SHRIMP Zircon Ages Research Articles

Cambrian Zawidów granodiorites in the Cadomian Lusatian Massif (Central European Variscides): what do the SHRIMP zircon ages mean?

The Lusatian Massif in the Central European Variscides, composed of upper Neoproterozoic (c. 570 Ma) greywacke intruded by c. 540 Ma old Cambrian granodiorites (and the somewhat younger Zawidow granodiorites), constitutes a fragment of the Cadomian basement of the Saxo-Thuringian Zone. The Lusatian Massif adheres on the east to the Karkonosze-Izera Massif composed of the c. 500 Ma Izera/Rumburk granites related to the Cambro-Ordovician rifting of the Cadomian basement, and narrow belts of micaschists. Trace-element and Sm-Nd isotope data suggest that the source rocks for the Lusatian greywacke, the Zawidow granodiorite and the Izera/Rumburk granite could have been similar, though not the same. The new SHRIMP U-Pb zircon data for the Zawidow granodiorite reveal, apart from the expected c. 540 Ma ages, three other zircon age populations of around 630 Ma, 600 Ma and 510 Ma, the latest being evidently younger than 540 Ma that was considered as the age of the granodiorite magma emplacement. Zircons with 206Pb/238U ages around 510 Ma, and somewhat older (up to c. 538 Ma), have also been reported from the Izera granites. This could mean that the granitic plutonism related to the Cadomian orogenic cycle and the Cambro-Ordovician rifting triggered two or more magmatic pulses during at least c. 30 My. During the prolonged period of igneous activity, the plate-tectonic environment at the Gondwana margin changed from collisional (Cadomian Orogeny), to initial rifting (Cambro-Ordovician).

Concordant U–Pb SHRIMP ages of U-rich zircon in granitoids from the Muruntau gold district (Uzbekistan): Timing of intrusion, alteration ages, or meaningless numbers

For decades, the U–Pb isotope system of zircon is routinely used in geosciences for the determination of intrusion ages of felsic magmatic rocks. It is, however, well-known that zircon and the related U–Pb isotope system may be influenced by secondary hydrothermal alteration and new (hydrothermal) zircon growth. As a result, age determination may yield meaningless or alteration ages. An example is given here for intensely altered late Paleozoic granitoid rocks from the Muruntau area, Central Kyzyl Kum, Uzbekistan. Pb–Pb single grain evaporation and U–Pb SHRIMP isotope systematics of zircon from six granitoid rocks yield roughly consistent results defining ages ranging between 288 and 303Ma. At first glance, these ages may be understood as intrusion ages of the magmatic rocks hosting the zircon. However, there is an apparent contradiction between the sequence of the measured U–Pb SHRIMP ages and the relative age sequences established in the field. Evaluation of data from comprehensive mineralogical and petrological studies reveals that the U–Pb system of magmatic zircon is complicated by inheritance in I-type granitoids, recrystallization, new hydrothermal zircon growth, and subsequent secondary alteration. Widespread albitization of the granitoids led to the formation of new, U-rich, hydrothermal zircon forming overgrowths on older grains or even new whole single crystals. Later alteration events caused direct and (apparent) reverse U–Pb isotope discordances. These late disturbances of the U–Pb system are mainly due to widespread sericitization. Consequently, precise concordant U–Pb zircon SHRIMP ages around 290–294Ma defined for U-rich, largely undisturbed crystal areas constrain the timing of albitization rather than that of the intrusion. The reliability of the concordant U–Pb SHRIMP ages measured on U-rich zircon is confirmed by a U–Pb SHRIMP age of hydrothermal monazite (292±8Ma). This monazite associates with extremely U-rich zircon (U–Pb SHRIMP age: 291±3Ma) in a sample of the Murun granite recovered beneath the giant Muruntau gold deposit by super-deep drilling. Obviously, detailed mineralogical studies are important for a correct interpretation of zircon ages when hydrothermal alteration and late re-mobilization of zirconium and uranium were significant as often observed in areas with intense ore mineralization. The extensive Au mineralization found at Muruntau is similar in age to albitization but apparently somewhat younger. Likewise, the intrusion age of Permian granites is at least somewhat older than the hydrothermal zircon ages confirmed by U–Pb ages of monazite. It is, however, difficult to constrain further granite intrusion ages based on presently available data.

Zircon ages and geochemical data of the Biluutiin ovoo ophiolite: implication for the tectonic evolution of South Mongolia

The early Palaeozoic tectonic evolution of South Mongolia is not well constrained due to the limited exposure of early Palaeozoic rocks in the area and the scarcity of both geochemical and geochronological data. In order to help rectify this situation, we have conducted detailed studies on the Biluutiin ovoo ophiolite in South Mongolia to provide constraints on the tectonic evolution of the region during this period. The Biluutiin ovoo ophiolite consists of ultramafic rocks, mylonitic gabbro, basalt, tuff sandstone, plagiogranite, calcite, and chert. Gabbro and plagiogranite samples from the ophiolitic complex yielded SHRIMP zircon ages of 525 ± 5 Ma and 503 ± 6 Ma, respectively. Biluutiin ovoo ophiolitic basalts display LREE and LILE enrichment and strong HFSE depletion, indicating that the ophiolite is supra-subduction zone (SSZ) type. Plagiogranite with adakite-like geochemical compositions suggests that palaeo-ocean subduction occurred in South Mongolia during Cambrian time. Intruding granite yielded a SHRIMP zircon age of 353 ± 2 Ma, indicating that the ophiolite was emplaced before early Carboniferous time. Identification of the Cambrian ophiolitic complex and the occurrence of Cambrian adakites indicate that southern Mongolia underwent a period of active volcanism during the Cambrian. The Cambrian formations are likely correlated to the early Palaeozoic subduction-accretion belt of Western Mongolia.

Age, composition, and source of continental arc- and syn-collision granites of the Neoproterozoic Sergipano Belt, Southern Borborema Province, Brazil

The Sergipano belt is the outcome of collision between the Pernambuco-Alagoas Domain (Massif) and the São Francisco Craton during Neoproterozoic assembly of West Gondwana. Although the understanding of the Sergipano belt evolution has improved significantly, the timing of emplacement, geochemistry and tectonic setting of granitic bodies in the belt is poorly known. We recognized two granite age groups: 630–618 Ma granites in the Canindé, Poço Redondo and Macururé domains, and 590–570 Ma granites in the Macururé metasedimentary domain. U–Pb SHRIMP zircon ages for granites of first age group indicated ages of 631 ± 4 Ma for the Sítios Novos granite, 623 ± 7 Ma for the Poço Redondo granite, 619 ± 3.3 Ma for the Lajedinho monzodiorite, and 618 ± 3 Ma for the Queimada Grande granodiorite. These granitoids are dominantly high-K calc-alkaline, magnesian, metaluminous, mafic enclave-rich (Queimada Grande and Lajedinho), or with abundant inherited zircon grains (Poço Redondo and Sitios Novos). Geochemical and isotope data allow us to propose that Sítios Novos and Poço Redondo granites are product of partial melting of Poço Redondo migmatites. Sr-Nd isotopes of the Queimada Grande granodiorite and Lajedinho monzodiorite suggest that their parental magma may have originated by mixing between a juvenile mafic source and a crustal component that could be the Poço Redondo migmatites or the Macururé metasediments. Other 630–618 Ma granites in the belt are the mafic enclave-rich Coronel João Sá granodiorite and the Camará tonalite in the Macururé sedimentary domain. These granites have similar geochemical and isotopic characteristics as the Lajedinho and Queimada Grande granitoids. We infer for the Camará tonalite and Coronel João Sá granodiorite that their parental magmas have had contributions from mafic lower crust and felsic upper crust, most probably from underthrust São Francisco Craton, or Pernambuco-Alagoas Domain. The younger 590–570 Ma granite group is confined to the Macururé metasedimentary domain. Although these granites do not show typical features of S-type granites, their U–Pb age, field relationships, geochemical and Sr-Nd data suggest that their parental magmas have originated from high degree melting of the Macururé micaschists. Field observations support a model in which the Macururé domain, limited by the Belo Monte-Jeremoabo and São Miguel do Aleixo shear zones, behaved as a ductile channel flow for magma migration and emplacement during the Neoproterozoic, very much like the channel flow model proposed for emplacement of leucogranites in the Himalayas.

The Sierra de Cachi (Salta, NW Argentina): geological evidence about a Famatinian retro-arc at mid crustal levels

The Cachi mountain ranges, located in Salta (NW Argentina), form an inlier where Neoproterozoic and Paleozoic materials of the An­des crop out in a plutono-metamorphic dome surrounded by Mesozoic and Cenozoic sedimentary rocks. The pre-Mesozoic rocks record a regional metamorphism of low pressure and high temperature conditions, which reached partial melting conditions and was coeval with the intrusion of sheeted granite and trondhjemite plutons. The metamorphic evolution and the emplacement of the plutons took place in an extensional tectonic setting. New U-Pb zircon dating by TIMS and SHRIMP methods yields similar ages for the migmatization (479.7 ± 3.5 Ma) and the emplacement of granites (472.1 ± 11 Ma), constraining the extensional event to Early Ordovician times. We also report the finding of gabbro boulders, which also yield a similar U-Pb SHRIMP zircon age of 477.5 ± 3.9 Ma. So, we propose that the extensional event recognized in Sierra de Cachi took place in a retro-arc setting linked to the Famatinian subduction along the western border of Gond­wana. The shortening produced by younger east-verging folds evidences the superposition of a compressional stress field, probably related with the Ocloyic orogeny (Middle/Late Ordovician-Devonian).

Mineralization Styles and Genesis of the Yinkeng Au‐Ag‐Pb‐Zn‐Cu‐Mn Polymetallic Orefield, Southern Jiangxi Province, SE China: Evidence from Geology, Fluid Inclusions, Isotopes and Chronology

AbstractThe Yinkeng orefield in Yudu County, Jiangxi Province, SE China, is a zone of concentrated Au‐Ag‐Pb‐Zn‐Cu‐Mn polymetallic ores. Based on summing up basic geology and ore geology of the orefield, the polymetallic deposits in the orefield have been divided into seven major substyles according to their occurring positions and control factors. The ore‐forming fluid inclusion styles in the orefield include those of two‐phase fluid, liquid CO2‐bearing three‐phase and daughter mineral‐bearing multi‐phase. The homogenization temperatures range from 382° to 122°C, falling into five clusters of 370° to 390°, 300° to 360°, 230° to 300°, 210° to 290° and 120° to 200°, and the clusters of 300° to 360°, 230° to 300° and 120° to 200° are three major mineralization stages, with fluid salinity peaks from 4.14% to 7.31%, 2.07% to 7.31% and 0.53% to 3.90%, respectively. The ore‐forming fluids are mainly type of NaCl‐H2O with medium to high density (0.74–1.02 g/cm3), or CO2‐bearing NaCl‐H2O with medium to low density (0.18–0.79 g/cm3). The fluid salinity and density both show a decline tendency with decreasing temperature. According to the measurement and calculation of H‐ and O‐isotopic compositions in the quartz of the quartz‐sulfide veins, δDV‐SMOW of the ore‐forming fluid is from –84‰ to –54‰, and δ18OV‐SMOW of that is from 6.75‰ to 9.21‰, indicating a magmatic fluid. The δ34SV‐SDT of sulfides in the ores fall into two groups, one is from –4.4‰ to 2.2‰ with average of –1.42‰, and the other from 18.8‰ to 21.6‰ with average of 19.8‰. The S‐isotopic data shows one peak at –4.4‰ to 2.2‰ (meaning –1.42‰) suggesting a simple magmatic sulfur source. The ore Pb‐isotopic ratios are 206Pb/204Pb from 17.817 to 17.983, 207Pb/204Pb from 15.470 to 15.620 and 208Pb/204Pb from 38.072 to 38.481, indicating characteristics of mantle‐derived lead. The data show that the major ore deposits in the orefield have a magmatic‐hydrothermal genesis and that the SHRIMP zircon age of the granodiorite porphyry, closely related to the mineralization, is 151.2±4.2 Ma (MSWD = 1.3), which can represent the formation ages of the ores and intrusion rocks. The study aids understanding of the ore‐forming processes of the major metallic ore deposits in the orefield.

Geochronological constraints on the polycyclic magmatism in the Bou Azzer-El Graara inlier (Central Anti-Atlas Morocco)

New U–Pb SHRIMP zircon ages from the Bou Azzer-El Graara onlier constrains the Neoproterozoic evolution of the Anti-Atlas during Pan-African orogenesis. Within the Central Anti-Atlas, the Bou Azzer-El Graara inlier exposes a dismembered ophiolite, long considered to mark a late Neoproterozoic suture between the West African Craton in the south, and Neoproterozoic arcs to the north. From north to south, this inlier includes four main geological units: a volcanic-arc, an ophiolite, a metamorphic complex and a continental platform. Several plutons intrude the volcanic-arc, the ophiolite, the metamorphic complex, and post-orogenic volcanic and sedimentary deposits unconformably cover these terranes.The age of the volcanic-arc is reported here for the first time. Analyses of zircon of two rhyolites provide ages of 761±7Ma and 767±7Ma. Zircons from two gneisses provide dates of 755±9Ma and 745±5Ma. Both dates are considered best estimates of the crystallization ages of their igneous protoliths. Analyses of zircon from two granitic bodies, which crosscut gneisses, provide younger dates of 702±5Ma and 695±7Ma. The age of an aplitic body of the ophiolite is reported here for the first time, as 658±8Ma (SHRIMP U–Pb on zircons). Theses ages suggest the existence of three distinct orogenic events during Cryogenian times: (i) 770–760Ma Tasriwine-Tichibanine orogeny with rollback of the subducting oceanic plate, leading to the formation of back-arc basins; (ii) 755–695Ma Iriri-n’Bougmmane orogeny; and (iii) the 660–640Ma Bou Azzer orogeny involving the formation and the emplacement of the Bou Azer ophiolite.During Ediacaran times, the Bou Azzer-El Graara inlier is characterized with the development of a continental volcanic arc between 630 and 580Ma (Bou Lbarod Group, 625±8Ma; Bleïda granodiorite, 586±15Ma), and strike-slip pull-apart basins (Tiddiline Group, 606±4Ma and 606±5Ma). These volcanic and sedimentary Lower Ediacaran sequences are deformed before the felsic pyroclastic deposits of the Ouarzazate Group (567±5Ma and 566±4Ma). Finally, the Ouarzazate Group is overlain by early Cambrian volcanic deposits of the Jbel Boho Formation (541±6Ma).

Microfabrics and zircon U–Pb (SHRIMP) chronology of mylonites from the Patos shear zone (Borborema Province, NE Brazil)

Microstructures, U–Pb SHRIMP zircon ages and temperature estimates were used to constrain the fabric evolution and timing of mylonitisation in the Patos shear zone (NE Brazil). Melt-bearing HT-mylonitic orthogneiss displays solid-state fabrics with coarse quartz ribbons and sutured grain boundaries. K-feldspar is often fractured and shows peripheral myrmekite. The textures typically become magmatic at the contacts with the Espinho Branco anatexite. Zircon crystals from two samples of the leucosome show Paleoproterozoic (ca. 2.2Ga) inherited cores that are enveloped by recrystallised Neoproterozoic rims. These zircon grains define a discordia with lower intercept ages of 558 and 562Ma but with large analytical errors. A leucogranite with transitional contacts with host diatexites provides a well-constrained zircon mean age of 566±6Ma (n=15, 95% conf.), which is considered the best estimate for the peak metamorphic conditions. The transition from HT-mylonites to fine-grained mylonites and ultramylonites is marked by a progressive grain size reduction assisted by recrystallisation. Quartz crystallographic fabrics display [0001] concentrations between Z and Y. K-feldspar and plagioclase fabrics record the activity of the (010)[001] and (010)[100] slip systems and, in the lower-temperature tectonites, the (100)[010] slip system. These results suggest a diachronic evolution in which an initial HT northern domain was deformed by intracrystalline slip and melt-assisted granular flow, followed by medium- to low-temperature solid-state creep in the southern shear zone domain. The mylonites therefore record the rheological heterogeneities responsible for the formation of a late continental shear zone that nucleated under partially molten conditions and was later reactivated through melt-absent strain localization following exhumation.

Petrogenesis of 1000 Ma Felsic Tuffs, Chhattisgarh and Indravati Basins, Bastar Craton, India: Geochemical and Hf Isotope Constraints

AbstractThe 1000 Ma felsic tuffs that occur near the top of both the Chhattisgarh and Indravati Basins in peninsular India have been attributed to a “rhyolite flare-up” occasioned by the collision of parts of East Antarctica with India, in what is now the Eastern Ghats Mobile Belt, and the suturing of the North and South Indian cratonic blocks along the Central Indian Tectonic Zone, all during the assembly of Rodinia. In this study we present (1) new U-Pb SHRIMP ages of zircons from the granitic basement; (2) Hf isotopic compositions (laser ablation multicollector ICP-MS) from previously dated zircons from a number of tuff samples and a sample of the Archean basement (Bastar craton); and (3) major, minor, trace, and rare earth element compositions of these and a randomly collected additional set of samples from the Bastar craton basement. We have used these data to infer the petrogenesis of the tuffs. The Hf isotopic data preclude derivation of the parent magmas of the tuffs from simple melting of the bas...

Tectonic and deformation history of the Gyeonggi Massif in and around the Hongcheon area, and its implications in the tectonic evolution of the North China Craton

The Precambrian rocks of the Gyeonggi Massif (GM) in and around the Hongcheon area, South Korea, consist of the Yongduri Gneiss Complex (YGC), Euiam Group (EG) and Euiam Gneiss Complex (EGC). The YGC and EG are composed mainly of partially migmatised metasedimentary rocks and the EGC is Paleoproterozoic intrusive rock that intruded the EG. At least three major folding (F1, F2 & F3) accompanied with the two-stages of ductile shearing and three-stages of metamorphic events (M1, M2 & M3) occurred in the study area. The F1 folds are extremely drawn out, isoclinal, intrafolial folds and have Class 2 to Class 1C geometry. The F1 folds and regional S1 foliation in the YGC and EG are the results of E-W compression during the D1 deformation. Ductile shearing in the southern part of the EG is marked by the Palbongsan Shear Zone that indicates top-to-the SW sheared movement during syn to post-F1 folding. The F2 folds are open to tight, SW plunging and inclined folds, and have Class 1A to 1C geometry. The F2 folding and subsequent NNE thrusting along multiple ductile shear zone parallel to S2 planar fabrics, represent the D2 deformation due to N-S progressive shortening. The D3 deformation was coaxial with the D1 deformation, leading to the development of the F3 kink bands in the mylonite zones. The SHRIMP zircon ages form the EG yield M1 metamorphism age of 1917–1925Ma. However ca. 1867–1883Ma, M2 metamorphism previously reported from the YGC is absent or weakly preserved in the EG representing that the M2 metamorphism was not strong enough to form new zircon in the EG. The igneous zircons from the augen gneiss in the EGC yield intrusion age of ca. 1867–1881Ma and the geochemistries of the EGC gneisses show post-collision tectonic origin. The D1 deformation observed in the YGC and EG but not in the EGC. The D1 deformation occurred during M1 metamorphism, and then the M2 metamorphism and 1867–1881Ma igneous activities could have occurred together during post-collision tectonic stage. The M1 metamorphism and ca. 1867–1881Ma post collision magmatism in the study area are well matched with the collision related metamorphism at ca. 1.90–1.93Ga and post collision igneous activities at ca. 1.80–1.86Ga along the Jiao-Liao-Ji belt in the North China Craton suggesting that M1 metamorphism could have caused by the collision along the Jiao-Liao-Ji belt. The M3 metamorphism occurred at ca. 230–260Ma and its peak metamorphic conditions were 720–730°C/13–14.5kbar. The D2 deformation and the M3 intermediate-P/T metamorphism occurred during the Permo-Triassic collision event between the North China Craton and South China Craton. The study area was not a collision boundary but located in the peripheral areas of the collision belt during the Paleoproterozoic and the Permo-Triassic time. The D3 deformation occurred at a time gap between the Permo-Triassic collision and the subduction related Jurassic intrusion in the Korean Peninsula.

Neoarchean crustal growth by accretionary processes: Evidence from combined zircon–titanite U–Pb isotope studies on granitoid rocks around the Hutti greenstone belt, eastern Dharwar Craton, India

The Neoarchean Hutti greenstone belt hosts mesothermal gold deposits and is surrounded by granitoid rocks on all sides. Combined U–Pb dating of zircon and titanite from the granitoid rocks constrains their emplacement history and subsequent geologic evolution. The Golapalli and Yelagatti granodiorites occurring to the north of the Hutti greenstone belt were emplaced at 2569±17Ma. The Yelagatti granodiorite yielded a younger titanite age of 2530±6Ma which indicates that it was affected by a post-crystallization thermal event that exceeded the titanite closure temperature. The western granodiorites from Kardikal have identical titanite and zircon ages of 2557±6Ma and 2559±19Ma, respectively. The eastern Kavital granodiorites yielded titanite ages of 2547±6Ma and 2544±24Ma which are identical to the published U–Pb zircon SHRIMP ages. These ages imply that the granitoid rocks surrounding the Hutti greenstone belt were formed as discrete batholiths within a short span of ca. 40Ma between 2570Ma and 2530Ma ago. They were juxtaposed by horizontal tectonic forces against the supracrustal rocks that had formed in oceanic settings at the end of the Archean. The first phase of gold mineralization coincided with the last phase of granodiorite intrusion in the Hutti area. A metamorphic overprint occurred at ca. 2300Ma ago that reset the Rb–Sr isotope system in biotites and possibly caused hydrothermal activity and enrichment of Au in the ore lodes. The eastern Dharwar Craton consists of quartz monzodiorite–granodiorite–granite suites of rocks that are younger than the greenstone belts that are older than ∼2650Ma reported from earlier studies. The granitoid magmatism took place between 2650 and 2510Ma ago indicating accretionary growth of the eastern Dharwar Craton.

Timing of post-collisional volcanism in the eastern part of the Variscan Belt: constraints from SHRIMP zircon dating of Permian rhyolites in the North-Sudetic Basin (SW Poland)

AbstractThe final stages of the Variscan orogeny in Central Europe were associated with voluminous granitic plutonism and widespread volcanism. Four samples representative of the main rhyolitic volcanic units from the Stephanian–Permian continental succession of the North-Sudetic Basin, in the eastern part of the Variscan Belt, were dated using the SIMS (SHRIMP) zircon method. Three samples show overlapping206Pb–238U mean ages of 294 ± 3, 293 ± 2 and 292 ± 2 Ma, and constrain the age of the rhyolitic volcanism in the North-Sudetic Basin at 294–292 Ma. This age corresponds to the Early Permian – Sakmarian Stage and is consistent with the stratigraphic position of the lava units. The fourth sample dated at 288 ± 4 Ma reflects a minor, younger stage of (sub)volcanic activity in the Artinskian. The silicic activity was shortly followed by mafic volcanism. The rhyolite samples contained very few inherited zircons, possibly owing to limited contribution of crustal sources to the silicic magma, or owing to processes involved in anatectic melting and magma differentiation (e.g. resorption of old zircon by Zr-undersaturated melts). The SHRIMP results and the stratigraphic evidence suggest that the bimodal volcanism terminated the early, short-lived (10–15 Ma) and vigorous stage of basin evolution. The Permian volcanism in the North-Sudetic Basin may be correlated with relatively late phases of the regional climax of Late Palaeozoic volcanism in Central Europe, constrained by 41 published SHRIMP zircon age determinations at 299–291 Ma. The Permian volcanism and coeval plutonism in the NE part of the Bohemian Massif can be linked to late Variscan, post-collisional extension.

Is the Ordos Block Archean or Paleoproterozoic in age? Implications for the Precambrian evolution of the North China Craton

The Ordos Block is a large but poorly exposed crustal segment in the western part of the North China Craton. Little is known about its Precambrian basement rocks and their evolution because of an extensive cover by younger sedimentary rocks. We report SHRIMP zircon ages and Hf-in-zircon isotopic compositions for five samples recovered from drill holes that penetrated into the Ordos basement. Based on these data, two age groups of rocks can be distinguished: an early Paleoproterozoic group that is represented by one sample from the northern margin of the Ordos Block and a late (or middle-late) Paleoproterozoic group consisting of the remaining four samples. The early Paleoproterozoic sample contains zircon cores and metamorphic rims with ages of >2.4 Ga and >2.28 Ga, respectively. The cores have eHf(t) values of −8.8 to 6.2 and Hf crustal model ages of 2606 to 3221 Ma. The remaining rocks of late (or middle-late) Paleoproterozoic age contain zircons that may also show a core-rim structure, but with the cores having ages of ∼2.08 Ga and the metamorphic rims recording ages of ∼1.9 Ga. The zircon cores have eHf(t) values of −9.1 to 10.1, and Hf crustal model ages of 2039 to 3068 Ma. Combined with data from earlier studies, we draw the conclusion that late Paleoproterozoic metasedimentary rocks are widespread in the Ordos basement. There is no evidence from our samples of an Archean component, and this calls into question the prevailing view that the Ordos basement is an Archean cratonic block. The fact that the Ordos basement was involved in a widespread late Paleoproterozoic tectono-thermal event indicates that most current models for the tectonic evolution of the western portion of the North China Craton need to be revised.

Late Eocene–Oligocene post-collisional monzonitic intrusions from the Alborz magmatic belt, NW Iran. An example of monzonite magma generation from a metasomatized mantle source

A potassic magmatic association in the Zagros hinterland of the Tethyan orogen in Iran is identified and characterized for relevant geochronologic and petrologic features. New data, including a combination of field relations, U–Pb zircon geochronology and rock geochemistry, come from seven plutons (Khankandi, Shaivar-Dagh, Yuseflu, Mizan, Saheb-Divan, Roudbar and Abhar) that form the Arasbaran–Taroum batholith (ATB), which forms part of the Alborz magmatic belt (AMB) of NW Iran. Zircon SHRIMP ages range from 38.32±0.17Ma, 38.94±0.42Ma and 37.78±0.28Ma for magma pulses of the Abhar pluton, at the East of the batholith, to 24.51±0.27Ma and 23.55±0.47Ma for pulses of the Mizan pluton at the West. Considering these ages and the previously published ones together, emplacement of the batholith took place during Late Eocene and Oligocene, from 38 to 23Ma, with an age progression from SE to NW at a rate of 2cm/year. The whole batholith is characterized by potassic rocks with K2O>2wt.% in gabbros and diorites (SiO2<50wt.%). Higher contents of K2O, of up to >6wt.%, are normally found in rocks with intermediate silica contents of about 60wt.% SiO2. These intermediate silica rocks are truly monzonites and are the most abundant in each pluton. With regard to trace elements, the monzonitic rocks of the ATB show some of the typical signatures of arc magmatism (depletion in Nb and Ti). Most samples contain moderate contents of Sr (500–800ppm), close to similar potassic magmas forming Cenozoic complexes in Central Iran. The relatively moderate Sr/Y and La/Yb ratios suggest that ATB magmas retain some adakitic signatures from the source region. Geochemical modeling is performed by using melt compositions and phase relations calculated with MELTS software, combined with experimental data and trace element signatures. We conclude that monzonitic and shoshonitic magmas of some plutons of the ATB (Shaivar-Dagh, Kahnkandi and Yuseflu) have an adakitic signature inherited from early melts that metasomatized the peridotite mantle. Decompression melting at a relatively low pressure of a metasomatized mantle source is the most plausible explanation for the generation of potassic magmas of the ATB. The lack of adakitic signature in three plutons supports the hypothesis that trace-element features are related to processes in the source. The tectonic implications are straightforward: First, subduction is needed to cook a metasomatized mantle source by fluxing of calc-alkaline melts (adakitic or not) from the subduction zone. Second, lithosphere extension is needed to carry metasomatized mantle to melt and to generate the potassic magmatism that formed the Arasbaran–Taroum batholith, as well as other Cenozoic potassic magmas of the Alborz magmatic belt in NW Iran.

Timing of deformational events in the Río San Juan complex: Implications for the tectonic controls on the exhumation of high-P rocks in the northern Caribbean subduction–accretionary prism

An integrated structural, petrological and geochronological study was undertaken to constrain the tectonic history and controls on the exhumation of the high-P rocks of the Río San Juan complex in the northern Caribbean subduction–accretionary wedge. In the main structural units of the complex, microtextural analyses were performed to identify the fabrics formed at peak of metamorphism in eclogite-facies conditions and during the main retrogressive event toward the low-P amphibolite or blueschist/greenschist-facies conditions.U–Pb SHRIMP dating on zircon rims (71.3±0.7Ma) coupled with 40Ar–39Ar analyses on phengite (~70–69Ma) in felsic sills placed temporal constraints on the exhumation of the Jagua Clara serpentinite-matrix mélange during the blueschist-facies stage at the early Maastrichtian. In the Cuaba unit, U–Pb TIMS zircon ages of 89.7±0.1Ma and 90.1±0.2Ma obtained for the crystallization of tonalitic/trondhjemitic melts in the lower Guaconejo and upper Jobito subunits, respectively, are similar. These ages coupled with a U–Pb SHRIMP zircon age of 87±1.8Ma obtained in a garnet amphibolite and a group of older 40Ar–39Ar cooling ages on calcic amphibole constrain the exhumation of the Guaconejo subunit from the high-P stage to the low-P stage at the ~90–83Ma time interval. Further, the age data collectively supports a genetic relationship between the distributed extensional ductile shearing, the related decompression and the local partial anatexis in the subunit, at least from the Turonian–Coniacian boundary to the early Campanian. A group of younger 40Ar–39Ar ages obtained in the mylonitized amphibolites of the basal Jobito detachment zone indicates late ductile deformation and exhumation/cooling in the late Campanian to Maastrichtian (~75–70Ma). Therefore, structural and age data established deformation partitioning and reworking of retrograde fabrics during ~20Ma in the Cuaba unit.The different exhumation rates obtained for the Jagua Clara mélange can be explained by uplift in two contrasting tectonic settings: a first stage of slow exhumation (1.4mm/yr) in the subduction channel, largely lower than plate velocities, and a second stage of relatively fast exhumation (7.6mm/yr) up to the surface. Therefore, the exhumation was temporally discontinuous and the velocity increase at ~70Ma probably was triggered in response to the entrance of buoyant material in the subduction zone, such as a Caribeana continental ribbon or the distal part of Yucatán–Bahamas continental margin. In contrast, the exhumation path of the Guaconejo subunit is composed of a first segment from the baric peak to the low-P amphibolite stage (at 84–83Ma) with an exhumation rate of 7.2mm/yr, a second segment around the closure temperature of calcic-amphibole (at 82–70Ma) with a rate of 0.4mm/yr, and a third segment to the surface exposure (at 60Ma) with a rate of 1.8mm/yr. These velocity differences can be correlated with the P–T path proposed for the exhumation of the subunit, with an initial isothermal decompression from the peak in the high-P amphibolite to the eclogite-facies produced by distributed extensional shearing, followed by a relatively slow cooling at low-P in the newly acquired structural position, and the final tectonics mainly partitioned in the late Jobito detachment zone. The initial fast exhumation can be related to a major modification in the convergence conditions across the intra-oceanic subduction zone as a jump of the basal subduction thrust beyond a previously accreted arc terrane.

SHRIMP U–Pb and Ar–Ar geochronology of major porphyry and skarn Cu deposits in the Balkhash Metallogenic Belt, Central Asia, and geological implications

The Balkhash Metallogenic Belt (BMB) in Kazakhstan, Central Asia, with the occurrence of the super-large Kounrad and Aktogai, the large Borly porphyry Cu–Mo deposits, and the large Sayak skarn polymetallic ore-field, is one of the central regions of the Paleozoic Central Asian metallogenic domain and orogenic belt. In this study, newly obtained SHRIMP zircon U–Pb ages of nine samples and 40Ar/39Ar ages of six mineral samples (inclding hornblende, biotite and K-feldspar) give more detailed constraints on the timing of the granitic intrusions and their metallogeny. Porphyritic monzonite granite and tonalite porphyry from the Kounrad deposit yield U–Pb zircon SHRIMP ages of 327.3±2.1Ma and 308.7±2.2Ma, respectively. Quartz diorite and porphyritic granodiorite from the Aktogai deposit yield U–Pb SHRIMP ages of 335.7±1.3Ma and 327.5±1.9Ma, respectively. Porphyritic granodiorite and granodiorite from the Borly deposit yield U–Pb SHRIMP ages of 316.3±0.8Ma and 305±3Ma, respectively. Diorite, granodiorite, and monzonite from the Sayak ore-field yield U–Pb SHRIMP ages of 335±2Ma, 308±10Ma, and 297±3Ma, respectively. Hornblende, biotite, and K-feldspar from the Aktogai deposit yield 40Ar/39Ar cooling ages of 310.6Ma, 271.5Ma, and 274.9Ma, respectively. Hornblende, biotite, and K-feldspar from the Sayak ore-field yield 40Ar/39Ar cooling ages of 287.3±2.8Ma, 307.9±1.8Ma, and 249.8±1.6Ma, respectively. The new ages constrain the timing of Late Paleozoic felsic magmatism to ∼336 to ∼297Ma. Skarn mineralization in the Sayak ore-field formed at ∼335 and ∼308Ma. Porphyry Cu–Mo mineralization in the Kounrad deposit and the Aktogai deposit formed at ∼327Ma, and in the Borly deposit at ∼316Ma. The Late Paleozoic regional cooling in the temperature range of ∼600°C to ∼150°C occurred from ∼307 to ∼257Ma.

U–Pb–Hf zircon study of two mylonitic granite complexes in the Talas-Fergana fault zone, Kyrgyzstan, and Ar–Ar age of deformations along the fault

A 2000km long dextral Talas-Fergana strike–slip fault separates eastern terranes in the Kyrgyz Tien Shan from western terranes. The aim of this study was to constrain an age of dextral shearing in the central part of the fault utilizing Ar–Ar dating of micas. We also carried out a U–Pb–Hf zircon study of two different deformed granitoid complexes in the fault zone from which the micas for Ar dating were separated. Two samples of the oldest deformed Neoproterozoic granitoids in the area of study yielded U–Pb zircon SHRIMP ages 728±11Ma and 778±11Ma, characteristic for the Cryogenian Bolshoi Naryn Formation, and zircon grains analyzed for their Lu–Hf isotopic compositions yielded εHf(t) values from −11.43 to −16.73, and their calculated tHfc ages varied from 2.42 to 2.71Ga. Thus varying Cryogenian ages and noticeable heterogeneity of Meso- to Paleoproterozoic crustal sources was established for mylonitic granites of the Bolshoi Naryn Formation. Two samples of mylonitized pegmatoidal granites of the Kyrgysh Complex yielded identical 206Pb/238U ages of 279±5Ma corresponding to the main peak of Late-Paleozoic post-collisional magmatism in the Tien Shan (Seltmann et al., 2011), and zircon grains analyzed for their Lu–Hf isotopic compositions yielded εHf(t) values from −11.43 to −16.73, and calculated tHfc ages from 2.42 to 2.71Ga indicating derivation from a Paleoproterozoic crustal source. Microstructural studies showed that ductile/brittle deformation of pegmatoidal granites of the Kyrgysh Complex occurred at temperatures of 300–400°C and caused resetting of the K–Ar isotope system of primary muscovite. Deformation of mylonitized granites of the Bolshoi Naryn Formation occurred under high temperature conditions and resulted in protracted growth and recrystallization of micas. The oldest Ar–Ar muscovite age of 241Ma with a well defined plateau from a pegmatoidal granite of the Kyrgysh Complex is considered as a “minimum” age of dextral motions along this section of the fault in the Triassic while younger ages varying from 227Ma to 199Ma with typical staircase patterns indicate protracted growth and recrystallization of micas during ductile deformations which continued until the end of the Triassic.

Variscan granitoid plutonism in the Strzelin Massif (SW Poland): petrology and age of the composite Strzelin granite intrusion

Petrological data and recently published U/Pb zircon SHRIMP ages reveal a protracted Variscan magmatic evolution in the Strzelin Massif (SW Poland), with three main stages of granitoid plutonism: 1 – tonalitic I, 2 – granodioritic and 3 – tonalitic II/granitic. The granitoids of the second and third stages form the Strzelin intrusion that is composed of three varieties: medium-grained biotite granite, fine-grained biotite granite and fine-grained biotite-muscovite granite. New SHRIMP data show that the medium-grained and fine-grained biotite granites comprise different zircon populations that reflect complex and prolonged plutonic processes. Two distinct magmatic events seem to be represented by well-defined zircon populations with apparent 206 Pb/ 238 U ages of 303 ± 2 Ma in the medium-grained biotite granite, and 283 ± 8 Ma in the fine-grained biotite granite. These dates, however, do not necessarily reflect the true magmatic ages, possibly being “rejuvenated” by radiogenic lead loss in zircons (impossible to resolve based on routine SHRIMP data). Based on field evidence, the third variety, the biotite-muscovite granite, postdates both types of biotite granites. The petrographic and geochemical features, including Nd isotope signature, along with various zircon inheritance patterns and ages, suggest that the parental magmas of the three granites originated from different crustal sources and were emplaced during three successive magmatic pulses.

SHRIMP zircon age for a K-bentonite in the top of the Laobao Formation at the Pingyin section, Guizhou, South China

The Lower Cambrian on the Yangtze Platform in South China (internationally equivalent to the Nerreneuvian and 2nd series of the Cambrian) is valuable for understanding the early evolution of life, the global biogeochemical circles, and the major changes of the ocean. In particular, a precise radiometric calibration of the Lower Cambrian of this region is a critical realm in its multidisciplinary studies. A SHRIMP U-Pb geochronology study of the K-bentonite in the topmost Laobao Formation at the Pingyin section, Guizhou, South China yielded an age of 536±5 Ma (MSWD = 0.75), suggesting that the K-bentonite here can be correlated with the intensely studied K-bentonite within the middle Zhongyicun Member (Bed 5) of the Zhujiaqing Formation at the Meishucun section in Yunnan. Such a regional correlation result reveals that the Bed 5 K-bentonite also extends into deeper-water environments rather than being deposited merely in the shallow platform interior of the Yangtze Platform. The age of the K-bentonite at the Pingyin section also implies that its overlying polymetallic Ni-Mo layer should be younger than 536±5 Ma. Hence the previous placement of the Precambrian/Cambrian boundary at this layer is inappropriate. Combined with the results of stratigraphic correlations, it is suggested that the K-bentonites in the middle Zhongyicun Member of the Zhujiaqing Formation (or the equivalents) and the base of the Shiyantou Formation (or its equivalents), together with the polymetallic Ni-Mo layer, serve as three important marker beds. Their self-consistent radiometric ages have established an improved geochronologic framework for the Lower Cambrian in South China.

Petrology, geochemistry, and geochronology of the Chah-Bazargan gabbroic intrusions in the south Sanandaj–Sirjan zone, Neyriz, Iran

The Chah-Bazargan gabbroic intrusions are located in the south of Sanandaj–Sirjan zone. Precise U–Pb zircon SHRIMP ages of the intrusions show magmatic ages of 170.5 ± 1.9 Ma. These intrusions consist primarily of gabbros, interspersed with lenticular bodies of anorthosite, troctolite, clinopyroxenite, and wehrlite. The lenticular bodies show gradational or sharp boundaries with the gabbros. In the gradational boundaries, gabbros are mineralogically transformed into anorthosites, wehrlites, and/or clinopyroxenites. On the other hand, where the boundaries are sharp, the mineral assemblages change abruptly. There is no obvious deformation in the intrusions. Hence, the changes in mineral compositions are interpreted as the result of crystallization processes, such as fractionation in the magma chamber. Rock types with sharp boundaries show abrupt chemical changes, but the changes exhibit the same patterns of increasing and decreasing elements, especially of rare earth elements, as the gradational boundaries. Therefore, it is possible that all parts of the intrusions were formed from the same parental magma. Parts showing signs of nonequilibrium crystallization, such as cumulate features and sub-solidification, underwent fracturing and were interspersed throughout the magma chamber by late injection pulses or mechanical movements under mush conditions. The geological and age data show that the intrusions were formed from an Al-, Sr-, Fe-enriched and K-, Nb-depleted tholeiitic magma. The magma resulted from the partial melting of a metasomatized spinel demonstrated by negative Nb, P, Hf, and Ti, and positive Ba, Sr, and U anomalies typical of subduction-related magmas.