Mineral Isochron Ages Research Articles

A crustal-scale, orogen-parallel strike-slip fault in the Middle Urals: age, magnitude of displacement, and geodynamic significance

The kinematic evolution of an orogen-parallel strike-slip fault in the Middle Urals demonstrates that orogen-parallel mass transfer was an important, previously underestimated process during the syncollisional evolution of the Middle Urals. The Kyshtym strike-slip fault extends NNE, parallel and adjacent to the Main Uralian fault, which is the main suture of the Uralide orogen. The Kyshtym fault is interpreted as one of two conjugate strike-slip fault zones that have accomodated the longitudinal transfer of material along the margins of a rigid indenter belonging to the East European craton. The dextral Kyshtym shear zone was active under retrograde lower amphibolite to middle/lower greenschist facies conditions. Four metagranitic, muscovite-bearing mylonites yielded Rb–Sr internal mineral isochron ages of 247.5±2.9, 244.5±6.5, 240.0±1.4, and 240.4±2.3 Ma, whereas a biotite-rich sample, without muscovite, gave a mineral isochron age of 229.1±3.2 Ma. The results indicate almost complete Sr-isotopic reequilibration on the hand specimen scale during mylonitization. The muscovite ages are interpreted as deformation ages and demonstrate a Late Permian/Early Triassic age for the Kyshtym shear zone. The shear zone transects a pre-orogenic syenite intrusion of Ordovician age. A maximum shear strain of γ=7±3 is estimated from the shape of the ductily deformed syenite body in map view and from the length/width ratios of deformed amphibolite bodies in the country rock. This shear strain suggests a maximum displacement of 28±12 km for the ~4-km-thick Kyshtym shear zone. A younger brittle fault, oriented subparallel to the shear zone, accomplished an additional horizontal displacement of 15±3 km; thus, the total displacement along the fault system is 43±15 km.

Mid-paleozoic collision in the north Qinling: Sm–Nd, Rb–Sr and 40Ar/39Ar ages and their tectonic implications

Different isotopic geochronometers have been used to study the nature of the mid-Paleozoic tectonothermal event in the North Qinling arc system at the southern margin of the North China Block. 40Ar/39Ar age of amphibole from sample QH-3 (426±2Ma), Rb–Sr mineral isochron age (411±5Ma) of sample QSP-10 of the Danfeng Group and Rb–Sr whole rock isochron age (406±22Ma) of a greenschist facies pillow lava sample of the Erlangping Group indicate a widespread Late Silurian tectonothermal event in the North Qinling area. The data also suggest a limited uplift in the North Qinling during the Late Silurian tectonothermal event. This event is restricted to the North Qinling area and the southern margin of North China Block, and not observed in the South Qinling belt and the northern margin of the South China Block. Consequently, it is related to the collision between the North China Block and a North Qinling arc system, which resulted in the closure of the North Qinling back-arc basin(s).

Neoproterozoic (∼800 Ma) orogeny in the Tuva-Mongolia Massif (Siberia): island arc–continent collision at the northeast Rodinia margin

The Tuva-Mongolia Massif is a composite Precambrian terrane incorporated into the Palaeozoic Sayany-Baikalian belt. Its Neoproterozoic amalgamation history involves early (∼800 Ma) and late Baikalian (600–550 Ma) orogenic phases. Two palaeogeographic elements are identified in the early Baikalian stage — the Gargan microcontinent and the Dunzhugur oceanic arc. They are represented by the Gargan Glyba (Block) and the island-arc ophiolites overthrusting it. The Gargan Glyba is a two-layer platform comprising an Early Precambrian crystalline basement and a Neoproterozoic passive-margin sedimentary cover. The upper part comprises olistostromes deposited in a foreland basin during the early Baikalian orogeny. The Dunzhugur arc ophiolite form klippen fringing the Gargan Glyba, and show a comprehensive oceanic-arc ophiolite succession. The Dunzhugur arc faced the microcontinent, as shown by the occurrence of forearc complexes. The arc–continent collision followed a pattern similar to Phanerozoic collisions. When the marginal basin lithosphere had been completely subducted, the microcontinental edge partially underthrust the arc, and the forearc ophiolite overrode it. Continued convergence caused a break of the arc lithosphere resulting in the uplift of the submerged microcontinental margin with the overthrust forearc ophiolites sliding into the foreland basin. Owing to the lithospheric break, a new subduction zone, inclined beneath the Gargan microcontinent, emerged. Initial melts of the newly-formed continental arc are represented by tonalites intruded into the Gargan microcontinent basement and its cover, and into the ophiolite nappe. The tonalite Rb–Sr mineral isochron age is 812±18 Ma, which is similar to a U–Pb zircon age of 785±11 Ma. A period of tonalite magmatism in Meso–Cenozoic orogenic belts is recognized some 1–10 m.y. after the collision. Accordingly, the Dunzhugur island arc–Gargan microcontinent collision is conventionally dated at around 800 Ma. It is highly probable that in the early Neoproterozoic, the Gargan continental block was part of the southern (in modern coordinates) margin of the Siberia craton. It is suggested that a chain of Precambrian massifs represents an elongate block separated from Siberia in the late Neoproterozoic. The Tuva-Mongolia Massif is situated in the northwest part of this chain. These events occurred on the NE Neoproterozoic margin of Rodinia, facing the World Ocean.

Sm–Nd and Rb–Sr dating of amphibolite from the Nellore–Khammam schist belt, SE India: constraints on the collision of the Eastern Ghats terrane and Dharwar–Bastar craton

The Nellore–Khammam schist belt, SE India, is sandwiched in between the Proterozoic Eastern Ghats terrane and the Archaean Dharwar–Bastar craton. We report Sm–Nd and Rb–Sr mineral isochron ages of amphibolite from the schist belt. The Sm–Nd and Rb–Sr ages are 824±43 Ma and 481±16 Ma, respectively. The Sm–Nd age indicates the timing of peak metamorphism, whereas the Rb–Sr age indicates the Pan-African thermal overprint. The peak metamorphism was related to collision of the Eastern Ghats terrane with the Dharwar-Bastar craton, which occurred during early Neoproterozoic time.

Cooling and inferred exhumation history of the Ryoke metamorphic belt in the Yanai district, south‐west Japan: Constraints from Rb–Sr and fission‐track ages of gneissose granitoid and numerical modeling

Abstract The Ryoke metamorphic belt in south‐west Japan consists mainly of I‐type granitoids and associated low‐pressure/high‐temperature metamorphic rocks. In the Yanai district, it has been divided into three structural units: northern, central and southern units. In this study, we measured the Rb–Sr whole‐rock–mineral isochron ages and fission‐track ages of the gneissose granodiorite in the central structural unit. Four Rb–Sr ages fall in a range of ca 89–87 Ma. The fission‐track ages of zircon and apatite are 68.9 ± 2.6 Ma and 57.4 ± 2.5 Ma (1σ error), respectively. Combining the newly obtained ages with previously reported (Th–)U–Pb ages from the same unit, thermochronologic study revealed two distinctive cooling stages; 1) a rapid cooling (> 40°C/Myr) for a period (~7 Myr) soon after the peak metamorphism (~ 95 Ma) and 2) the subsequent slow cooling stage (~ 5°C/Myr) after ca 88 Ma. The first rapid cooling stage corresponds to thermal relaxation of the intruded granodiorite magma and its associated metamorphic rocks, and to the uplift by a displacement along low‐angle faults which initiated soon after the intrusion of the magma. Uplift by the later stage deformation having formed large‐scale upright folds resulted in progress of the exhumation during the first stage. The average exhumation velocity of the stage is ≥ 2 mm/yr. During the second stage, the rocks were not accompanied by ductile deformation and were exhumed with the rate of 0.1–0.2 mm/yr. The difference in the exhumation velocity between the first and second cooling stages resulted from the difference in the thickness of the crust and in the activity of ductile deformation between the early and later stages of the orogenesis.

Relationship between cooling rate and cooling age of a mineral: Theory and applications to meteorites

Abstract— We reviewed here the recent development on the mathematical formulation of closure temperature of a cooling geochronological system, which permits direct retrieval of cooling rate from cooling age when the diffusion parameters, grain size and initial temperature are known. This formulation is used to show how the cooling rate can be retrieved by comparing the core and bulk age of a mineral determined by a single decay system. The cooling rates of seven H chondrites of the metamorphic types H4, H5 and H6 were retrieved from the available data on the Pb‐Pb model ages of the phosphates and the diffusion kinetic data of Pb in apatite. The results are in excellent agreement with the metallographic cooling rates and show an inverse relation with the metamorphic grade of these chondrites. We also addressed the problem of ∼90 Ma younger Sm‐Nd mineral isochron age, defined by orthopyroxene, phosphate and plagioclase, of the Morristown mesosiderite compared to the Pb‐Pb age of the Estherville mesosiderite. It is shown that this younger age could have been a consequence of resetting during cooling instead of an “impulsive heating” event, as suggested earlier.

The emplacement age of gabbroic rocks and associated granitoids of the Liganga-Msanyo complex, south eastern Tanzania.

The Msanyo Gabbro and the associated Mdando granitoids, which intrude Paleoproterozoic high-grade metamorphic rocks of the Ubendian Belt in southern Tanzania, yield imprecise Sm-Nd whole rock isochron ages of 1608±134 Ma (e (Nd) = 2.1) and 1642±100 Ma (e (Nd) = 1.6), respectively. These ages are indistinguishable at the 95% confidence level, indicating that the two rock suites were intruded at the same time. A more precise estimate of the time of emplacement is provided by a 5-point mineral isochron age of 1505±42 obtained from one of the gabbro samples. Calculated initial e (Nd) values and mean crustal residence ages are similar for both rock suites and range from –0.7 to 1.5 and 1.7 to 1.9 Ga. Respectively. Initial Sr isotope ratios lie between 0.7032 and 0.7039 and these values are similar to those obtained in the mantle and the lower continental crust. The rather evolved Nd isotope systematics, combined with the relatively non-radiogenic Sr isotopic signature, suggest that both suites cystallised from mantle-derived magma that had assimilated a significant amount of lower continental crustal material. Subsequent to their emplacement, the rocks experienced localised shearing and sericitisation during the Ukingan tectonic event. A Rb-Sr plagioclase-whole rock age dates this event at 1137±50 Ma. Tanzania Journal of Science Volume 27A (Special Issue) 2001, pp. 1-14

Phanerozoic crustal growth: U–Pb and Sr–Nd isotopic evidence from the granites in northeastern China

Northeastern China (NE China) is an eastern part of the Central Asian Orogenic Belt (CAOB). Its geology is distinguished by extensive distribution of Phanerozoic granitic and acid volcanic rocks. The granitoids comprise mainly I- (especially the highly fractionated felsic I-) and A-type granites; S-type granites are conspicuously absent. New zircon U–Pb and Rb–Sr mineral isochron ages, together with previously published results, indicate that these granites were emplaced in Late Paleozoic to Late Mesozoic times. Most of the granites are characterized by low initial 87Sr/ 86Sr ratios (0.705±0.001), positive ε Nd( T) values (+4 to 0) and fairly young T DM model ages (500–1300 Ma), suggesting that a significant proportion of the granites is of juvenile origin. In contrast, some granites emplaced within Precambrian terranes (or microcontinents) in this area show negative ε Nd( T) values of 0 to −8, and higher T DM ages of 1500–2000 Ma. This suggests that the Precambrian basement rocks have exerted a significant role in the generation of these granites. Sr–Nd isotopic modeling shows that these granites are likely to have been produced by partial melting of mixed sources, mostly probably of Phanerozoic underplated basaltic and lower crustal rocks. The generation of the granites with negative ε Nd( T) values involved a smaller proportion of basaltic component (≈40%), whereas the dominant granites with positive ε Nd( T) values require a much greater amount of basaltic material (55–85%). The emplacement of granitic rocks with such a large proportion of mantle-derived component indicates a massive addition of juvenile continental crust in NE China. We propose that the granites were formed in an anorogenic tectonic setting and that underplating of basaltic liquid as a result of extension is an important mechanism for the continental growth.

U-Pb geochronology of the Southern Black Forest Batholith (Central Variscan Belt): timing of exhumation and granite emplacement

Some granites, granitoid dykes and volcanic rocks of the Southern Black Forest were dated by U–Pb techniques using zircon and monazite. An effusive rhyolite, which is interbedded in upper Visean sedimentary sequences of the Badenweiler-Lenzkirch zone, was dated at 340 ±2 Ma. This weakly metamorphic zone of supracrustal rocks borders high-grade gneiss terrains in the north and the south, which are intruded by a series of granitoid intrusions: the strongly sheared Schlachtenhaus granite is dated by monazite at 334±2 Ma and the hypothesis of a Devonian emplacement is therefore discarded. The emplacement of all other granites, crosscutting dykes and of an ignimbrite were all within analytical uncertainty: St. Blasien granite 333±2 Ma; Barhalde granite 332±3 Ma; Albtal granite 334±3 Ma; and a porphyry dyke at Prag 332+2/-4 Ma. Deformation and thrusting of the basement units near the Badenweiler-Lenzkirch zone occurred after the emplacement of the Schlachtenhaus granite, but before the intrusion of the other granitoids, and may therefore be constrained to the time period unresolved between 334±2 and 333±2 Ma. The ignimbritic rhyolite of Scharfenstein was deposited in a caldera 333±3 Ma ago. This age coincides within error limits with published U–Pb monazite and Rb–Sr small slab ages of mimatitic gneisses, Ar–Ar hornblende ages of metabasites and Sm–Nd mineral isochron ages of eclogitic rocks in the underlying basement. This suggests that exhumation and cooling of this basement unit must have been active at rates of approximately 20 km and a few 100°C per million years. The silicic melts are interpreted to be of hybrid crust/mantle origin and their formation was most likely linked to these exhumation tectonics. A phase of mantle upwelling and heat advection into the crust is proposed to be the reason for this short-episodic magmatic pulse.

Rb–Sr isochron ages of the Cretaceous granitoids in the Ryoke belt, Kinki district, Southwest Japan

Abstract Granitoids are widely distributed in the Ryoke belt and have been divided into four main igneous stages based on their field setting. In this paper, we present Rb–Sr isochron ages for the younger Ryoke granitoids (second stage to fourth stage) in the Kinki district. The Yagyu granite (second stage) gave a Rb–Sr whole‐rock isochron age of 74.6 ± 10.9 Ma with an initial 87Sr/86Sr ratio of 0.70938 ± 0.00016, and a Rb–Sr mineral isochron age of 71.8 ± 0.1 Ma. The Narukawa granite (second stage) yielded a Rb–Sr mineral isochron age of 79.5 ± 0.4 Ma. A Rb–Sr whole‐rock isochron age of 78.3 ± 3.0 Ma with an initial 87Sr/86Sr ratio of 0.70764 ± 0.00014 was obtained for the Takijiri adamellite (third stage). The Katsuragi quartzdiorite (fourth stage) gave a Rb–Sr whole‐rock isochron age of 85.1 ± 18.3 Ma (initial 87Sr/86Sr ratio of 0.70728 ± 0.00006), and mineral isochron ages of 76.9 ± 0.5 Ma and 74.8 ± 0.5 Ma. The Minamikawachi granite (fourth stage) gave a Rb–Sr whole‐rock isochron age of 72.8 ± 2.0 Ma with an initial 87Sr/86Sr ratio of 0.70891 ± 0.00021. These age data indicate that the igneous activity in younger Ryoke granitoids of Kinki district occurred between 80 and 70 Ma, except for the Katsuragi quartz diorite. The isotopic data on the various igneous stages in Kinki district correspond with the relative timing from field observations. Based on the initial 87Sr/86Sr ratios, the granitoids of the Ryoke belt in Kinki district are spatially divided into two groups. One is granitoids with initial 87Sr/86Sr ratio of 0.707–0.708, distributed in the southern part of the Ryoke belt. The other is granitoids with initial 87Sr/86Sr ratio of 0.708–0.710 distributed in the northern part of the Ryoke belt. The initial 87Sr/86Sr ratios of granitoids increase with decreasing (becoming younger) Rb–Sr whole‐rock isochron ages.

Timing of magmatic and metamorphic events in the Jijal complex of the Kohistan arc deduced from Sm-Nd dating of mafic granulites

Abstract Mafic to ultramafic granulites in the northeastern part of the Jijal complex include two-pyroxene granulite, garnet-clinopyroxene granulite and garnet hornblendite. Field and textural relations indicate that two-pyroxene granulite is a relict left after formation of the garnet-clinopyroxene granulite and garnet hornblendite was an originally intrusive rock which dissected the protoliths of mafic granulites. Sm-Nd mineral isochron ages of 118 ± 12 Ma, 94.0 ± 4.7 Ma and 83 ± 10 Ma were determined for two-pyroxene granulite, garnet-clinopyroxene granulite and garnet hornblendite respectively. These ages, together with previously reported chronological data, led to the following tectonic implications: (1) crystallization of the granulite protoliths predates, or is coeval with, the tectonic accretion of the Kohistan arc to the Asian continent; (2) crustal thickening related to the accretion was probably responsible for the high-pressure granulite-facies metamorphism in the Jijal complex; (3) formation of the garnet hornblendite assemblage was probably after crystallization of garnet-clinopyroxene granulite.

Petrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism

The origin of the Sondalo gabbroic complex has been unravelled by means of a petrological study of the least evolved rocks, troctolites to norites containing up to 20% of anhedral clinopyroxene and titanian pargasite. Pyroxenes and titanian pargasite from the troctolites have higher Mg, Al and Cr, and lower Mn than those from the norites, whereas plagioclase does not show systematic compositional variations (An ca. 65 mol%). The variation trend of anorthite content of plagioclase versus the forsterite content of olivine differs from that of arc-related gabbroic rocks. Plagioclase, clinopyroxene, orthopyroxene and titanian pargasite were analyzed for REE and selected trace elements by ion microprobe. Application of crystal/liquid partition coefficients to trace element mineral compositions suggests that the parental liquids of both troctolites and norites had tholeiitic affinity and were slightly LREE and LILE enriched relative to N-MORB. A troctolite and a norite give Sm-Nd mineral isochron ages of 300 ± 12 Ma and 280 ± 10 Ma. Plagioclase-amphibole Rb-Sr isochron ages are 266 ± 10 Ma and 269 ± 16 Ma for the same rock samples, and they are interpreted to represent cooling ages. The Nd-Sr-O isotopic compositions indicate that a substantial crustal contribution was involved in the petrogenesis of the norite, which has low ɛNd(290 Ma), high 87Sr/86Sr(290 Ma) and high δ18OPx (−2.6, 0.7057 and +7.9‰, respectively) compared with the troctolites. We thus conclude that the troctolite/norite association formed by concomitant fractional crystallization and crustal assimilation. The somewhat elevated δ18OCpx (+6.4‰) and the relatively low forsterite contents in olivine suggest that the parental liquids of the troctolites had already been evolved through an AFC type process. The ɛNd(290 Ma) and 87Sr/86Sr(290 Ma) of these rocks (ranging from +2.8 to +4.4 and from 0.7037 to 0.7040, respectively) probably do not reflect the ɛNd and Sr isotopic compositions of their mantle source, and it is thus unclear whether the primary melts were derived from a slightly enriched or from a depleted mantle source. The Sondalo gabbroic complex was most likely associated with the post-Variscan gabbroic complexes of the Alpine belt. These gabbroic complexes can be ascribed to the intrusion at different crustal levels of tholeiitic mantle-derived melts and were emplaced in the time span of 300–270 Ma.

Samarium‐neodymium and rubidium‐strontium systematics of nakhlite Governador Valadares

Abstract— The Sm‐Nd systematics of whole‐rock and mineral separate samples from nakhlite Governador Valadares define a good 147Sm‐143Nd mineral isochron age of 1.37 ± 0.02 Ga. This age is in excellent agreement with the 39Ar‐40Ar and Rb‐Sr ages obtained previously for this meteorite. However, the Rb‐Sr isotopic data for our sample show that the isotopic system is disturbed. The lack of isotopic equilibrium is probably caused by the weathering of the sample as indicated by the presence of secondary alteration phases. The whole‐rock and acid‐washed mineral data yield a Rb‐Sr age of 1.20 ± 0.05 Ga, which probably represents a lower limit to the crystallization age of the rock. The petrographic evidence indicates that this meteorite is a clinopyroxene cumulate that probably crystallized in a subsurface sill (McSween, 1994). Thus, the Sm‐Nd isotopic age probably represents the age of such a magmatic event. The initial ε143Nd value determined for the rock at 1.37 Ga is +17 ± 1, indicating that the parent magma of the rock came from a light‐rare‐earth‐element‐depleted source of 147Sm/144Nd = ∼0.237 based on a simple two‐stage evolution model. Results of the same model calculation for the initial 87Sr/86Sr ratio of the rock suggest that its source material was depleted in 87Rb/86Sr by ∼50% relative to the estimated martian value at 1.37 Ga. Both the high Sm/Nd and low Rb/Sr values support a clinopyroxene‐rich cumulate source for the genesis of the nakhlite Governador Valadares. Furthermore, our Sm‐Nd age and ε143Nd data and the previously published ε142Nd datum for the rock (Harper et al., 1995) are consistent with early differentiation of the parent planet, formation of cumulate sources ∼4.56 Ga ago, and late melting of the sources and formation of the rock ∼1.37 Ga ago. The good agreement of isotopic ages and petrographic features among Governador Valadares, Nakhla, and Lafayette strongly suggests that all three nakhlites have undergone similar evolutionary histories. The nakhlite age data suggest that isotopic heterogeneity in the martian mantle sources existed up to ∼1.37 Ga ago and early mantle structures probably have not been disturbed for a significant portion of martian history.

Metamorphism, isotopic ages and composition of lower crustal granulite xenoliths from the Cretaceous Salta Rift, Argentina

Crustal xenoliths from basanitic dikes and necks that intruded into continental sediments of the Cretaceous Salta Rift at Quebrada de Las Conchas, Provincia Salta, Argentina were investigated to get information about the age and the chemical composition of the lower crust. Most of the xenoliths have a granitoid composition with quartz-plagioclase-garnet-rutile ± K-feldspar as major minerals. The exceedingly rare mafic xenoliths consist of plagioclase-clinopyroxene-garnet ± hornblende. All xenoliths show a well equilibrated granoblastic fabric and the minerals are compositionally unzoned. Thermobarometric calculations indicate equilibration of the mafic xenoliths in the granulite facies at temperatures of ca. 900 °C and pressures of ca. 10 kbar. The Sm-Nd mineral isochron ages are 95.1 ± 10.4 Ma, 91.5 ± 13.0 Ma, 89.0 ± 4.2 Ma (granitoid xenoliths), and 110.7 ± 23.6 Ma (mafic xenolith). These ages are in agreement with the age of basanitic volcanism (ca. 130–100 and 80–75 Ma) and are interpreted as minimum ages of metamorphism. Lower crustal temperature at the time given by the isochrons was above the closure temperature of the Sm-Nd system (>600–700 °C). The Sm-Nd and Rb-Sr isotopic signatures (147Sm/144Nd = 0.1225–0.1608; 143Nd/144Ndt0 = 0.512000–0.512324; 87Rb/86Sr = 0.099–0.172; 87Sr/86Srt0 = 0.708188–0.7143161) and common lead isotopic signatures (206Pb/204Pb = 18.43–18.48; 207Pb/204Pb = 15.62–15.70; 208Pb/204Pb = 38.22 –38.97) of the granitoid xenoliths are indistinguishable from the isotopic composition of the Early Paleozoic metamorphic basement from NW Argentina, apart from the lower 208Pb/204Pb ratio of the basement. The Sm-Nd depleted mantle model ages of ca. 1.8 Ga from granitoid xenoliths and Early Paleozoic basement point to a similar Proterozoic protolith. Time constraints, the well equilibrated granulite fabric, P-T conditions and lack of chemical zoning of minerals point to a high temperature in a crust of nearly normal thickness at ca. 90 Ma and to a prominent thermal anomaly in the lithosphere. The composition of the xenoliths is similar to the composition of the Early Paleozoic basement in the Andes of NW Argentina and northern Chile. A thick mafic lower crust seems unlikely considering low abundance of mafic xenoliths and the predominance of granitoid xenoliths.

TRACE ELEMENT AND ND-SR ISOTOPE GEOCHEMISTRY OF THE GABBROIC ROCKS FROM THE NORTHERN APENNINE OPHIOLITES: GEOCHRONOLOGY AND RELATIONS WITH ASSOCIATED PERIDOTITES

The Northern Apennine ophiolites crop out in different stratigraphic settings. The petrological and geochemical features of the mantle-gabbro association are well known for the ophiolites of the Internal Liguride Units (Rampone et al., 1998). The mantle ultramafics are mainly clinopyroxene- poor plagioclase-bearing lherzolites, residual after the extraction in Permian times of MORB-type liquids, which were intruded in Middle Jurassic times by N-MORB-type gabbros. The mantle ultramafics of the ophiolites from the External Liguride Units dominantly consist of rather fertile plagioclase-bearing lherzolites with pyroxenite bands (Rampone et al., 1995), and they are considered as slices of subcontinental mantle, probably accreted to the lithosphere in the Proterozoic. Little is known about the rare gabbroic rocks from the External Liguride ophiolites, as well as about the mantle ultramafics from the ophiolites which crop out in Southern Tuscany. In this work, we have carried out traceelement and Nd-Sr isotopic investigations of i) the gabbroperidotite association from Southern Tuscany ophiolites (Lanciaia Unit), and ii) the gabbroic rocks from the External Liguride ophiolites. This has allowed us to depict a complete overview of the gabbro-peridotite association of the Northern Apennine ophiolites. The gabbroic rocks from the different Units contain clinopyroxenes with similar trace element compositions, that yield N-MORB equilibrium liquids. Moreover, clinopyroxenes show marked trace element zonings, similarly to what was observed for the gabbroic rocks from ODP Leg 153 in the MARK area (Ross and Elthon, 1997), most likely related to the entrapment of interstitial liquid (Tribuzio et al., 1999). The gabbroic rocks considered in this study give Sm/Nd mineral isochron ages ranging from 179 to 170 Ma, interpreted to date the primary magmatic crystallization. These ages are significantly older than those recently found (163 ± 2 to 166 ± 2 Ma) on the basis of U/Pb and Ar/Ar geochronological determinations on zircons and amphiboles for the gabbroic rocks of the Western Alps ophiolites (Bill et al., 1997; Rubatto et al., 1998). 87Sr/86Sr of clinopyroxenes and initial eNd (8.5 to 8.9) are consistent with a formation from N-MORB. The gabbroic rocks from Internal Liguride ophiolites and the basalts (Rampone et al., 1998) from both Internal and External Liguride ophiolites show closely similar initial eNd. The incompatible trace element compositions of basalts from the different Units resemble those calculated for the parental liquids of the gabbroic rocks. The studied peridotites from Southern Tuscany ophiolites are porphyroclastic spinel-lherzolites. These rocks have a residual geochemical fingerprint, as shown by the marked depletion in incompatible trace elements of clinopyroxenes, similarly to what was observed for the Internal Liguride lherzolites. The clinopyroxenes from the southern Tuscany lherzolites indeed have slightly lower Sm/Nd values and slightly higher Sr contents than those from the Internal Liguride lherzolites. Major element mineral compositions of Southern Tuscany lherzolites yield relatively low-temperature estimates for the spinel-facies equilibration (ca. 1000 °C), similarly to those found for the External Liguride lherzolites. On the other hand, the latter rocks cleary differ from the Southern Tuscany lherzolites in the occurrence of accessory Ti-rich amphibole and in the fact that clinopyroxenes have markedly higher concentrations of incompatible trace elements. Nd and Sr isotopic compositions of clinopyroxenes from the Southern Tuscany lherzolites calculated at 170 Ma are respectively higher (eNd ca. 11.0) and lower than those of associated gabbros. Therefore, there is not a genetic meltresidue relationship for this gabbro-peridotite association, as inferred by Rampone et al. (1998) for the Internal Liguride ophiolites. The clinopyroxenes from the External Liguride lherzolites are characterized by markedly heterogeneous Nd and Sr isotopic compositions, coupled with a relatively constant Sm/Nd value (Rampone et al., 1995). Similar geochemical relations have been also observed for the Lanzo lherzolitic massif in the Western Alps (Bodinier et al., 1991). In the External Liguride Units, the clinopyroxenes from the Monte Nero lherzolite body, which is locally intruded by gabbroic to basaltic dykes, display Nd and Sr isotopic compositions that are consistent with those of analyzed gabbroic rocks. As a whole, the ophiolites of the Northern Apennine/Western Alps system are characterized by a homogeneous composition of the gabbroic complex and by a marked heterogeneity of the associated mantle section. The compositional spread of the mantle lherzolites can be related to two major events: 1) partial melting at 300-270 Ma, in relation to the post-Variscan extensional regime; 2) partial re-equilibration with N-MORB liquids derived from deeper astenospheric sources at 180-160 Ma.

Deepest exposed crust of Brazil-SHRIMP establishes three events

Research Article| October 01, 1999 Deepest exposed crust of Brazil-SHRIMP establishes three events Léo A. Hartmann; Léo A. Hartmann 1Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves, 9500, 91500-000 Porto Alegre, Rio Grande do Sul, Brazil Search for other works by this author on: GSW Google Scholar Jayme A. D. Leite; Jayme A. D. Leite 2Departamento de Recursos Minerais, Universidade Federal de Mato Grosso, Avenida Fernando Correa da Costa s/n, Cuiabá, Mato Grosso, Brazil Search for other works by this author on: GSW Google Scholar Neal J. McNaughton; Neal J. McNaughton 3Centre for Strategic Mineral Deposits, University of Western Australia, Nedlands 6907, Western Australia Search for other works by this author on: GSW Google Scholar João Orestes S. Santos João Orestes S. Santos 4Geological Survey of Brazil (CPRM), Rua Banco da Provìncia, 105, Porto Alegre, Rio Grande do Sul, Brazil Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (10): 947–950. https://doi.org/10.1130/0091-7613(1999)027<0947:DECOBS>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Léo A. Hartmann, Jayme A. D. Leite, Neal J. McNaughton, João Orestes S. Santos; Deepest exposed crust of Brazil-SHRIMP establishes three events. Geology 1999;; 27 (10): 947–950. doi: https://doi.org/10.1130/0091-7613(1999)027<0947:DECOBS>2.3.CO;2 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 The deepest exposed crust of Brazil is in the western portion of the exposed Precambrian shield of southernmost Rio Grande do Sul State and comprises a bimodal sequence of mafic garnet granulites and metatrondhjemites, intercalated with smaller volumes of metamorphosed pyroxenites, anorthosites, sillimanite gneisses, and banded iron formation. As determined by zircon U/Pb sensitive high-resolution ion microprobe (SHRIMP), this granulite terrain accreted from the mantle at the end of the Archean (ca. 2.55 Ga) and was deformed in high-pressure granulite facies metamorphic conditions in the Paleoproterozoic (ca. 2.02 Ga). A younger event, possibly ca. 900 Ma, caused local amphibolite facies retrogression in the complex. Zircons from a metabasalt and a metatrondhjemite show external morphologies typical of high-grade metamorphic rocks, whereas backscattered electron images reveal internal oscillatory zoning of their igneous precursors. Spot ages from zircon agree well with previous model Nd and Sm/Nd mineral isochron ages. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

一の目潟と黒瀬のマントルカンラン岩捕獲岩のＲｂ‐Ｓｒ鉱物アイソクロン年代

Petrology, geochemistry and Rb-Sr chronology of seven spinel lherzolite-harzburgite xenolith samples from Ichinomegata, northeast Japan and Kurose, southwest Japan were studied. Whole rocks and the constituent minerals were analyzed for Rb and Sr contents and 87Sr/86Sr ratios. The pure minerals were obtained by hand-picking and repetitive washing with hot acids (2M HCl, 5% HF) and distilled water. The peridotite xenoliths from Ichinomegata show wide variations in mineral and chemical compositions; clinopyroxene contents range from 4 to 13 volume%, Cr/(Cr+Al) ratios of spinel range from 0.19 to 0.41 and MgO contents of whole rock range from 38 to 46 weight%. The chondrite-normalized patterns of whole rock vary from LREE-depleted to flat or U-shaped. In spite of above petrographical and geochemical variations, 87Sr/86Sr ratios of the clinopyroxenes are very uniform (0.7033∼0.7034). Rb-Sr mineral isochron ages of 208.9±80.1 Ma, 233.8±8.4 Ma, 252.9±19.8 Ma and 310.8±23.3 Ma are given by olivine, orthopyroxene, clinopyroxene and whole rock from each of four peridotite xenoliths. Since the peridotites have the relatively narrow range of ages and wide range of equilibrium temperatures, it is believed that a large-scale isotopic equilibration has occurred at various depths in the upper mantle beneath Ichinomegata. The peridotite xenoliths from Kurose show narrow ranges in mineral and chemical compositions; clinopyroxene contents range from 7 to 9 vol.%, Cr/(Cr+Al) ratios of spinel range from 0.28 to 0.31 and MgO contents of whole rock range from 41 to 43 wt.%. On the other hand, 87Sr/86Sr ratios of the clinopyroxenes are very diverse (0.7034, 0.7039 and 0.7043), compared with those in Ichinomegata peridotites. The mineral isochrons based on the data of pyroxenes and whole rock also show very variable ages and initial 87Sr/86Sr ratios. The Rb-Sr mineral isochron ages of 129.9±4.8 Ma, 312.8±16.7 Ma and 487.2±9.1 Ma indicate large intervals between the equilibration events.

Isotopic signature of Pan-African rejuvenation in the Kerala Khondalite Belt, southern India: implications for East Gondwana reassembly : Unnikrishnan-Warrier C., Journal - Geological Society of India, 1997, 50/2 (179–190)

Sm-Nd isotope systematics on mineral separates from sillimanite-and cordierite-bearing metapelite (khondalite), and garnet-and biotite-bearing gneiss (leptynite) from the Kerala Khondalite Belt (KKB), southern India, yielded mineral isochron ages (wr-feld-bio-gar) of 537 ± 27 Ma (MSWD = 0.9) and 534 ± 26 Ma (MSWD = 1.23) respectively. Rb-Sr systematics in the same samples gave wr-feld-bio mineral isochron ages of 437 ± 9 Ma (MSWD = 0.67) and 467 ± 9 Ma (MSWD = 0.76), These results provide the first mineral isochron ages for the regional metasedimentaries in the KKB. The e (Nd T) values at 550 Ma for Khondalite and leptynite are -22.7 and -21.8 respectively. These results demonstrate a complete rejuvenation of the crust during Pan-African times. Coeval alkaline plutons emplaced along fault-lineaments in this area suggest an extensional tectonic regime. Geochronologic correlations with the Lutzow-Holm Bay Complexes in East Antarctica, and the Highland and Southwestern Complex of Sri Lanka show that a similar Pan-African tectonothermal event manifested in all the East Gondwana crustal fragments.

The Santa Catarina Granulite Complex of Southern Brazil: A Review

The Santa Catarina granulite complex (SCGC) is mainly composed of intermediate to felsic orthogneisses of trondhjemitic, tonalitic and granodioritic compositions (TTG) intruded by subsidiary basic-ultrabasic rocks, and a small metasedimentary component. The ortho-derived rocks belong to different igneous suites. Nd model ages TDM of 2.7-2.8 Ga date the first major mantle-crust differentiation in the area, while ∼2.6 Ga Rb-Sr whole rock isochron ages in both depleted and undepleted granulites and Nd model ages of 2.4-2.3 Ga record subsequent magmatic events. Regional granulite-facies which reached ∼800°C at ca. 5-8 kb finished at ∼2.3 Ga according to U-Pb zircon and Sm-Nd mineral isochron ages, and was also followed by a regional amphibolite facies metamorphism at ∼2.0 Ga, recorded by Pb-Pb, Rb-Sr whole rock isochrons in rocks with little orthopyroxene and predominant amphibole. Tectonic stabilization was complete at the end of the Transamazonian Cycle at ∼1.9 Ga as indicated by K-Ar mineral ages, and apart from local reactivation along faults at the margins of the complex and granite intrusion, the SCGC was not affected by the Neoproterozoic Brasiliano Cycle, during which it acted as a microplate and was incorporated onto the eastern border of the Paraná Craton. Isotopic and geochemical characteristics of Brasiliano crustal granites which intrude the complex suggest that it is not an adequate source for the melts, and thus the SCGC may be underlain by different Paleoproterozoic (2.0 Ga) crustal material.

Ion microprobe U-Pb zircon geochronology and correlation of Archaean gneisses from the Lewisian Complex of Gruinard Bay, northwestern Scotland

We present ion microprobe U-Pb zircon geochronology for three samples from the late Archaean Lewisian gneiss complex at Gruinard Bay, northwestern Scotland, in an attempt to resolve the discrepancy between Sm-Nd and Pb-Pb isochron ages for trondhjemite and hornblendite components. Whole-rock samples imply ca. 2.8 Ga. ages for both components (Whitehouse et al., 1996), but separated minerals have yielded ca. 2.4–2.5 Ga for trondhjemite and 3.3 Ga for hornblendite (Burton et. al., 1994). Zircons separated from three representative lithologies at Gruinard Bay (trondhjemite, hornblendite, and banded tonalite) yield concordant or near concordant U-Pb data with 207Pb206Pb ages mostly between ca. 2.85 Ga and ca. 2.75 Ga, and a few outliers at ca. 2.65 Ga. Backscattered electron images of zircons from the trondhjemite allow differentiation between ca. 2.85 Ga cores and ca. 2.75 Ga overgrowths; the latter are interpreted as igneous on the basis of field relationships of the trondhjemite to the other samples. Morphology of the hornblendite zircons is more consistent with crystallisation under metamorphic rather than igneous conditions at ca. 2.8 Ga, although the previous arguments of Whitehouse et al. (1996) for a protolith age not greatly in excess of this remain valid. The new zircon data suggest that magmatism at Gruinard Bay occurred over a relatively short time interval, in accord with the previous observations from whole-rock isotopic data. There is no evidence from the analysed zircons for ca. 2.4–2.5 Ga or 3.3 Ga magmatic ages in the trondhjemite and hornblendite, respectively. The mineral isochron ages for the trondhjemite probably represent metamorphic resetting related to the regional ca. 2.5 Ga high-grade event recorded in the central region. For the hornblendite, the mineral Sm-Nd ages are confirmed as spurious, resulting from post-crystallisation open-system isotope behaviour on the mineral scale. Regional comparison of ages within the Lewisian Complex suggests that the ≤5 ca. 2.85 Ga Gruinard Bay TTG gneisses might have been amalgamated with ≥ 2.96 Ga gneisses of the central region before or during a ca. 2.76 Ga event and subsequently have experienced a similar geological evolution.