Rubidium‐strontium geochronology of the encounter bay granite and adjacent Metasedimentary rocks,South Australia

Rubidium‐strontium and strontium isotope data for eight whole‐rock samples of granite varieties from the Encounter Bay area, South Australia, yield an isochron age of 487 ± 37 m.y. Two specimens of albitised granite, formed as a result of late‐stage metasomatic alteration of original megacrystic granite, conform to this isochron. These data support a genetic relation between granites and late‐stage metasomatic alteration as suspected from field, petrographical and geochemical studies. Eight samples from contiguous Kanmantoo Group metasedimentary rocks have an isochron age of 487 ± 60 m.y. Thus this metamorphic event is coincident with emplacement of the Encounter Bay Granite.

The initial Sr⁸⁷Sr⁸⁶ ratio for the Encounter Bay Granite (0.719) is significantly higher than initial ratios for the Palmer (0.709) and Anabama (0.705) Granites from the same region and can be attributed to either remobilisation or incorporation of strontium from older crustal material in the intrusion. The apparent initial Sr⁸⁷/Sr⁸⁶ ratio for the Kanmantoo Group metasedimentary rocks (0.722) can not be distinguished from that for the Encounter Bay Granite within the analytical uncertainties. Compatability of ages and high initial Sr⁸⁷Sr⁸⁶ ratios suggest that the granites formed by remobilisation of associated crustal rock.     

Strontium isotopic studies of alkalic rocks: Localities from Australia,Spain, and the Western United States

Initial Sr⁸⁷/Sr⁸⁶ ratios and rubidium and strontium contents have been measured in 83 specimens from 8 suites of alkalic and ultrabasic rocks. The range of initial Sr⁸⁷/Sr⁸⁶ ratio observed and the number of specimens (in parentheses) analysed for each suite are: West Kimberley, Australia (8), 0.7125–0.7215; Jumilla, Spain (6), 0.7136–0.7158; Bearpaw Mountains, Montana (9), 0.7062–0.7086; Highwood Mountains, Montana (10), 0.7072–0.7087; Hopi Buttes, Arizona (8), 0.7038–0.7094; Leucite Hills, Wyoming (17), 0.7055–0.7070; Montana diatremes (13), 0.7034–0.7073; Navajo Province, Arizona and New Mexico (12), 0.7052–0.7099.The initial Sr⁸⁷/Sr⁸⁶ ratios of the West Kimberley and Jumilla rocks are the highest yet found in strontium-rich basic rocks. Some of the individual specimens from West Kimberley have initial ratios as high as some estimates of the present Sr⁸⁷/Sr⁸⁶ ratio of average crustal material. This is interpreted to mean that the West Kimberley and Jumilla rocks contain substantial amounts of radiogenic strontium, and possibly other elements, from a crustal source.     

A Rb-Sr geochronologic study of the pegmatites of the Middletown area,Connecticut

The pegmatites of Eastern Connecticut have a mineralogy consistent with a magmatic origin yet occur in a non-igneous environment. Various theories of genesis have been investigated by the Rb-Sr geochronologic method.Rb-Sr measurements on early stage pegmatite minerals indicate an age of 258±1 m.y. with initial Sr⁸⁷/Sr⁸⁶=0.734±0.0096. Previously reported K-Ar and U, Th-Pb ages for pegmatite minerals are 249±8 m.y. and 260±3 m.y. respectively. Rb-Sr whole rock data for the host rocks vary between 285±10 m.y. and 472±15 m.y. in age and between 0.705±002 and 0.7167±0.0016 in initial Sr⁸⁷/Sr⁸⁶. A direct genetic relationship between the pegmatites and their host rocks is thus precluded. In addition, whole rock samples of the Brimfield schist taken at variable distances from the Strickland Quarry pegmatite have remained essentially closed systems with respect to Rb and Sr and thus an in situ origin for this pegmatite is unlikely. Mixing of pegmatite and country rock systems has occurred only locally, and isotopic studies of these mixed rocks yield a date of 231±4 m.y. with initial Sr⁸⁷/Sr⁸⁶=0.7188±0.004, an age not inconsistent with previously reported K-Ar and Rb-Sr mineral dates on host rock minerals (approximately 220 to 240 m.y.).Late stage cleavelandites are anomalously enriched in radiogenic Sr-87, the source of which was most probably other zones within the crystallizing pegmatite. This is indicated by analyses of pegmatite whole rocks which show both enrichment and depletion of radiogenic Sr-87 in local systems. The conclusion is drawn that there was widespread movement of radiogenic Sr-87 within each pegmatite system, but that pegmatite-host rock reactions were minimal.     

The Origin of Associated Basic and Acid Rocks in the Lebombo--Nuanetsi Igneous Province, Southern Africa, as implied by Strontium Isotopes

Whole rock Rb–Sr measurements were made on associatedbasic and acid intrusives and extrusives from the northern andsouthern ends of the Lebombo monocline with a view to determineif the acid rocks came from the mantle or from the crust. Inthe south, in Swaziland (now called Ngwame) and Zululand, rhyolitesand granophyres (age 202 14 m.y.) have an initial Si⁸⁷–Sr⁸⁶ratio of 0.7042 0.0005; in the north, in the Nuanetsi syncline,rhyolites (age 206 13 m.y.) and granites from ring complexes(age 177 7 m.y.) have initial ratios of 0.7081 0.0008 and0.7085 0.0007, respectively. The initial ratios of the basicrocks, basalts and gabbros, vary in both areas and have a rangefrom 0.7042 to 0.7125 which can be explained by crustal contamination.The low initial ratios and isotopic homogeneity of the acidmagmas over long distances (160 km) in Swaziland and Zululandare interpreted as implying that they were derived from themantle.     

The geochemistry of the Dunedin Volcano: Strontium isotope chemistry

The isotopic composition of strontium has been determined for samples from the alkaline lavas of the Dunedin Volcano covering the range basalt, basanite, intermediate compositions, phonolite and quartz normative trachyte. The basaltic, intermediate and phonolitic rocks appear to be comagmatic and have similar low initial Sr⁸⁷/Sr⁸⁶ ratios around 0.7030, comparable with those of other alkaline provinces. The quartz normative trachytes have initial ratios significantly higher than those of the other rocks (0.7040) although their age is comparable. Contamination by sea water or crustal material could explain the higher initial ratios of the trachytes but it does not account for important features of their chemistry. It is suggested that the trachytes formed by partial melting involving an alkali feldspar-rich portion of older igneous rocks. Rb-Sr ages obtained are comparable with published K-Ar dates. The Rb-Sr age for the trachytes is 14.± 7 m.y. and the other alkali-enriched rocks give ages ranging within the limits of 14.4 to 12.0 m.y.     

Age and strontium‐isotope geochemistry of differentiated rocks from the newer Volcanics,MT Macedon Area,Victoria, Australia

The Newer Volcanics Province of Victoria and South Australia consists of a major region of mainly alkaline basalts within which are two restricted areas containing strongly differentiated flow‐rocks. Typical alkalic basalts from this widespread province have K‐Ar ages from 4.5 to 0.5 m.y. and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7038 to 0.7045. Contrastingly, in the Macedon area of differentiated lavas, flow compositions range from limburgite to soda trachyte, with K‐Ar ages from 6.8 to 4.6 m.y. and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7052 to 0.7127. These differentiated rocks therefore are older, and some of them may have been contaminated by reaction with more radiogenic basement rocks during differentiation. Alternatively, the variation in initial Sr‐isotope composition may have resulted from varying isotopic composition of partial melts from the immediate source rocks. The most felsic of the differentiated rocks, soda trachyte, is extremely enriched with Rb relative to Sr; one of the three restricted outcrops of this rock (Camel's Hump) yields a total‐rock Rb‐Sr isochron age of 6.3 ± 0.6 m.y. with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7127. K‐Ar sanidine ages reported for the three outcrops of trachyte are identical to each other and to the Rb‐Sr isochron result.     

A Rubidium-Strontium study of the Twilight Gneiss,West Needle Mountains,Colorado

The Precambrian trondhjemitic Twilight Gneiss (Twilight Granite of Cross and Howe, 1905b) of the West Needle Mountains, southwestern Colorado, and its interlayered amphibolite and metarhyodacite yield a Rb-Sr isochron of 1,805±35 m.y. A low initial Sr⁸⁷/Sr⁸⁶ ratio of 0.7015 implies that metamorphism of these rocks to amphibolite facies took place soon after their emplacement. The mild metamorphism of Uncompahgran age, prior to 1,460 m.y. ago, and Laramide volcanism did not affect the Rb-Sr system in the Twilight. Rb contents of 26.5 to 108 ppm, Sr contents of 114 to 251 ppm, and K₂O percentages of 1.23 to 3.64 in the Twilight Gneiss, in conjunction with high K/Rb ratios and the low initial ratio of Sr⁸⁷/Sr⁸⁶, lend support to geologic data that suggest the Twilight originated as volcanic or hypabyssal igneous rocks in a basaltic volcanic pile.Publication authorized by the Director, U.S. Geological Survey.     

A crustal origin for eclogites and a mantle origin for garnet peridotites: Strontium isotopic evidence from clinopyroxenes

Strontium isotopic data suggest that the classic eclogite-facies rocks of western south Norway described by Eskola (1921) formed from several parental materials in a variety of environments. Mineral separates from essentially basic, bi-minerallic (clinopyroxene and garnet) eclogites that occur as lens-shaped masses within high grade gneisses (country rock eclogites) have Sr⁸⁷/Sr⁸⁶ values that range from 0.704 for fine-grained varieties to 0.716 for coarse-grained, orthopyroxene-bearing varieties. These high, varied ratios contrast with the very low, restricted ratios (0.701 to 0.704) of similar minerals from ultrabasic, garnet-clinopyroxene-orthopyroxene-olivine assemblages (garnet peridotites) that occur as lenses within large peridotite bodies. The eclogite-facies metamorphism that generated the garnet peridotites may have occurred in the mantle. However, the metamorphism that generated at least the more radiogenic country-rock eclogites must have occurred in the crust. The high Sr⁸⁷/Sr⁸⁶ ratios of these eclogites could be generated either by forming them from crustal parental rocks or by contaminating mantle-derived parental rocks with radiogenic strontium from the country rocks. If this contamination occurred after intrusion and before eclogite-facies metamorphism, a rather contrived history must be postulated that involves intrusion, contamination accompanied by hydration, subsequent dehydration, and finally eclogite-facies metamorphism. These processes could have occurred within the long, complicated history of the enclosing country rocks. Alternatively, if the contamination occurred during eclogite-facies metamorphism, the presence of some hydrous fluid appears to be required to transport the radiogenic strontium from the enclosing country rocks. The eclogites with the highest Sr⁸⁷/Sr⁸⁶ ratios are also the most coarse-grained and it is possible that the presence of some intergranular fluid enabled these eclogites to recrystallize to a much larger grain size than would have been possible in a totally anhydrous environment. The garnet peridotites and fine-grained country rock eclogites may have formed from mantle material in the crust but escaped contamination by radiogenic strontium as a result of their position in a dry environment in the crust.Lamont-Doherty Geological Observatory Contribution No. 2443     

Strontium and neodymium isotopic study of the western Mogollon-Datil volcanic region,New Mexico,USA

The Sr- and Nd-isotopic compositions of large mid-Cenozoic caldera-forming eruptions, and related rocks, from the western portion of the Mogollon-Datil volcanic field have been determined. The average initial ⁸⁷Sr/⁸⁶Sr ratios of 27 samples from felsic flows range from 0.70629 to 0.72872; however, all but two flows are 0.71337 or less. Ten analyses of intermediate and mafic rocks showed a tendency towards lower initial ⁸⁷Sr/⁸⁶Sr ranging from 0.70363 to 0.70968. Initial ¹⁴³Nd/¹⁴⁴Nd ratios of II felsic and intermediate rocks range from 0.51216 to 0.51231. Two basalts analyzed for ¹⁴³Nd/¹⁴⁴Nd have ratios of 0.51250 and 0.51291. During the course of the volcanic activity from 34 Ma to the present, there was a shift towards lower initial ⁸⁷Sr/⁸⁶Sr ratios, and lower SiO₂ contents. A number of models of crustal melting, fractionation, mixing, and assimilation and fractional crystallization (AFC), using a variety of possible endmembers, were tested, to see if they could explain the isotopic and geochemical characteristics of the Mogollon-Datil volcanic rocks. The best fit was an AFC model using two components, one a mantle-sourced primary magma, with isotopic ratios of the Kilbourne Hole, N. M., basanite, and the other an upper crust with average continental isotopic ratios, and Sr and Nd abundances similar to the Texas Canyon pluton of Arizona.     

Contact and fracture ultramafic assemblages from Norway: Rb-Sr evidence for crustal contamination

Strontium isotopes are used as tracers of crustal contamination of alpine-type ultramafic rocks from the Basal Gneiss Complex of the Caledonides of southern Norway. Minerals from anhydrous assemblages that occur in the cores of these ultramafic lenses give Sr⁸⁷/Sr⁸⁶ ratios (0.7011 to 0.7047) that reflect the expected ambient Sr⁸⁷/Sr⁸⁶ conditions of the ancient upper mantle. Rb-Sr evidence for crustal contamination is found in hydrous assemblages that occur within fractures and around the margins of the ultramafio bodies. Olivine, enstatite, amphibole, and magnesite from these assemblages have present-day Sr⁸⁷/Sr⁸⁶ ratios (0.7049 to 0.7085) that are significantly higher than those of compositionally equivalent minerals from the interiors of the ultramafic bodies. The high Sr⁸⁷/Sr⁸⁶ values were acquired as a result of the reaction between the ultramafic rock and ion-charged hydrous solutions carrying strontium with the ambient Sr⁸⁷/Sr⁸⁶ ratio (around 0.713) of the enclosing country rook during the waning phases of the Caledonian Orogeny. Mineral separates from the interiors of these ultramafic bodies can yield useful information on the ancient upper mantle. Wholerock samples, however, will show some evidence of contamination from the crust as a result of the formation of at least trace amounts of secondary hydrous minerals. Most whole-rook Sr⁸⁷/Sr⁸⁶ ratios from alpine-type ultramafic rocks from other orogenic belts show evidence of this contamination.     

Strontium Isotope Abundance Studies and Rubidium--Strontium Age Determinations on Tertiary Igneous Rocks from the Isle of Skye North-West Scotland

The isotopic abundance of strontium has been measured in representativerock-types from the Tertiary igneous suite in the Isle of Skye,north-west Scotland. The isotopic abundance of Sr⁸⁷, expressedas the Sr⁸⁷/Sr⁸⁶ ratio, for twelve basalts, dolerites, gabbros,and peridotites is 0.7058?0.0010. On the other hand, the averageinitial Sr⁸⁷/Sr⁸⁶ ratio for one granite from the Eastern RedhillsComplex and for three granites from the Western Redhills Complexis 0.7124?0.0015. The three principal members of the marscoitesuite of the Western Redhills Centre, namely, ferrodiorite,porphyritic felsite, and marscoite, also have significantlyhigher initial Sr⁸⁷/Sr⁸⁶ ratios than the basaltic rocks. Itis concluded that the rocks with high initial ratios were derivedfrom a source with significantly higher Rb/Sr ratio than thatof the basaltic rocks. The hypothesis which is most in accordwith the isotopic evidence is that the granitic rocks and themarscoite suite were produced by partial melting of ancientLewisian rocks, which form the underlying basement at no greatdepth in this area. Rb—Sr (and K-Ar) age determinations were carried out onsome representative rocks from the Eastern and Western RedhillsCentres. It was not possible to establish a significant agedifference between any of the rock units from either Centres.The mean age is 54?3 m.y. and suggests that intrusion of thegranitic rocks of Skye occurred during a relatively short periodof time within the Lower or Middle Eocene peroid.     

Petrogenesis of the early Cretaceous Valle Chico igneous complex (SE Uruguay): Relationships with Paraná–Etendeka magmatism

The early Cretaceous (130 Ma) igneous complex of Valle Chico (SE Uruguay) is made up of felsic plutonic and subordinate volcanic rocks and dykes cropping out over an area of about 250 km². This complex is strictly linked with the formation of the Paraná–Etendeka Igneous Province and the first stages of the South Atlantic Ocean rifting. The plutonic rocks range from quartz-monzonite to syenite, quartz-syenite and granite. The volcanic rocks and the dykes range from quartz-latite to trachyte and rhyolite; no substantial differences in term of chemical composition have been found between plutonic and volcanic rocks. Only a sample of basaltic composition (with tholeiitic affinity) has been sampled associated with the felsic rocks. The Agpaitic Index of the Valle Chico felsic rocks range from 0.72 to 1.34, with the peralkaline terms confined in the most evolved samples (SiO₂>65 wt.%). Initial ⁸⁷Sr/⁸⁶Sr₍₁₃₀₎ of the felsic rocks range from 0.7046 to 0.7201, but the range of ⁸⁷Sr/⁸⁶Sr of low-Rb/Sr samples cluster at 0.7083; ¹⁴³Nd/¹⁴⁴Nd₍₁₃₀₎ ratios range from 0.5121 (syenite) to 0.5117 (granite). The tholeiitic basalt show more depleted isotopic compositions (⁸⁷Sr/⁸⁶Sr₍₁₃₀₎=0.7061; ¹⁴³Nd/¹⁴⁴Nd₍₁₃₀₎=0.5122), and plots in the field of other early Cretaceous low-Ti basaltic rocks of SE Uruguay. The radiogenic Sr and unradiogenic Nd of the Valle Chico felsic rocks require involvement of lower crustal material in their genesis either as melt contaminant or as protolith (crustal anatexis). In particular, most of the Valle Chico (VC) felsic rocks define a near-vertical array in Sr–Nd isotopic spaces, pointing toward classical EMI-type composition; this feature is considered to reflect a lower crust involvement as observed for other mafic and felsic rocks of the Paraná–Etendeka Igneous Province. Decompression melting of the lower crust related to Gondwana continental rifting before the opening of the South Atlantic Ocean or the presence of thermal anomalies related to the Tristan plume may have induced the lower crust to partially melt. Alternative hypothesis considers contamination of upper mantle by a mafic/ultramafic keel composed of lower crust and uppermost mantle after delamination and detachment processes. This interaction may have occurred after the continent–continent collision during the last stages of the Panafrican Orogeny. This “lower crust” model does not exclude active involvement of upper crust as contaminant, necessary to explain the strongly radiogenic ⁸⁷Sr/⁸⁶Sr₍₁₃₀₎ isotopic composition of some VC SiO₂-rich rocks. Mineralogical (sporadic presence of pigeonite, Ca–Na and Na clinopyroxene, calcic- and calco-sodic amphibole) and geochemical evidences (major and trace element as well as Sr–Nd isotopic similarities with the felsic early Cretaceous volcanic rocks of the Arequita Formation in SE Uruguay) allow us to propose for the VC rocks a transitional rock series (the most abundant rock types are of syenitic/trachytic composition) preferentially evolving towards SiO₂-oversaturated compositions (granite/rhyolite) also with a strong upper crustal contribution as melt contaminant. This conclusion is in contrast with previous studies according which the VC complex had clear alkaline affinity. Many similarities between VC and the coeval Paresis granitoids (Etendeka, Namibia) are evidenced in this paper. The genetic similarities between VC and the rhyolites (s.l.) of SE Uruguay may find counterparts with the genetic link existing between the early Cretaceous tholeiitic-alkaline Messum complex and the quartz latites (s.l.) of the Awahab Formation (Etendeka region, Namibia).     

Origin of the Cenozoic–Mesozoic magnetite-series and ilmenite-series granitoids in East Asia

Among the Phanerozoic granitoids of East Asia, the most prevailing Cenozoic–Mesozoic rocks are reviewed with respect to gabbro/granite ratio, bulk composition of granitoids, redox state, and O- and Sr-isotopic ratios. Quaternary volcanic rocks, ranging from basalt to rhyolite, but typically felsic andesite in terms of bulk composition in island arcs, are oxidized type, possibly due to oxidants from subducting oceanic crust into the source regions. Miocene plutonic rocks in the back-arc of Japan could be a root zone for such volcanism but are more felsic in composition. Cenozoic–Mesozoic plutonic zones are classified by (1) the redox state (magnetite/ilmenite series), and (2) average bulk composition (granodiorite/granite). The granodioritic magnetite series occur with fairly abundant gabbro and diorite in the back-arc of island arcs (Greentuff Belt) and intercontinental rapture zones (Yangtze Block). These rocks are mostly juvenile in terms of the ⁸⁷Sr/⁸⁶SrI and δ¹⁸O values.The granitic magnetite series with some gabbroids occur in rapture zones along the continental coast (Gyeongsang Basin, Fujian Coast) and the back-arc of island arc (Sanin Belt). They were generated mostly in felsic continental crust, with the help of heat and magmas from upper mantle. The generated granitic magmas had little interaction with C- and S-bearing reducing materials, due probably to extensional tectonic settings. The δ¹⁸O value gives narrow ranges but the ⁸⁷Sr/⁸⁶SrI ratio varies greatly depending upon the age and composition of the continental crust. Granitic ilmenite-series are characterized by high δ¹⁸O values, implying much contribution of sediments. The ⁸⁷Sr/⁸⁶SrI ratios are low in island arcs but very high in continental interior settings. Amount of mafic magmas from the upper mantle seems a key to control the composition of granitoid series in island arc settings, while original composition of the protolith may be the key to control granitoid composition in continental interiors.     

On the age of the Onverwacht Group,Swaziland Sequence,South Africa

Some rocks of the Onverwacht Group, South Africa, have been analyzed for Rb and Sr concentrations and Sr isotopie composition. These rocks include volcanic rocks, layered ultramafic differentiates and cherty sediments. Whole rock data indicate that the Rb-Sr isotopie systems in many samples were open and yield no reasonable isochron relationships. However, the data of mineral separates from a basaltic komatiite define a good isochron of $\text{t = 3.50 ± 0.20}$ (2δ) b.y. with an initial Sr⁸⁷/Sr⁸⁶ ratio of $\text{0.70048 ± 5}$(2δ). The orthodox interpretation of this age is the time of the low grade metamorphism. Since the basaltic komatiite is stratigraphically lower than the Middle Marker Horizon (dated as $\text{3.36 ± 0.07}$ b.y. Hurley et al., 1972), and since it is commonly found that volcanism, sedimentary deposition, metamorphism and igneous intrusion in many Archean greenstone-granite terrain all took place in a relatively short time interval (less than 100 m.y.), it is reasonable to assume that the age of 3.50 b.y. might also represent the time of initial Onverwacht volcanism and deposition. The initial Sr⁸⁷/Sr⁸⁶ ratio obtained above is important to an understanding of the Sr isotopic composition of the Archean upper mantle. If the komatiite represents a large degree of partial melt (40–80 per cent) of the Archean upper mantle material, then the initial ratio obtained from the metamorphic komatiite should define an upper limit for the Sr isotopic composition of the upper mantle under the African crustal segment.     

The significance of rubidium-strontium age of sedimentary rock

The isotopic composition of strontium and the rubidium and strontium contents were measured for samples of the Lower Permian Havensville Shale collected at four different localities across a distance of approximately 200 miles in Kansas and northern Oklahoma. The Rb-Sr dates ranged from 320 m.y. to 395 m.y. The initial Sr⁸⁷/Sr⁸⁶ ratios were between 0.709 and 0.717. The ages are interpreted to be intermediate between the age of the source material and the time of sedimentation and, therefore, are not indicative of any geologic event. A model has been proposed to explain the colinearity of the Rb-Sr data and the evolution of different Rb-Sr dates for the same stratigraphic unit at different localities.     

Strontium Isotope Studies related to Petrogenesis in the Caledonian Basic Igneous Province of NE. Scotland

Rubidium-strontium isotope analyses are reported for 90 whole-rocksfrom the Newer Gabbros and associated rocks of NE. Scotland.The Sr⁸⁷/Sr⁸⁶ ratio of the regional gabbro magma at Insch andBelhelvie underwent a progressive increase, in the former casefrom 0·703 to 0·712 during differentiation tosyenogabbros and syenites. This is ascribed to a process ofisotopic equilibration with country rocks of the Dalradian Series.The wider implications of a process by which the Sr⁸⁷/Sr⁸⁶ ratioof a basic magma can be increased in the crust without any othersigns of petrological contamination are considered. Cordierite-noritesand related rocks of the Haddo House and Arnage masses haveinitial Sr⁸⁷sol;Sr⁸⁶ ratios of 0·720–0·730,essentially the same range as for the country rocks at the timeof intrusion. This is interpreted as support for the hypothesisthat such rocks were formed by the partial fusion of peliticsediments adjacent to the basic igneous sheet.     

Young Alpine dykes south of the Tauern Window (Austria): a K-Ar and Sr isotope study

Fortyfive new K-Ar ages and Sr isotope data on amphiboles, biotites, clinopyroxenes and whole rock samples from subvolcanic dykes south of the Tauern Window establish, that alkalibasaltic dykes were intruded 30 m.y. ago and shoshonitic volcanism occured between 30 and 24 m.y. ago. Two calc-alkaline rocks of high-potassium composition yielded ages of 40 and 26 m.y. resp., a spread which may or may not be real. Calc-alkaline dykes with medium and low potassium contain excess argon and are hence undatable. Alkalibasaltic dykes have ⁸⁷Sr/⁸⁶Sr ratios of 0.7056–0.7070, shoshonitic rocks 0.7075–0.7133, potassium rich calc-alkaline dykes 0.7077–0.7100. ⁸⁷Sr/⁸⁶Sr of all other calc-alkaline rocks scatter between 0.7074 and 0.7150. Sr data indicate that dykes studied do not represent closed Sr systems, but that Sr characteristics result from selective strontium assimilation en route to surface. Primary Sr isotopic ratios of alkalibasaltic dykes point to an origin of these rocks in enriched sub-continental upper mantle. The source region of shoshonitic and high-potassium calcalkaline rocks could have ⁸⁷Sr/⁸⁶Sr around 0.707, which is assigned to the input of a component rich in alkalies, LREE and LIL elements. Genetic relationships with other Tertiary magmatites of similar geotectonic position are explained in terms of plate tectonic models of the Eastern Alps.     

Rubidium-Strontium Age of some Metamorphic Rocks from the Llano Uplift, Texas1

Four total-rock samples of the Valley Spring Gneiss give a Rb-Srisochron age of 1120?25 million years and an initial Sr⁸⁷/Sr⁸⁶ratio of 0.706?0.002. This age is significantly older than the1,000–1,050 and 920?20 million year dates previously determinedfor the igneous rocks of the Llano Uplift. Mineral isochrons for the individual gneiss samples yield anage of metamorphism of 1,000?15 million years, in good agreementwith the time of major plutonism. The strontium in each of theolder metamorphic rocks was isotopically homogenized at thistime, resulting in Sr⁸⁷/Sr⁸⁶ ratios ranging from 0?716 to 0?775.From these data it is concluded that the plutons, which appearto have formed contemporaneously with a Sr⁸⁷/Sr⁸⁶ ratio of 0?706,could not have obtained their strontium from the surroundingmetamorphic rocks unless some special mechanism operated toremove excess radiogenic strontium. Only if the granite remainedan open system, unable to retain radiogenic strontium duringits early history, could it be as old as the total-rock ageof the gneiss.     

Lead and strontium isotopes in rocks of the Absaroka volcanic field,Wyoming

The Absaroka volcanic field is comprised of predominant andesitic volcaniclastic rocks and less abundant potassium-rich mafic lavas (shoshonites and absarokites). Strontium and lead isotopic variations preclude a simple derivation from an isotopically uniform source: Sr⁸⁷/Sr⁸⁶, 0.7042 to 0.7090; Pb²⁰⁶/Pb²⁰⁴, 16.31 to 17.30; Pb²⁰⁸/Pb²⁰⁴, 36.82 to 37.64. We postulate that these rocks were derived from a lower crust or upper mantle which underwent a preferential loss of uranium relative to lead approximately 2800±200 m.y. ago. Variations in lead and strontium isotopic compositions are thought to reflect small inhomogeneities in U/Pb and Rb/Sr ratios in the source.Publication authorized by the Director, U.S. Geological Survey.     

The Isotopic Composition of Strontium in the Skaergaard Intrusion, East Greenland

The isotopic composition of strontium in the different partsof the differentiated Skaergaard intrusion has been determined.The average Sr⁸⁷/Sr⁸⁶ ratio for the basic rocks was found tobe 0?7065?0?002. Higher values, between 0?7101 and 0?7303, wererestricted to the late-stage acid granophyres. The Sr⁸⁷/Sr⁸⁶ratios for the basic Skaergaard rocks are similar to those foundby previous workers. The enrichment in Sr⁸⁷ expressed in theSr⁸⁷/Sr⁸⁶ ratio is taken to indicate contamination of the acidgranophyres by a source enriched in Sr⁸⁷. From considerationsbased upon circumstantial evidence the average country rock,composed of old Precambrian acid to intermediate gneiss, isnot sufficiently enriched in.Sr⁸⁷ to account for the Sr⁸⁷ enrichmentobserved in the acid granophyres by a simple assimilation process.At the present stage of the investigation the enrichment ofthe acid granophyres in Sr⁸⁷ is unexplained.