Rock Model Ages Research Articles

The oldest crust in the Ukrainian Shield – Eoarchaean U–Pb ages and Hf–Nd constraints from enderbites and metasediments

Abstract The oldest crust in the Ukrainian Shield occurs in the Podolian and Azov domains, which both include Eoarchaean components. U–Pb age data for Dniestr–Bug enderbites, Podolian Domain, indicate that these are c. 3.75 Ga old, and Lu–Hf isotope data indicate extraction from chondritic to mildly isotopically depleted sources with ɛ Hf up to c. +2. Nd model ages support their Eoarchaean age, while model ages for Dniestr–Bug metasedimentary gneisses indicate that these also include younger crustal material. Most of the Hf-age data for metasedimentary zircon from the Soroki greenstone belt, Azov Domain, reflects Eoarchaean primary crustal sources with chondritic to mildly depleted Hf isotope signatures at 3.75 Ga. A minor portion is derived from Mesoarchaean crust with a depleted ɛ Hf signature of c. +4 at 3.1 Ga. U–Pb zircon ages from Fedorivka greenstone belt metasediments are consistent with the Soroki age data, but also include a 2.7–2.9 Ga component. Nd whole rock model ages provide support for a younger crustal component in the latter. Both domains have been subject to Neoarchaean, c. 2.8 Ga, and Palaeoproterozoic, c. 2.0 Ga, metamorphism. The spatial distribution indicates that the Podolian and Azov domains evolved independently of each other before the amalgamation of the Ukrainian Shield.

Geochemical characteristics and U–Pb zircon LA-ICPMS ages of granitoids from the Pan-African Dahomeyide orogen, West Africa

The Dahomeyide orogen in southeastern Ghana and adjoining parts of Togo and Benin records the suture of West African craton (WAC) into northwest Gondwana. The suture zone is characterized by distinctive high-pressure mafic rocks that mark the collision of WAC with exotic blocks to the east. Granitoids to the east of the suture zone, which are postulated to be juvenile crust representing the arc terrane that formed during subduction and oceanic closure, were investigated. We compiled a map of granitoid gneisses in Ghana, Togo, and Benin which allows us to distinguish between two main rock types: (1) migmatitic gneisses with biotite as the dominant mafic mineral and (2) dioritic gneiss characterized by hornblende as the main mafic mineral and with occasional garnet. These rocks types are separated by a shear zone inferred to be a splay of Kandi shear zone, a segment of the Trans-Saharan shear zone that extends for ∼2500km from the Sahara to the Gulf of Guinea. The granitoids are characterized by I-type signatures, display variable amounts of LREE enrichment, Eu depletion and Sr/Y ratios that are consistent with a subduction origin. The migmatitic gneiss unit, within a zone of highly strained granitoids (straight gneiss) that forms a sliver just east of the suture zone, yielded U–Pb zircon ages ranging from 615 to 589Ma. This age range is similar to the metamorphic crystallization age of the suture zone rocks. The zircons analyzed display concentric zoning (in CL images) and are inferred to be magmatic. On the other hand, zircon separates from the dioritic gneiss, with grains also showing igneous zoning in CL images, yielded U–Pb zircon crystallization ages ranging from 2.19 to 2.14Ga. These are the first U–Pb zircon ages to document Paleoproterozoic rocks in the postulated juvenile terrane, west of the Kandi fault zone. Nd model ages of rocks in the suture zone are also consistent with the existence of widespread involvement of older crust in the Pan-African collision zone, similar to the isotopic data from the correlative Medio Coreau domain of the northwest Borborema province in Brazil.

Emplacement and deformation of the Vinukonda meta-granite (Eastern Ghats, India)—Implications for the geological evolution of peninsular India and for Rodinia reconstructions

The U–Pb and Rb–Sr systematics of zircon populations from two samples and biotite–white mica pairs from four samples constrain the time of emplacement and deformation of the Vinukonda meta-granite, Eastern Ghats, India. Sm–Nd and Lu–Hf systematics of whole rock samples provide information on the magma source and its evolution. The granitic magma intruded into granitic gneisses of the Vinjamuru domain (Krishna Province, Andhra Pradesh, India), which were already deformed and metamorphosed at epidote–amphibolite facies peak conditions. Consistent Sm–Nd and Lu–Hf whole rock model ages suggest an isotopically homogeneous crustal source and a multi-stage evolution of the granitic magma. U–Pb analyses of prismatic zircon multi-grain fractions define two discordias with identical upper intercepts at 1588.4 ± 7.1 and 1589.7 ± 5.7 Ma, interpreted to date magma emplacement. Rb–Sr ages of strongly annitic biotite ( X Mg 0.07–0.20) and phengitic white mica (3.13–3.23 Si pfu) show a complex age pattern. Due to low temperatures during deformation, the Rb–Sr system only partly re-equilibrated in biotite. Synkinematically grown phengitic white mica evidence a low-grade metamorphic overprint at 501–474 Ma. The deformation was associated with W/NW-directed nappe stacking, which caused the actual geological configuration of southern Andhra Pradesh with the arcuate Nallamalai fold belt as most prominent feature. Regional considerations lead to the proposition that thrusting in the Krishna Province is the expression of intracratonic deformation in response to the amalgamation of the Eastern Ghats-Rayner Province terrane with subcontinental India in early Palaeozoic time.

P– T– t evolution of the Wilson Terrane metamorphic basement at Oates Coast, Antarctica

Within the basement of the northern Wilson Terrane at Oates Coast, a very-high-grade central zone is distinguished from high-grade zones to the east and west. In the central zone, P– T estimates of 8 kbar and 800°C derive from the relic assemblage: (1) Crd+Bt+Sil+Spl+Pl+Qtz for an earlier medium-pressure granulite-facies metamorphism which is also documented by relic assemblages Qtz+Pl+Bt+Opx (±Grt±Cpx). A subsequent low-pressure granulite-facies to upper-amphibolite-facies stage with pervasive migmatization took place at 4–5.5 kbar and minimum 700°C, as derived from mineral reactions and thermodynamic calculations on the assemblages (2) Grt+Crd+Bt+Pl+Qtz and (3) Grt+Bt+Sil+Pl+Qtz±Spl. Decompression at still high temperatures and a clockwise directed P– T– t path are indicated by reactions Bt+Sil+Qtz=Crd+Grt+Kfs+V and Grt+Sil+Qtz+V=Crd. The low-pressure granulite-facies to upper-amphibolite-facies stage is dated by six nearly concordant U–Pb monazite ages of 484–494 Ma from three migmatite samples and correlates to the late Pan-African Ross Orogeny in Cambro-Ordovician times. The age of the medium-pressure granulite-facies assemblages is not constrained by geochronological data. Either they form relics of the Precambrian Antarctic Craton, or they represent an early metamorphic stage of the Ross Orogeny. Rb–Sr and K–Ar dating on biotites yielded 470, 468, 470 Ma and 473, 469, 470 Ma (±5 Ma each), indicating the time of cooling to 450–300°C. This is confirmed by 40Ar– 39Ar plateau ages of 476±2, 472±3 and 470±2 Ma for these biotites. A late tectonic pegmatite yielded a concordant U–Pb monazite age of 489±3 Ma, while slightly discordant U–Pb data of two zircon fractions are explained by recent minor lead loss of ca 490 Ma old zircons. Cooling to ca 500–350°C is dated to 472±2 Ma by concordant 40Ar– 39Ar plateau ages of two muscovite fractions. The cooling history of the basement from high-grade conditions to the blocking temperature of Rb–Sr and K–Ar in micas took place within ca 15–20 Ma. Cooling rates of 18–25°C Ma −1 can be derived, if continuous cooling is assumed. U–Pb data points of zircons as well as Sm–Nd whole rock model ages between 1.8 and 1.9 Ga indicate that at least part of the migmatites derive from Early Proterozoic crustal protoliths. Comparing the new P– T– t data from the northern Wilson Terrane with those from the southern Wilson Terrane, a common tectono-metamorphic history becomes evident for a 600 km long sector of the Ross Orogenic belt at the Pacific end of the Transantarctic Mountains, at least since the granulite-facies metamorphic event.

Caledonian eclogite-facies metamorphism of Early Proterozoic protoliths from the North-East Greenland Eclogite Province

High-pressure metamorphic assemblages occur in mafic, ultramafic and a few intermediate rocks in a gneiss complex that covers an area of approximately 400 × 100 km in the North-East Greenland Caledonides. Detailed petrologic and geochronologic studies were carried out on three samples in order to clarify the P-T-t evolution of this eclogite province. Geothermobarometry yields temperature estimates of 700–800 °C and pressure estimates of at least 1.5 GPa from an eclogite sensu stricto and as high as 2.35 GPa for a garnet websterite. The eclogite defines a garnet-clinopyroxene-amphibole-whole rock Sm-Nd isochron age of 405 ± 24 Ma (MSWD 0.9). Isofacial garnet websterites define garnet-clinopyroxene-orthopyroxene-amphibole-whole rock-(biotite) ages of 439 ± 8 Ma (MSWD =2.1) for a coarse-grained sample and 370 ± 12 Ma (MSWD=0.6) for a finer-grained variety. Overgrowths on zircons from the fine-grained pyroxenite and the eclogite give a pooled 206Pb/238U SHRIMP age of 377 ± 7 Ma (n=4). Significantly younger Rb-Sr biotite ages of 357 ± 8, 330 ± 6 and 326 ± 6 agree with young Rb-Sr, K-Ar and 40Ar/39Ar mineral ages from the gneiss complex and indicate slow cooling of the eclogitic rocks. High-pressure metamorphism may have been at least 439 Ma old (Siluro-Ordovician) with cooling through amphibolite-facies conditions in the Devonian and continued crustal thinning and exhumation well into the Carboniferous. Sm-Nd whole rock model ages indicate the eclogite protoliths are Early Proterozoic in age, while 207Pb/206Pb SHRIMP ages of 1889 ± 18 and 1981 ± 8 from anhedral zircon cores probably reflect Proterozoic metasomatism. The samples have negative ɛNd values (−5 to −16) and elevated 87Sr/86Sr ratios (0.708–0.715), consistent with field evidence that the eclogite protoliths were an integral part of the continental crust long before Caledonian metamorphism. The presence of a large Caledonian eclogite terrane in Greenland requires modification of current tectonic models that postulate subduction of Baltica beneath Laurentia during the Caledonian orogeny.

Nd isotopic evidence for the position of southernmost Indian terranes within East Gondwana

Sm-Nd model ages of orthopyroxene-bearing massif charnockites from the Cardamom Hills Massif and adjoining supracrustal rocks from the Kerala Khondalite Belt in southernmost India are used to infer some of the relationships within these rocks and between them and neighboring areas. Most of these rocks have model ages of 2.1–2.8 Ga with most charnockites in the range 2.2–2.6 Ga. Thus, 3.0–3.4 Ga Archean rocks to their north did not contribute material to either suite and the two suites may have been juxtaposed after formation of the supracrustal rocks. The similarity of Sm-Nd isotope systems in the two units studied here supports an argument that the massif charnockites were the primary sole source of the detritus incorporated into the supracrustal rocks. A cordierite gneiss, representative of a relatively minor lithology in the supracrustal belt, has a model age of ∼ 1.3 Ga. The protolith of this gneiss not only formed from much younger material than the rest of the belt but also formed significantly after the other metasedimentary rocks. The source material of the gneiss protolith may have been located in the Wanni and Vijayan Complexes of Sri Lanka. The overlap of the model ages of rocks in this area and those in the Highland Complex of Sri Lanka supports the notion that these two sets of rocks were joined to each other in Gondwana. They belong to a belt that ran from Antarctica through Sri Lanka and India into Madagascar. This belt was involved in Pan-African tectono-metamorphism, as reflected in the ∼ 550 Ma age of the last, granulite-forming, event throughout the belt.

Pb-Sr-Nd isotopic behavior of deeply subducted crustal rocks from the Dora Maira Massif, Western Alps, Italy

Pb, Nd and Sr isotope data are reported on mineral separates and whole rock samples of Mg-rich metapelites and associated rocks that have been subducted to depths of ca. 100 km in the mantle within the last 100 Ma and returned to the surface, and for which metamorphic conditions were estimated to be 700–800°C at > 28 kilobars pressure. Three rock types, fine- and coarse-grained pyrope quartzites and NaFe-rich jadeite-kyanite quartz layers, were collected from a single outcrop in the coesite-bearing portion of the Dora Maira massif. Samples analyzed include pyrope, pseudomorphs after pyrope, phengite, zircon, the new mineral ellenbergerite, and whole rocks. Isotope data from all three decay systems indicate minimal, if any, isotopic exchange with mantle rocks. Isotope ratios from minerals with low parent/daughter ratios cluster around 143Nd/ 144Nd = 0.51225: ϵ(Nd) = −7.6; 87Sr/ 87/Sr = 0.7 = 15.75–15.80 with 206Pb/ 204Pb = 19.4–20.7. These compare closely with ratios from Precambrian granitic rocks or their derived sediments. From the Pb isotope data and the Sm/Nd whole rock model ages we estimate a mean age of 2000 ± 500 Ma for the sediment protoliths. Pyrope mineral separates record approximate ages for Alpine metamorphism of 34–38 Ma by the Sm/Nd method; the U/Pb method applied to zircon and ellenbergerite yields possible additional metamorphic ages of 50–55 Ma. These data illustrate a possible mechanism for creating isotopically enriched mantle. Had these rocks yielded their isotopes to the mantle, “enriched mantle” could have resulted, the magnitude of the enrichment depending upon the total volume attained by the mixing process. Similarly the metasediments could also provide the high potassium abundances and appropriate Pb, Sr and Nd isotope ratios that are postulated to accompany mantle metasomatic processes.

Diagenetic Resetting of Sm-Nd Isotope Systematics in Wilcox Group Sandstones and Shales, San Marcos Arch, South-Central Texas: ABSTRACT

ABSTRACT Sm-Nd analyses of sandstones and shales from the Wilcox Group (Upper Paleocene-Lower Eocene) suggest that the whole rock Sm-Nd signature is changed by burial diagenesis. The samples are from the San Marcos arch, south-central Texas, at depths ranging from outcrop to 15,000 feet. Sandstones from above the depth of intense feldspar reactions (ca. 10,000 feet) have whole rock Nd model ages of 1400 - 1500 Ma, whereas sandstones buried deeper than ca. 10,000 feet have Nd model ages of 1500 - 2000 Ma. Subsurface shale samples have whole rock model ages that increase from 1300 Ma at 5,000 feet of burial to 1500 Ma at 15,000 feet. Model ages increase most between 5,000 and 10,000 feet, within the zone of intense smectite illitization. The similar depositional age of all samples makes change in provenance an unlikely cause of the observed Nd model age variation. Sm and Nd reservoirs within the shale change considerably with progressive diagenesis. Outcrop samples contain subequal concentrations of Sm and Nd in the acid-soluble and silicate fractions. The acid-soluble reservoir in deeper samples contains progressively less Sm and Nd, and higher Sm/Nd. In all samples, the acid-soluble Nd is more radiogenic than the Nd in the corresponding silicate fraction, and the difference between the two reservoirs increases with depth. Diagenetic enrichment of Sm relative to Nd in the authigenic phases is the most likely explanation for the observed increase in Nd model ages in the sandstones. Decrease in Nd causes the increase in model age in the shales.

U-Th-Pb systematics in three Apollo 14 basalts and the problem of initial Pb in lunar rocks

The isotopic composition of Pb and the elemental concentration of U, Th and Pb were measured on ‘total’ rock samples 14053, 14073 and 14310 and on mineral separates of 14310 and 14053. Sample #73 appears to be quite similar to #310. Sample #310 yields total rock model ages of T( 206Pb/ 238U) = 4.24AE, T( 207Pb/ 235U) = 4.27AE, and T( 208Pb/ 232Th) = 4.13AE. These are nearly concordant and distinct from the Rb-Sr and K-Ar crystallization ages of 3.88 AE. Mineral separates from 14310 show a wide spread in 207Pb/ 206Pb ranging from 0.483 to 0.995. The data points define a reasonable linear array on the coupled Pb-U evolution diagram. Similar analyses of 14053 give high, discordant total rock model ages of T( 206Pb/ 238U) = 5.60AE, T( 207Pb/ 235U) = 5.18AE, and T( 208Pb/ 232Th) = 5.48AE . Mineral separates show a range of 207Pb/ 206Pb from 0.716 to 1.209. These data also define a reasonable linear array on the coupled Pb-U evolution diagram. These are the first Pb-U isochrons obtained for lunar basalts and indicate a reasonable solution to the previous discrepancy between the different methods of ‘absolute’ age determination. The resulting U-Pb isochron ages are compatible with the Rb-Sr and K-Ar ages on the same rocks. However, it is not possible to establish a precise time of ‘crystallization’ from the Pb-U data because of the small angle of intersection between the linear arrays and the concordia curve. These data show that total rock model ages do not in general yield crystallization ages. The data on #310 and #053 show that these rocks were formed containing a highly radiogenic initial lead ( 207Pb/ 206Pb) I 0 ≈ 1.46 which accounts for the excessively high total rock model ages by the U-Th-Pb method. The only significant discrepancy in the data is the apparent variability of ( 208Pb/ 206Pb) I 0 in #053 which remains to be resolved. The ( 207Pb/ 206Pb) I 0 in these rocks corresponds to the radiogenic lead evolved between 4.51 and 3.88 AE in a U-rich environment. Such data from initial Pb may provide a new chronometer for early lunar evolution. The high 207Pb/ 206Pb ages in some total lunar soils as well as in treated fractions may be partly explained as a consequence of the contribution of lunar basalts with radiogenic initial Pb. The data prove that at the time of extrusion of some basalts, unsupported lead with extremely high 207Pb/ 206Pb ratios was added to the lunar surface.