding with the Honshu arc since the middle Miocene.The KGC includes rocks ranging in compositions from biotite-bearing granite (the Shosenkyo and Mizugaki plutons), and hornblende^biotitebearing granodiorite, tonalite, quartz-diorite, and granite (the Shiodaira, Sanpo, Hirose and Sasago plutons), to hornblende-bearing tonalite and trondhjemite (the Ashigawa^Tonogi pluton), indicating that it was constructed from multiple intrusions of magma with different bulk chemistry. The Sr-isotopic compositions corrected to sensitive high-resolution ion microprobe (SHRIMP) zircon ages (SrI) suggest that the primary magmas of each pluton were formed by anatexis of mixed lower crustal sources involving both juvenile basalt of the IBM arc and Shimanto sedimentary rocks of the Honshu arc. After the primary magmas had formed, the individual plutons evolved by crystal fractionation processes without significant crustal assimilation or additional mantle contribution. SHRIMP zircon U^Pb ages in the KGC range from 16 8 to 10 6Ma and overlap the resumption of magmatic activity in the IBMand Honshu arcs at c. 17Ma and the onset of IBM arc^Honshu arc collision at c. 15Ma.The age of the granite plutons is closely related to the episodic activity of arc magmatism and distinct granitic magma batches could be formed by lower crustal anatexis induced by intrusion of underplated mantle-derived arc magmas. Based on pressures determined with the Al-in-hornblende geobarometer, the KGC magmas intruded into the middle crust. Thus, the KGC could represent an example of the middle-crust layer indicated throughout the IBM arc by 6 0^6 5 km/s seismic velocities. This granitic middle-crust layer acted buoyantly during the IBM arc^Honshu arc collision, leading to accretion of buoyant IBMarc middle crust to the Honshu arc.

Carboniferous granitic plutons from the northern margin of the North China block: implications for a late Palaeozoic active continental margin

We report four late Palaeozoic zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb ages for granitic plutons from the Inner Mongolia Palaeo-uplift on the northern margin of the North China block. These cast a new light on the poorly understood tectonic history of the northern margin of the North China block and the Central Asian Orogenic Belt during the late Palaeozoic. The plutons have for a long time been considered to belong to the early Precambrian basement of the North China block. Our new SHRIMP U–Pb zircon dating of four plutons at Longhua, Daguangding, Boluonuo and Hushiha has yielded intrusive ages of 311 ± 2 Ma, 324 ± 6 Ma, 302 ± 4 Ma and 310 ± 5 Ma, respectively. Geochemical data suggest that these granitoids have a calc-alkaline, subduction-related I-type signature, indicating the existence of an Andean-style continental arc along the northern margin of the North China block during the late Palaeozoic. Our results also indicate that the Palaeo-Asian Ocean still existed during latest Carboniferous–earliest Permian time, and that the final collision between the southern Mongolia composite terranes and the North China block occurred later than c . 290 Ma. We suggest that the northern margin of the North China block was an active continental margin and the Inner Mongolia Palaeo-uplift is a deeply exhumed mid-crustal ‘root’ of a late Palaeozoic Andean-style continental arc.

Origin of the Mesozoic magmatism in the North China Craton: constraints from petrological and geochemical data

Abstract Voluminous plutonic and volcanic rocks were emplaced in the eastern part of the North China Craton (NCC) in the Mesozoic. The Mesozoic igneous rocks include a variety of rock types ranging from monzogabbroic, through monzonitic to monzogranitic, and locally to syenitic. Monzonitic rocks are dominant, and frequently contain mafic enclaves (dioritic in composition). The principal geochemical signatures of these Mesozoic rocks include high-K calc-alkaline to shoshonitic affinity, high Sr–Ba abundances and high Sr/Y, La/Yb, and highly enriched Sr–Nd isotopic compositions with ε Nd ( t ) ranging from −8 to −20 and I Sr from 0.7053 to 0.710. Zircon sensitive high-resolution ion microprobe dating reveals that these Mesozoic rocks formed between 180 Ma and 120 Ma, but are predominantly confined to a narrow range of 135–127 Ma. The sudden surge of Mesozoic magmatism was genetically linked to the upwelling of asthenosphere in a back-arc extensional regime that was caused by subduction of the palaeo-pacific plate beneath the eastern NCC. Upwelling of hot asthenospheric mantle material triggered partial melting of enriched subcontinental lithosperic mantle, generating voluminous mafic magmas. The mafic magmas underplated in the lower crust and sparked melting of the latter, producting granitic melts. We suggest that the Mesozoic rocks in the NCC probably originated from mixing between the coeval mafic and granitic melts, followed by fractionation of ferromagnesian phases and subordinate plagioclase, rather than from melting of mafic lower crust as previously suggested by many others.

Fluid evolution and large-scale gold metallogeny during Mesozoic tectonic transition in the Jiaodong Peninsula, eastern China

Abstract The Jiaodong Peninsula or eastern Shandong Province, the most important gold producing in region China, is located in the southeastern margin of the North China Craton. The gold deposits in the Jiaodong Peninsula are divided into three gold belts, from west to east, the Zhaoyuan–Laizhou, Penglai–Qixia and Muping–Rushan belts. The deposits occur as gold-bearing quartz veins and disseminated- and stockwork-style ores adjacent to fault zones. Most of the gold deposits can be classified in four stages: stage I, quartz–(minor) pyrite; stage II, pyrite–quartz–gold; stage III, quartz–base metal sulphide minerals; stage IV, quartz–carbonate. Ar–Ar ages, Rb–Sr isochrons, and hydrothermal zircon sensitive high-resolution ion microprobe U–Pb ages obtained from these deposits suggest a gold mineralization time of 120±10 Ma. The Sr–Nd isotopic compositions of pyrites and the associated rocks suggest that the ore-forming materials were probably derived from a mixed source. Fluid inclusion studies show that ore-forming fluids of gold deposits are consistent throughout the Jiaodong Peninsula, with similar mineralizing temperature and pressure conditions. Ore-forming fluids are characterized by H 2 O–CO 2 –NaCl±CH 4 . The optimal mineralizing temperature and pressure ranges are 170–335 °C and 0.7–2.5 kbar. Oxygen and hydrogen isotope data show that ore fluids are of magmatic origin. Gold deposits in the Jiaodong Peninsula formed in the same mineralizing–geodynamic conditions, and are related to the Mesozoic tectonic transition in the eastern North China Craton. Gold metallogeny is only one expression of the Mesozoic tectonic transition.

Miocene to Holocene landscape evolution of the western Snake River Plain region, Idaho: Using the SHRIMP detrital zircon provenance record to track eastward migration of the Yellowstone hotspot

We report new U-Pb detrital zircon sensitive high-resolution ion microprobe (SHRIMP) age data (702 grains) from 13 samples collected from Miocene to Holocene sedimentary deposits in the western Snake River Plain region. These samples effectively show that modern stream sediments of the Snake River system reliably and repeatedly record the detrital zircon age populations that are present as sources in their drainage basins across the Cordilleran thrust belt and Basin and Range Province. We use this framework and the provenance of Neogene sedimentary rocks in the region to test the effect of the migrating Yellowstone hotspot on regional drainage patterns in southern Idaho since the middle Miocene. Our results indicate that Neogene paleodrainages were fi rst directed radially away from the tumescent Yellowstone highland, then subsequently reversed their fl ow toward the subsiding Snake River Plain basin. This occurred in east-progressing time-constrained intervals starting at 16 Ma. In northern Nevada, the drainage divide is represented by a northeast-trending, southeast-migrating crest of high topography. Specifi cally, middle to late Miocene (16‐ 10 Ma) sedimentary deposits of the western Snake River Plain and Oregon-Idaho graben contain early to middle Eocene (52‐42 Ma) detrital zircon populations sourced in Challis magmatic rocks north of the Snake River Plain. Middle Jurassic (160 Ma) and middle to late Eocene (42‐35 Ma) detrital zircons, sourced from rocks in northern Nevada, are not present. Late Eocene detrital zircons from Nevada are present in two younger than 7 Ma sedimentary units of the Idaho Group along the Oregon-Idaho border. This indicates that by the late Miocene, southeastward headward erosion of the paleo‐Owyhee River into the Owyhee Plateau had captured drainage from north-central Nevada and directed it northwestward toward the subsiding western Snake River Plain. The modern Owyhee Plateau is still a topographic high, in contrast to the modern Snake River Plain, suggesting that lowering of the regional Snake River Plain base level, rather than crustal subsidence, drove stream capture. By the late Pliocene (3 Ma), Middle Jurassic detrital zircons are recorded in the Glenns Ferry Formation and Tuana Gravel of the central Snake River Plain, suggesting that surface subsidence reversed the fl ow direction of paleo‐Salmon Falls Creek from southward into Nevada to northward toward Idaho. Miocene strata of the western Snake River Plain lack recycled Proterozoic detrital zircons that are ubiquitous in sedimentary rocks of the central and southeast Idaho thrust belts. Such detrital zircons appear on the central and western Snake River Plain in early Pliocene to Holocene (4‐0 Ma) deposits. This records capture of drainage from the eastern Snake River Plain. The Yellowstone hotspot controlled the east-migrating continental divide, in the wake of which formed the western-draining, and progressively eastward-collecting, Snake River system.

SHRIMP Zircon U-Pb Age, Geochemistry, and Nd-Sr Isotopes of the Gaojiacun Mafic-Ultramafic Intrusive Complex, Southwest China

The Gaojiacun mafic-ultramafic intrusive complex in the Yanbian area, Sichuan Province, Southwest China, is a layered intrusive body that underwent intensive magmatic differentiation during two cumulative cycles. SHRIMP (sensitive high-resolution ion microprobe) zircon U-Pb data show that the Gaojiacun complex was formed at 825 ± 12 Ma. These mafic-ultramafic rocks are relatively enriched in light rare-earth elements (LREE) and large-ion lithophile elements (LILE), but are relatively depleted in high-field-strength elements (HFSE). They are characterized by low initial 87Sr/86Sr (0.7045 to 0.7050) and positive εNd(t) values (+1.3 to +4.5). The geochemical data indicate that the parental magma of the mafic/ultramafic rocks was derived from a depleted mantle source, and underwent fractional crystallization and crustal contamination. We suggest that the Gaojiacun complex was formed in a continental rift related to mantle superplume activity beneath the Neoproterozoic supercontinent Rodinia.

New insights into the history and origin of the southern Maya block, SE México: U–Pb–SHRIMP zircon geochronology from metamorphic rocks of the Chiapas massif

The histories of the pre-Mesozoic landmasses in southern Mexico and their connections with Laurentia, Gondwana, and among themselves are crucial for the understanding of the Late Paleozoic assembly of Pangea. The Permian igneous and metamorphic rocks from the Chiapas massif as part of the southern Maya block, Mexico, were dated by U–Pb zircon geochronology employing the SHRIMP (sensitive high resolution ion microprobe) facility at Stanford University. The Chiapas massif is composed of deformed granitoids and orthogneisses with inliers of metasedimentary rocks. SHRIMP data from an anatectic orthogneiss demonstrate that the Chiapas massif was part of a Permian (∼ 272 Ma) active continental margin established on the Pacific margin of Gondwana after the Ouachita orogeny. Latest Permian (252–254 Ma) medium- to high-grade metamorphism and deformation affected the entire Chiapas massif, resulting in anatexis and intrusion of syntectonic granitoids. This unique orogenic event is interpreted as the result of compression due to flat subduction and accretionary tectonics. SHRIMP data of zircon cores from a metapelite from the NE Chiapas massif yielded a single Grenvillian source for sediments. The majority of the zircon cores from a para-amphibolite from the SE part of the massif yielded either 1.0–1.2 or 1.4–1.5 Ga sources, indicating provenance from South American Sunsas and Rondonian-San Ignacio provinces.

Magmatic and Hydrothermal Chronology of the Giant Rio Blanco Porphyry Copper Deposit, Central Chile: Implications of an Integrated U-Pb and 40Ar/39Ar Database

The history of hypabyssal intrusion and hydrothermal activity in the northeastern and central parts of the be-hemothian (sensu Clark, 1993) Rio Blanco-Los Bronces porphyry copper-molybdenum deposit is clarified on the basis of integrated U-Pb and 40Ar/39Ar geochronology. Isotope dilution thermal ion mass spectrometry (ID-TIMS) U-Pb dates for zircon separates and ID-TIMS and sensitive high resolution ion microprobe (SHRIMP) dates for single zircon grains in pre-, syn- and late-mineralization volcanic and intrusive host rocks in the Rio Blanco, Don Luis, and Sur-Sur mining sectors provide a temporal framework for interpretation of incremental-heating and spot-fusion 40Ar/39Ar dates for, respectively, magmatic biotite and hydrothermal biotite, muscovite, and orthoclase. The ore deposit is hosted in part by 16.77 ± 0.25 to 17.20 ± 0.05 (2σ) Ma andesitic volcanic strata of the Farellones Formation, but the major host rocks are units of the San Francisco batholith, including the 11.96 ± 0.40 Ma Rio Blanco granodiorite (mine terminology), the 8.40 ± 0.23 Ma Cascada granodiorite, and the 8.16 ± 0.45 Ma diorite. Hypabyssal dacitic intrusions (late porphyries) emplaced into the batholith yield 206Pb/238U ID-TIMS dates ranging from 6.32 ± 0.09 Ma (quartz monzonite porphyry), through 5.84 ± 0.03 Ma (feldspar porphyry) to 5.23 ± 0.07 Ma (Don Luis porphyry). The late-mineralization Rio Blanco dacite plug yields a SHRIMP zircon age of 4.92 ± 0.09 Ma. The 40Ar/39Ar plateau ages for phenocrystic biotites in quartz monzonite porphyry, feldspar porphyry, and Don Luis porphyry, as well as the preore diorite, range only from 5.12 ± 0.07 to 4.57 ± 0.06 Ma. All are significantly younger than the corresponding zircons and exhibit no correlation with intrusive sequence. The 40Ar/39Ar ages for hydrothermal biotite and orthoclase veins within the San Francisco batholith units fall in a narrow interval from 5.32 ± 0.27 to 4.59 ± 0.11 Ma. Hydrothermal sericites (muscovite), one associated with chalcopyrite, yielded spot-fusion ages of 4.40 ± 0.15 Ma (Rio Blanco granodiorite hosted) and 4.37 ± 0.06 Ma (Don Luis porphyry hosted). Comparison with the ID-TIMS and SHRIMP zircon ages indicates that most of the 40Ar/39Ar ages, even 95 percent plateaus, do not record initial magmatic cooling or hydrothermal alteration-mineralization events, evidence for quasipervasive reheating to at least 300°C by successive intrusions. Published Re-Os ages for two molybdenite samples range from 5.4 to 6.3 Ma and overlap extensively with the zircon U-Pb ages for the late porphyries. They imply that Cu-Mo mineralization overlapped temporally with the emplacement of, at least, quartz monzonite porphyry and feldspar porphyry units of the late porphyry suite and was, therefore, contemporaneous with the rise of dacitic melts to subvolcanic levels. Hydrothermal activity is inferred to have continued until 4.37 ± 0.06 Ma, following intrusion of the Don Luis porphyry and the early stages of emplacement of the Rio Blanco dacite plug complex. Hypogene Cu-Mo mineralization therefore probably persisted for 2 m.y. The geochronologic data do not resolve whether ore formation was continuous or episodic, but the observed crosscutting relationships between intensely altered and mineralized country rocks and less altered and mineralized late porphyry bodies support a model in which the ascent of metal-rich brines from an unexposed zone of the parental magma chamber was periodically stimulated by magma perturbation and hypabyssal intrusion.

Alkaline syenites in eastern Cathaysia (South China): link to Permian–Triassic transtension

Two alkaline syenite plutons, the Tieshan and Yangfang plutons, have recently been recognized within NE-trending fault zones in eastern Cathaysia, South China. The rocks are very enriched in K 2O (6.28–9.39 wt.%), rare earth elements (REE; particularly light REE) and large ion lithophile elements, but are relatively low in high field strength elements. Isotopically, they are characterized by high initial 87Sr/ 86Sr (0.7093 to 0.7123) and low ɛ Nd( t) values (−5.64 to −10.63). The geochemical data suggest that the alkaline syenites most likely formed via fractional crystallization of enriched mantle-derived magmas. Sensitive High-Resolution Ion Microprobe zircon U–Pb dating indicates that these two intrusions have Late Permian (254±4 Ma) and Early Triassic (242±4 Ma) crystallization ages, respectively. Our data suggest that a tectonic regime dominated by transtension probably existed from at least the latest Permian into the Triassic and was responsible for the formation of the Tieshan and Yangfang alkaline syenites. When combined with previous paleomagnetic, structural, and sedimentology data, we suggest that the transtension along the NE-trending strike-slip fault zones was related to oblique subduction of the Pacific plate underneath South China.

Grenville-age magmatism at the South Tasman Rise (Australia): A new piercing point for the reconstruction of Rodinia

Research Article| October 01, 2005 Grenville-age magmatism at the South Tasman Rise (Australia): A new piercing point for the reconstruction of Rodinia A.M. Fioretti; A.M. Fioretti 1Consiglio Nazionale delle Ricerche (CNR), Istituto di Geoscienze e Georisorse, Corso Garibaldi 37, I-35137 Padova, Italy, and Visiting Fellows at Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar L.P. Black; L.P. Black 2Minerals Division, Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia, and Visiting Fellows at Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar J. Foden; J. Foden 3Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5005, Australia Search for other works by this author on: GSW Google Scholar D. Visonà D. Visonà 4Dipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi 37, I-35137 Padova, Italy Search for other works by this author on: GSW Google Scholar Author and Article Information A.M. Fioretti 1Consiglio Nazionale delle Ricerche (CNR), Istituto di Geoscienze e Georisorse, Corso Garibaldi 37, I-35137 Padova, Italy, and Visiting Fellows at Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia L.P. Black 2Minerals Division, Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia, and Visiting Fellows at Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia J. Foden 3Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5005, Australia D. Visonà 4Dipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi 37, I-35137 Padova, Italy Publisher: Geological Society of America Received: 02 Mar 2005 Revision Received: 03 May 2005 Accepted: 03 May 2005 First Online: 03 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2005) 33 (10): 769–772. https://doi.org/10.1130/G21671.1 Article history Received: 02 Mar 2005 Revision Received: 03 May 2005 Accepted: 03 May 2005 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A.M. Fioretti, L.P. Black, J. Foden, D. Visonà; Grenville-age magmatism at the South Tasman Rise (Australia): A new piercing point for the reconstruction of Rodinia. Geology 2005;; 33 (10): 769–772. doi: https://doi.org/10.1130/G21671.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract A U-Pb zircon sensitive high-resolution ion microprobe age of 1119.4 ± 8.5 Ma, obtained from a quartz syenite dredged from the South Tasman Rise (Australia), provides the first direct evidence of the presence of Grenville-age magmatic rocks along the central part of the hypothesized Australia–East Antarctica conjugate margin of Laurentia. The distinctive mineralogy and geochemistry of the rock and its Sm-Nd and Pb isotopic signatures 1) indicate that it represents a juvenile Grenville-age addition to the crust, 2) support a correlation with the Grenville magmatic province of the western United States, and 3) set a unique pivotal point for a precise reconstruction of Rodinia. The resulting scenario implies the presence of a new magmatic province crossing the East Antarctic craton, the extension of Proterozoic belts of southwest Laurentia to East Antarctica, and appears consistent with the Australia–western United States (AUSWUS) model. This tectonic setting envisages a near-local source for the ubiquitous Grenville-age detrital zircon population in marginal Neoproterozoic to early Paleozoic Gondwanan sequences and suggests a possible direct source for the widespread Grenville-age inherited zircon component observed in most Paleozoic granites in northern Victoria Land (Antarctica) and east Tasmania (Australia). You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

North-south extension in the Tibetan crust triggered by granite emplacement

We combine zircon sensitive high-resolution ion microprobe U-Pb spot dating and mica 4 0 Ar- 3 9 Ar plateau ages with field-geological and geochemical constraints from the Malashan area of Southern Tibet to show that the deformed granite core of the North Himalayan metamorphic domes in this area is not Indian basement, but was intruded and deformed during the Himalayan orogeny. Microstructural observations reveal that a transition from top-to-the-south thrust-related to top-to-the-north extension-related deformation occurred during granite intrusion and related metamorphism. This suggests that intrusion triggered the onset of extensional tectonics in the Tibetan middle to upper crust. Expected positive feedback mechanisms between decompression melting leading to more intrusion and more extensional deformation suggest that this mechanism may have been important on a regional scale.

K-bentonites in the Argentine Precordillera contemporaneous with rhyolite volcanism in the Famatinian Arc

New U–Pb radiometric dates for K-bentonite horizons within the Lower Cambrian to Middle Ordovician platform carbonates from the Precordillera terrane of NW Argentina provide further constraints on models for the allochthonous or parautochthonous accretion of this terrane. Two K-bentonite layers from the Talacasto section yield indistinguishable sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon dates of 469.5 ± 3.2 Ma and 470.1 ± 3.3 Ma respectively. These are within uncertainty of the U–Pb SHRIMP zircon date of 468.3 ± 3.4 Ma for a porphyritic rhyolite from the Famatinian magmatic arc, Sierra de las Planchadas, near Rio Chaschuil. Geochemical and isotope data also demonstrate the similarity of the K-bentonite and Chaschuil rhyolite parent magmas. Thus, it is highly probable that the Famatinian arc volcanoes provided the ash for the K-bentonite horizons, suggesting proximity to the Precordillera terrane during the deposition of the Lower Cambrian to Middle Ordovician platform carbonates. This implication supports a mid-Ordovician collision model, but could also be compatible with a parautochthonous model for docking of the Precordillera terrane, by movement along the Pacific margin of Gondwana, rather than across the Iapetus Ocean.

U–Pb geochronology of late Neoproterozoic augen granites in the Moine Supergroup, NW Scotland: dating of rift-related, felsic magmatism during supercontinent break-up?

Within the Caledonides of Caithness, Scotland, the Neoproterozoic metasedimentary rocks of the Moine Supergroup are intruded by minor sheets of strongly deformed granite. U–Pb sensitive high-resolution ion microprobe zircon ages of 599 ± 9 Ma (Berriedale augen granite) and 588 ± 8 Ma (Braeval augen granite) are interpreted to date emplacement during the late Neoproterozoic. These augen granites are therefore unrelated to either Knoydartian ( c . 870–790 Ma) or Caledonian ( c . 470–420 Ma) orogenesis in the Scottish Highlands. Intrusion was, however, broadly contemporaneous with late Neoproterozoic extension of the Laurentian margin during continental break-up and the opening of the Iapetus Ocean. In the context of evidence for rift-related mafic magmatism within the Dalradian basin in Scotland at c . 600 Ma, as well as contemporaneous anorogenic magmatism along the margins of the developing Iapetus Ocean in the Appalachians, we propose that the protoliths of the augen granites were probably emplaced during continental break-up. The new data broaden the extent of this late Neoproterozoic magmatic event in the Scottish Highlands: other deformed granites that are at present undated but have been assumed to be of Caledonian age therefore require reinvestigation.