Low Initial Sr Isotopic Ratios Research Articles

Prolonged Neo-Tethyan magmatic arc in Myanmar: evidence from geochemistry and Sr–Nd–Hf isotopes of Cretaceous mafic–felsic intrusions in the Banmauk–Kawlin area

Cretaceous mafic–felsic intrusions are extensively distributed in the West Burma terrane (Myanmar), but their petrogenesis and tectonic setting still remain unclear. In this study, whole-rock geochemical and Sr–Nd as well as zircon U–Pb and Hf isotopic data for the Cretaceous mafic–felsic intrusions from the Banmauk–Kawlin area (northern Myanmar) are presented. Precise zircon U–Pb dating results indicate that they emplaced at Cretaceous (~ 110–90 Ma) and likely belong to an eastward extension of the coeval Gangdese magmatic belt in the southern Lhasa terrane (Tibet). The studied Cretaceous intrusions (SiO2 = 41.97 to 74.54 wt%) mostly have calc-alkaline and Na-rich characteristics, and strong enrichments in large ion lithophile elements (e.g., Cs, Rb, and K), depletions in Nb and Ta on primitive mantle-normalized diagrams, consistent with geochemical characteristics of arc-type magmas. Most of Cretaceous gabbroic and dioritic intrusions with relatively depleted mantle Sr–Nd–Hf isotopic compositions (87Sr/86Sri = 0.7041–0.7048, eNd(t) = 0.2–6.8, and eHf(t) = 4.3–15.1) and high Ba/La ratios were possibly derived from partial melting of mantle wedge metasomatized by slab-derived fluids and underwent a certain degree of fractional crystallization. Whereas Cretaceous granodiorite and granite probably formed by partial melting of juvenile arc lower crust on the basis of positive eNd(t)–eHf(t) values (− 2.4 to 6.6 and − 2.8 to 15.3) and low initial Sr isotopic ratios (87Sr/86Sri = 0.7045–0.7063). A wide range of Th/Nb ratios and Sr–Nd–Hf isotopic values in these intrusions suggests that their juvenile arc lower crust possibly formed by melting of slab-derived fluids and/or sediment melts metasomatized mantle. Overall, Cretaceous (~ 110–90 Ma) mafic–felsic intrusions in the West Burma terrane may have formed in an arc setting during subduction of the Neo-Tethyan oceanic lithosphere, suggesting a prolonged Neo-Tethyan magmatic arc system from southern Tibet to Southeast Asia.

Geochemistry and apatite U–Pb geochronology of alkaline gabbros from the Nodoushan plutonic complex, Sanandaj–Sirjan Zone, Central Iran: Evidence for Early Palaeozoic rifting of northern Gondwana

The Zagros Orogen formed during the Cenozoic collision of Arabia with Eurasia and resulted in the closure of the Neo‐Tethys Ocean. Collision was preceded by a complicated tectonic history involving Pan‐African orogenesis, Late Palaeozoic rifting and the formation of Neo‐Tethys, and subsequent Mesozoic convergence on the northern margin of the ocean contemporaneous with ophiolite obduction on its southern margin. The Sanandaj–Sirjan Zone (SaSZ) is a metamorphic belt within the Zagros Orogen of Gondwanan provenance. U–Pb zircon geochronology has revealed the presence of Pan‐African igneous and metamorphic basement complexes, in addition to minor late Palaeozoic plutons and abundant Jurassic plutonic rocks. This study presents a LA‐ICP‐MS U–Pb apatite age of 440.7 ± 3.7 Ma from a gabbroic body on the central north‐eastern margin of the SaSZ (Nodoushan region). Based on petrography and geochemistry, en route fractional crystallization of ascending magma was an important process in the evolution of the intrusive rocks. Geochemical characteristics imply that the south‐west Nodoushan alkaline gabbroic magma was originated from the asthenospheric mantle source, whereas the high ratios of (La/Yb)N and (Dy/Yb)N are related to the low degree of partial melting from the garnet‐bearing mantle source. Enrichment pattern of Nb, Ta, and Ti and depletion of Rb, Th, and Y are similar to the OIB pattern and intraplate alkaline magmatic rocks. High εNd(t) values (+1.29 to +1.50), low initial Sr isotopic ratios (0.703679–0.704098), and trace element ratios (e.g., Nb/La, Ce/Pb, Ba/Nb, and Th/Nb) indicate that crustal contamination was insignificant in its petrogenesis. The petrogenesis of the Nodoushan alkaline gabbroic rocks could be related to the presence of extensional phase, upwelling and decompressional melting of asthenospheric mantle in the rift basin which was associated with the opening of the Paleo‐Tethys Ocean in Late Ordovician to Silurian times.

Paleoproterozoic plume-related basaltic rocks in the Mana gold district in western Burkina Faso, West Africa: Implications for exploration and the source of gold in orogenic deposits

Birimian volcanic rocks of the Mana District are located in the an important gold-mineralized segment of the Paleoproterozoic Houndé greenstone belt, western Burkina Faso, which contains cumulative resources of ∼11 Moz. Five orogenic gold deposits (∼8 Moz) are hosted in or close to basaltic rocks. Theses rocks were studied to investigate their possible role as a gold source in younger orogenic gold deposits. They are Fe-rich tholeiitic basalts with flat REE patterns, with (La/Yb)N = 0.96–1.3 and without negative Eu anomaly (Eu/Eu* = 0.92–1.26). The basalts also have low initial Sr isotopic ratios (0.693612–0.702190) and positive εNd values (+2.25 to +3.14). Using a Ce/Nb vs. Th/Nb diagram and various plume-related basalts worldwide for comparison, the Mana basalts are shown to be plume-related. In addition, using Zr/Nb vs. Nb/Th and Nb/Y vs. Zr/Y binary diagrams and reference fields, the Mana basaltic rocks appear to have formed directly above the plume head. Because plume-related basalts tend to be enriched in gold relative to MORB, we propose that the gold endowment of the Mana district is mostly related to the occurrence of plume-related basaltic rocks, which may have served as an important metal stock during subsequent remobilization for forming the orogenic gold deposits. We also propose that for gold exploration, two simple geochemical diagrams involving Zr, Y, Nb and Th could be used at an early stage to test the origin of the basaltic rocks and hence indirectly establish the fertility of a specific belt for hosting orogenic gold deposits.

Geochemical characteristics and tectonic setting of the Middle Permian Tiaohu Formation mafic-ultramafic rocks of Santanghu area, Xinjiang, Northwest China

Santanghu area in northeastern Xinjiang region of Northwest China is an important component of the Central Asian Orogenic Belt (CAOB), in which the dynamic mechanism of Permian magmatism is controversial. In Santanghu area is exposed a thick succession of the Middle Permian basalts, including a small amount of picritic basalts and andesites, known as the Tiaohu Formation. The picritic basalts contain cumulate olivine, and have whole-rock Mg# up to 0.68–0.77; the basalts exhibit porphyritic or doleritic textures, and have relatively low Mg# of 0.41–0.54, typical of evolved magmas. The mafic-ultramafic rocks of the Tiaohu Formation are slightly enriched in Light Rare Earth Elements (LREEs), and exhibit negative Nb and Ti anomalies. They also have high TiO2 content, and Nb/Y and Zr/Yb ratios greater than those of island arc volcanic rocks. Relatively low initial Sr isotopic ratios and high positive ɛ Nd(t) and ɛ Hf(t) values argue against contamination by ancient continental crust, and suggest formation of the Tiaohu Formation by partial melting of relatively refractory depleted lithospheric mantle that underwent metasomatism and extraction by fluid from the subducted slab. In addition, up to 38% olivine in picritic basalts indicates high-degree partial melting of lithospheric mantle, and the underlying Lucaogou Formation contains fragments of ultra-alkaline magmatic rocks that originated in the deep mantle. These observations imply wide-spread underplating in Santanghu area, which may have been associated with a mantle plume.

Petrogenesis of the Yangchang Mo-bearing granite in the Xilamulun metallogenic belt, NE China: geochemistry, zircon U-Pb ages and Sr-Nd-Pb isotopes

The Yangchang granite-hosted Mo deposit is typical of the Xilamulun metallogenic belt, which is one of the important Mo–Pb–Zn–Ag producers in China. A combination of major and trace element, Sr, Nd and Pb isotope, and zircon U–Pb age data are reported for the Yangchang batholith to constrain its petrogenesis and Mo mineralization. Zircon LA-ICPMS U–Pb dating yields mean ages of 138 ± 2 and 132 ± 2 Ma for monzogranite and granite porphyry, respectively. The monzogranites and granite porphyries are calc-alkaline with K2O/Na2O ratios of 0.75–0.92 and 1.75–4.42, respectively. They are all enriched in large-ion lithophile elements (LILEs) and depleted in high-field-strength elements (HFSEs) with negative Nb and Ta anomalies in primitive-mantle-normalized trace element diagrams. The monzogranites have relatively high Sr (380–499 ppm) and Y (14–18 ppm) concentrations, and the granite porphyries have lower Sr (31–71 ppm) and Y (5–11 ppm) concentrations than those of monzogranites. The monzogranites and granite porphyries have relatively low initial Sr isotope ratios of 0.704573–0.705627 and 0.704281, respectively, and similar 206Pb/204Pb ratios of 18.75–18.98 and 18.48–18.71, respectively. In contrast, the εNd(t) value (−3.7) of granite porphyry is lower than those of monzogranites (−1.5 to −2.7) with Nd model ages of about 1.0 Ga. These geochemical features suggest that the monzogranite and granite porphyries were derived from juvenile crustal rocks related to subduction of the Paleo-Pacific plate under east China. Copyright © 2013 John Wiley & Sons, Ltd.

Tectonic Setting of the Cu-Ni Sulfide-Bearing Mafic-Ultramafic Complexes in Northern Jilin Province, NE China

There are series of Cu-Ni sulfide-bearing mafic-ultramafic intrusions widespread in north JilinProvince,Northeastern China. The intrusions formed in Xing’an-Mongolian Orogenic Belt near to the northeastern margin of North China Craton. The complexes were formed in almost same period according to the zircon U-Pb dating reported recently, which means that the complexes were formed in same tectonic period and belong to one tectonic magmatic event. The rock assemblages are different from the ophiolite type and Yidun type in orogenic belt. The mafic-ultramafic complexes formed in the range from 217 Ma to 232 Ma coeval with A-type granites in the area, which formed bimodal igneous rock assemblage. According to the regional angular unconformities, there were existed the orogenies of Caledonian, Hercynian, Early Indosinian, Late Indosinian and Yanshanian. The Early Indosinian coeval with orogenic I-type granites and sanukitie that suggesting the lithosphere thickening in the extrusion tectonic setting of orogenic processes, however the Late Indosinian coeval with bimodal igneous rock assemblage that suggesting the lithosphere thinning in the extension tectonic setting of post-orogenic processes in the Xing’an-Mongolian Orogenic Belt. Chemical composition of the mafic-ultramafic rocks has the characteristics of high-Mg and low-K tholeiites related with inter-continental post-orogenic tectonic setting. The trace elements indicate their formed in conditions of continental extension belt or initial rift and has the characteristics of revolution from oceanic island arc, volcanic arc of continental margin to continental extended belt. The low initial Sr isotopic ratios and positive εNd(t) values suggest that the initial magma of the complexes come from the parting melting of depleted lithospheric mantle. The depleted ithospheric mantle was new formed supported by zircon Hf isotope in Hongqiling complex. The depleted lithospheric mantle may be caused by the asthenosheric mantle upwelling and underplating in the tectonic setting of extension during the Late Indosinian post-orogenic processes.

Archean mantle fluids preserved in fibrous diamonds from Wawa, Superior craton

We analyzed the first samples of Neoarchean mantle fluid captured as inclusions in fibrous diamonds from the 2.701–2.697 Ga Wawa metaconglomerate (Canada). The K-, Na-, and Cl-rich carbonate-bearing saline fluid inclusions bear a strong resemblance to those of Phanerozoic fibrous diamonds. This similarity in major and trace elements, volatile components, and nitrogen characteristics implies the uniformitarian extension of some mantle processes, including the formation of fluid-rich fibrous diamonds, back to the Neoarchean. Low initial Sr isotope ratios ( 87 Sr/ 86 Sr i = 0.700–0.702) for the diamonds at 2.7 Ga are within error of estimates of Archean depleted mantle or bulk silicate Earth at 2.7 Ga, consistent with a non-enriched, convecting mantle/group I kimberlite-like source of the fluid. Eu anomalies occur in both Neoarchean and Phanerozoic saline fibrous diamond fluid, but not in silicic or carbonatitic fluid varieties. This restriction suggests that interaction with Cl-rich fluid may lead to Eu anomalies in mantle rocks, which therefore should not be automatically assigned to a subduction origin when a petrogenetic history involving saline fluids is suspected.

First occurrence of Paleoproterozoic oceanic plateau in the Guiana Shield: The gold-bearing El Callao Formation, Venezuela

The El Callao Formation is part of the Guasipati-El Callao Greenstone Belt, located in the northernmost part of the Guiana Shield, Venezuela. It consists of volcanic rocks, which have been analyzed for major and trace elements, REE concentrations and Sr–Nd isotopic composition. Based on these data, they have been characterized as Fe-rich tholeiitic basalts. On a primitive mantle-normalized diagram, all basalt samples display fairly flat patterns for the REE (La/SmCN=0.77–1.13 and La/YbCN=0.87–1.42) as well as for other trace elements, without negative anomalies for either Eu (Eu/Eu*=0.88–1.19) or Nb. The basalts have low initial Sr isotopic ratios (∼0.7014 to ∼0.7019) and low positive ɛNdi values (+0.7 to +2.1). Based on these geochemical data we propose that the tholeiitic basalts were formed in an oceanic plateau tectonic setting from a mantle plume source, possibly from the plume-head peripheral part, and that they have a primitive mantle source. Samples show isotopically uniform juvenile features, which rule out the possibility of significant contamination of the protolith magmas by ancient continental crust. These results are similar to those obtained from other greenstone belts from the Guiana Shield and Birimian domains of the West African Craton. As noted recently by Bierlein and Craw (2009), metabasalts derived from an oceanic plateau, when accreted into a collisional orogen, may result in significant gold enrichment; metamorphic terranes hosting large volumes of these rocks, such as the Guasipati-El Callao Greenstone Belt, are consequently prospective for gold mineralization.

Extrusion of high-pressure Cache Creek rocks into the Triassic Stikinia–Quesnellia arc of the Canadian Cordillera: implications for terrane analysis of ancient orogens and palaeogeography

AbstractThe volcanic Triassic Takla Group constitutes a significant part of Stikinia and Quesnellia, two major terranes of the Canadian Cordillera that are separated by high-pressure rocks of the Cache Creek terrane containing Asian fauna. The geochemical and isotopic characteristics of the Takla Group in Quesnellia and Stikinia are similar, that is, tholeiitic basalts characterized by low abundances of strongly incompatible trace elements, negative Nb anomalies, +6 to +8 εNd values, the low initial Sr isotopic ratios, and relatively horizontal chondrite-normalized heavy REE patterns, all features typical of relatively primitive arcs with little or no continental crust involvement. These similarities have led to several geometric models: post-Middle Jurassic duplication by strike-slip faulting, and oroclinal or synformal folding. However, they are all inconsistent with either palaeomagnetic or faunal data, and the presence of a Triassic overstep sequence, which indicates amalgamation c. 50 ma before emplacement of the youngest oceanic rocks of the Cache Creek terrane. An alternative model is proposed: oblique eastward subduction of the Cache Creek accretionary prism and fore-arc producing high-pressure metamorphism, followed by extrusion into the arc and exhumation by the Middle Jurassic. This model implies that these high-pressure rocks, rather than marking an oceanic suture between disparate arc terranes, support a para-autochthonous origin.

The Eocene–Oligocene magmatic hiatus in the south-central Canadian Cordillera: a capture of the Kula Plate by the Pacific Plate?

The Tertiary (Paleogene and Neogene) geological record in south-central Canadian Cordillera is dominated by the 350–400 km wide, lower Eocene volcanic arc and the overlying Miocene–Recent back-arc lavas that are separated by a hiatus in magmatic activity between 48 and 24 Ma. In the Black Dome area (~240 km north of Vancouver), the Eocene volcanic rocks are mainly continental margin calc-alkaline andesite and dacite, resulting from the melting of a juvenile mafic source at the base of the crust. In contrast, the Miocene volcanic rocks resemble continental flood basalts. Both Eocene and Miocene rocks from the Black Dome volcanic complex have high positive εNd values (+7.2 to +7.4 and +6.4 to +7.6, respectively) and low initial Sr isotopic ratios (0.702 516 – 0.703 528 and 0.703 376 – 0.703 392, respectively) comparable to modern oceanic basalts. The onset of the hiatus in magmatism at 48 Ma coincides with capture of the Kula Plate by the Pacific Plate resulting in a change in convergence direction with the North American Plate from orthogonal to margin-parallel. The margin-parallel motion is inferred to have removed a 50–100 km sliver of the Eocene forearc that formed the boundary between the Pacific and subducted Kula Plate. Reinitiation of arc magmatism at 24 Ma is related to subduction of the Farallon and associated plates and it superimposed back-arc tholeiitic magmatism on top of the Eocene arc.

Petrogenesis of Cretaceous Peraluminous Granite Suites with Low Initial Sr Isotopic Ratios, Kyushu Island, Southwest Japan Arc

Cretaceous granitic rocks are widely distributed in the northern to central parts of Kyushu Island, southwest Japan arc, and are divided into two types, granite and tonalite-granodiorite. These rocks are clearly defined by their mineral assemblage, that is, the granite contains mostly muscovite and/or garnet and does not have hornblende, whereas the tonalite-granodiorite is always observed with hornblende. Many workers suggest that most of the granite has low initial Sr isotopic ratios (SrI 90 Ma: 0.7052-0.7059) whose values are similar to those of tonalite-granodiorite (SrI 90 Ma: 0.7050-0.7056). The SiO 2 contents of the granite (67.9 to 78.0 wt.%) are higher than that of tonalite-granodiorite (54.2 to 68.2 wt.%). The granite is characterized by peraluminous compositions, while the tonalite-granodiorite shows metaluminous to peraluminous compositions. The petrogenetic relation between granite and tonalite-granodiorite is examined by a modal batch melting model, which referred to results of already published experiments. The result of the model suggests that the petrogenesis of the peraluminous granitic magma can be explained by an anatexis of metaluminous tonalite in the tonalite-granodiorite under lower to middle crustal conditions and variable fH 2O. Rocks of tonalite to granodiorite in a subduction system generally have low initial Sr isotopic ratios (<0.705), which imply that the magma derived from a highly metaluminous origin such as mafic magma, mafic lower crust and/or subducting slab. In this study, it is regarded that the peraluminous granitic magma can be directly produced by an anatexis of metaluminous tonalite. Therefore, peraluminous granite with low initial Sr isotopic ratios may imply to a first step of recycling of the granitic layer in an active plate margin.

Granitoids of the Central Asian Orogenic Belt and continental growth in the Phanerozoic

The Central Asian Orogenic Belt (CAOB), also known as the Altaid Tectonic Collage, is characterised by a vast distribution of Paleozoic and Mesozoic granitic intrusions. The granitoids have a wide range of compositions and roughly show a temporal evolution from calcalkaline to alkaline to peralkaline series. The emplacement times for most granitic plutons fall between 500 Ma and 100 Ma, but only a small proportion of plutons have been precisely dated. The Nd-Sr isotopic compositions of these granitoids suggest their juvenile characteristics, hence implying a massive addition of new continental crust in the Phanerozoic. In this paper we document the available isotopic data to support this conclusion.Most Phanerozoic granitoids of Central Asia are characterised by low initial Sr isotopic ratios, positive εNd(T) values and young Sm—Nd model ages (TDM) of 300-1200 Ma. This is in strong contrast with the coeval granitoids emplaced in the European Caledonides and Hercynides. The isotope data indicate their ‘juvenile’ character and suggest their derivation from source rocks or magmas separated shortly before from the upper mantle. Granitoids with negative εNd(T) values also exist, but they occur in the environs of Precambrian microcontinental blocks and their isotope compositions may reflect contamination by the older crust in the magma generation processes.The evolution of the CAOB is probably related to accretion of young arc complexes and old terranes (microcontinents). However, the emplacement of large volumes of post-tectonic granites requires another mechanism, probably through a series of processes including underplating of massive basaltic magma, intercalation of basaltic magma with lower crustal granulites, partial melting of the mixed lithologic assemblages leading to generation of granitic liquids, followed by extensive fractional crystallisation. The proportions of the juvenile or mantle component for most granitoids of Central Asia are estimated to vary from 70% to 100%.

Isotope Tracing and Prospecting Assessment of Gold–Silver Deposits in Zhejiang Province

AbstractThree different types of gold and silver deposits in Zhejiang Province (Huangshan gold deposit, Zhilingtou gold–silver deposit and Haoshi silver deposit) show marked differences in lead and strontium isotopic composition, suggesting three different sources and geneses of these deposits. The Huangshan gold deposit features low initial Sr isotope ratios and low μ values or low content of radiogenic Pb and its ore–forming materials came primarily from the upper mantle; the Zhilingtou gold–silver deposit shows high initial Sr isotope ratios and high μ values or high content of radiogenic Pb and the ore–forming materials were derived mainly from the upper crust; and the Haoshi silver deposit has its Pb and Sr isotope ratios between the above two cases with the ore—forming materials stemming from both the mantle and the crust. The characteristic Pb isotopic composition may serve as an indicator for prospecting for different types of ore deposits.

Spatial variations of Sr and Nd isotope ratios of Cretaceous-Paleogene granitoid rocks, Southwest Japan Arc

Cretaceous-Paleogene granitoid rocks and contemporaneous volcanic rocks are widely distributed in the Inner Zone of Southwest Japan. This intense intermediate to felsic magmatism is considered to have taken place on the eastern margin of the Eurasian Continent, before the Southwest Japan Arc drifted away from the continent in the middle Miocene, resulting in the opening of the Japan Sea. The granitoid rocks show regional variations in terms of their radiometric age, petrography, Sr, Nd and O isotope ratios. Based on Sr and Nd isotope ratios, granitoid rocks can be divided into three zones (South, Transitional and North) between the Median Tectonic Line and the Japan Sea. Granitoid rocks and associated gabbros of the North Zone have low initial Sr isotope ratios (0.7048 to 0.7068) and high initial eNd values (+3 to-2.2), whereas granitoid rocks and gabbros from the South Zone have high initial Sr isotope ratios (0.7070 to 0.7088) and low initial eNd values (-3.0to-8.0). Most granitoid rocks from the Transitional Zone have Sr and Nd isotope ratios that lie between those of the North and South Zones, although there is some overlap. Contamination of magmas by upper crust cannot explain this geographical variation in Sr and Nd isotopes. Instead, the regional variation is attributed to compositionally different, magma sources (probably upper mantle and lower crust), beneath the North and South Zones. This is supported by the Sr and Nd isotopic ratios of upper mantle and lower crustal xenoliths included in Cenozoic volcanic rocks in the North and South Zones. These ratios are similar to those of the granitoid rocks in the respective zones. It is suggested that a micro-continent or island arc consisting of continental crust was underthrust beneath the South Zone before or during the Cretaceous, resulting in compositionally distinct sources for granitoid rocks of the North and South Zones. The large variation observed in Sr and Nd isotope ratios for Transitional Zone granitoid rocks is explained by variable proportions of the two different crustal and upper mantle components.

Plutonic and metasedimentary rocks from the Coastal Range of northern Chile: Rb[sbnd]Sr and U[sbnd]Pb isotopic systematics

More than 50% of the surface in the Coastal Range of northern Chile between the towns of Taltal and Chan˜aral consists of granitoid rocks. Age determinations by means of UPb isotopic analyses on sized zircons and by means of RbSr isotopic measurements, in conjuction with other geochemical studies, reveal a complex pattern for the development of the plutonites. Though closely associated, two groups of intrusive rocks can be distinguished on the basis of isotopic and geochemical data:In a west-east transect southeast of Chan˜aral, several calc-alkaline plutonites with basic to acid members intruded mainly into Paleozoic metasedimentary rocks. The UPb data of the zircons from the plutonites yielded concordant ages ranging from Lower Jurassic to Lower Cretaceous. UPb ages are identical with RbSr biotite ages within the error limits and almost coincide with the KAr mineral ages. Both the ages and the initial87Sr86Sr ratios decrease continuously from west to east. The low initial Sr isotopic ratios (0.703–0.705) and other geochemical parameters indicate I-type characteristics.In the west-east segment northeast of Chan˜aral, mainly alkaline plutonic rocks intruded in an interval from Upper Permian to Upper Triassic. The UPb data of zircons are slightly discordant and indicate a small portion of inherited crustal material> 1 Ga old. RbSr biotite ages were reset by thermal pulses due to the emplacement of younger granitoid rocks. The initial 87Sr86Sr ratios of < 0.710 point to an origin of these rocks by anatexis of crustal rocks (S-type).The oldest rocks in the area of Chan˜aral consist of thick sequences of metasediments. The material for these rocks was derived from a source> 2.5 Ga old as indicated by the UPb data of the zircons. The lower intercept age of the zircons points to metamorphism of the source area at about 600 Ma. Due to uplift in the Early Paleozoic the source area supplied the material for the sediments which were deposited in the Devonian. Contact metamorphism in the Triassic transformed the sediments into metasedimentary rocks as demonstrated by RbSr whole-rock data. This event was caused by the emplacement of the S-type granitoid rocks which probably originated, however, from a deeper level of the crust.

Oxygen-isotope geochemistry of Archean granitoid gneisses and related rocks in the English River Subprovince, northwestern Ontario

Whole-rock δ 18O values for Archean granitoid gneisses from the southern domain of the English River Subprovince, northwestern Ontario, increase from an average of +6.5 for 3000 Ma old tonalites in the eastern Lac Seul area to an average of +7.9 for 2750–2800 Ma old tonalites, trondhjemites and granodiorites that occur in the Kenora and eastern Lac Seul areas. Still younger granitoid plutons and dykes (2660-2560 Ma) that intrude the older gneisses also have average δ 18O = +7.9. These values lie towards the low end of the range known for normal granitoid rocks. Oxygen-isotope data for minerals from the granitoid rocks indicate minor isotopic disequilibrium, but no evidence exists for large-scale depletion in 18O subsequent to granitoid formation. Rocks rich in 18O are abundant only in the northern domain of the English River Subprovince, which is dominated by paragneiss and metasedimentary migmatite (δ 18O +9−12). Within the Kenora area of the southern domain, however, a remnant supracrustal sequence characterized by a thin metachert/iron formation unit (δ 18O = +8−10) has been recognized. The low whole-rock oxygen-isotope values indicate that pre-existing 18O-rich supracrustal rocks were not involved in the genesis of the 3000 Ma old gneisses and have made only limited, if any, contribution to the magmas that formed the younger units. This conclusion is supported by the low initial Sr-isotope ratios of these rocks (0.7009-0.7027). The oxygen- and strontium-isotope data require that most of the granitoid rocks in the southern domain of the English River Subprovince comprise juvenile or near-juvenile additions to the continental crust.

The geochemistry of central Australian granulites in relation to the chemical and isotopic effects of granulite facies metamorphism

Metamorphism to intermediate-pressure granulite grade had a minimal effect on the geochemistry of layered gneisses in central Australia. The overall composition of the terrain is granodioritic and major element compositions have equivalents in igneous and sedimentary supracrustal rocks. K, Rb, Sr and probably Th concentrations, and K/Rb ratios are normal; the initial isotopic composition of Sr shows the usual range of crustal rocks. However, U is strongly depleted and was lost by a pervasive process, probably dehydration, rather than by anatexis. Comparison with other areas in which major chemical depletions and unusually low initial Sr isotopic ratios are postulated leads to alternative interpretations of these areas which do not involve large scale chemical migration. An intermediate composition for the lower crust may result from a high density of basic intrusions rather than chemical processes.

Strontium isotope study of the composite batholith between Princeton and Okanagan Lake

Twenty-seven whole-rock samples of granitic rocks and associated mafic inclusions define a Rb–Sr ispchron of 156 ± 6 m.y. with an initial ratio of 0.70415 ± 0.00017 (95% confidence limits). K–Ar and Rb–Sr data for granodiorite, quartz monzonite, and granite are concordant with this Upper Jurassic age. K–Ar dates suggest that diorite and quartz diorite are distinctly older, 165 to 185 m.y. old. The Rb–Sr data for these more mafic rocks alone (166 ± 53 m.y.) is not accurate enough to confirm this, but is not in conflict. Country rock samples are generally similar to the batholith in Sr-isotopic composition, and nearly all mafic inclusions are equilibrated with their enclosing granitic host. The Cordilleran transition zone between relatively low initial Sr isotope ratios (0.704 and less) and relatively high ratios (0.706 and greater) appears to lie just east of the batholith studied.

A Discussion on the evolution of the Precambrian crust - Geochemistry of granulite facies rocks and problems of their origin

Occurrences of granulite facies rocks are widespread in continental regions where they mostly are parts of stable shield areas. Granulite facies terrains are classified as low-, medium- or high-pressure terrains on the basis of critical mineral associations. Special interest is attached to the medium- and highpressure terrains, as they are representative of the deepest crustal levels available for study in any areal extent on the surface, and may give information about the composition of the lower continental crust. Granulite facies terrains are mainly composed of metamorphic and metasomatic rocks, but magmatic rocks with primary igneous textures interpreted as formed by crystallization of magmas under granulite facies conditions are frequent in some areas. Examples of such rocks are anorthosites, gabbros and mangerites. The low-pressure—high-temperature granulite facies rocks are chemically indistinguishable from the amphibolite facies gneisses with which they characteristically occur. It is therefore important to make a distinction between these and the higher pressure types. The medium- to high-pressure granulite facies terrains are characterized by a less ‘acidic’ average major element compositions, and significant depletions in Rb, Cs, Th and U compared with average surface shield compositions. Available data also indicate low initial Sr isotope ratios, even in the gneissic types. In the author’s opinion the important problem associated with granulite facies rocks is not that of their origin, but rather of their importance as constituents of the continental crust, and how they attained their present chemistry.