Petrogenesis and paleotectonic history of the Wild Bight Group,an Ordovician rifted island arc in central Newfoundland

The Wild Bight Group (WBG) is a sequence of early and middle Ordovician volcanic, subvolcanic and epiclastic rocks, part of the Dunnage Tectonostratigraphic Zone of the Newfoundland Appalachians. A detailed geochemical and Nd-isotopic study of the volcanic and subvolcanic rocks has been carried out to determine the geochemical characteristics of the rocks, interpret their palcotectonic environments and constrain their petrogenetic history. The lower and central stratigraphic levels of the WBG contain mafic volcanic rocks with island-arc geochemical signatures, including LREE-enriched are tholeiites with _Nd(t) =-0.1 to +2.2 (type A-I), LREE-depleted arc tholeiites with _Nd(t) =+5.6 to +7.1 (type A-II) and an unusual suite of strongly incompatible-element depleted tholeiites in which _Nd(t) ranges from-0.9 to +4.6 and is negatively correlated with¹⁴⁷Sm/¹⁴⁴Nd (type A-III). High-silica, low-K rhyolites occur locally in the central part of the stratigraphy, associated with mafic rocks of arc affinity, and have _Nd(t) =+4.7 to +5.4. The upper stratigraphic levels of the WBG dominantly contain rocks with non-arc geochemical signatures, including alkalic basalts with _Nd(t) =+4.6 to +5.5 (type N-I), strongly LREE- and incompatible element-enriched tholeiites that are transitional between alkalic and non-alkalic rocks with _Nd(t) =+4.4 to +7.0 (type N-II) and rocks with flat to slightly LREE-enriched patterns and _Nd(t) =+5.1 to +7.4 (type N-III). Rocks with non-arc and arc signatures are locally interbedded near the stratigraphic type of the WBG. Nd-isotopic data in the type A-I and A-II rocks are generally compatible with mixing/partial melting models involving depleted mantle, variably contaminated by a subducted crustally-derived sediment. The petrogenesis of type A-III rocks must involve source mixing and multi-stage partial melting, but the details are not clear. The geochemistry and Nd isotope data for types N-I, N-II and N-III rocks are compatible with petrogenetic models involving variable partial melting of a source similar to that postulated for modern oceanic island basalts. Comparison of the WBG with modern analogues suggests a 3-stage developmental model: stage 1) island-arc volcanism (eruption of type mafic volcancs); stage 2) arc-rifting (continued eruption of type A-I, A-I, eruption of types A-II and A-III mafic volcanics and high-silica, low-K rhyolites); and stage 3) back-arc basin volcanism (continued minor eruption of type A-I basalts, eruption of types N-I, N-II, N-III basalts). Stages 1 and 2 volcanism involved partial melting of subduction contaminated mantle, while stage 3 volcanism utilized depleted-mantle sources not affected by the subducting slab. This model provides a basis for interpreting coeval sequences in central Newfoundland and a comparative framework for some early Paleozoic oceanic volcanic sequences elsewhere in the Appalachian orogen.     

High precision intercalibration of 40Ar-39Ar standards

Intercalibration of one intralaboratory and three interlaboratory standards used in ⁴⁰Ar-³⁹Ar dating has been carried out. In order to provide homogeneous values for ${}^{40}\text{Ar}{}^1{}^{40}\text{K}$ the standards were prepared by careful handpicking. To control the neutron fluence in the Herald Reactor (A.W.R.E.) 16 aliquots of the standards were arranged along 0.6 × 60 cm of a single silica tube. The corrections for all known interferences from K, Ca, Cl were carefully assessed. Two of the hornblende standards, Hb3gr and MMHb-1 appear homogeneous at the 0.1% level while the other two standards, LP-6 and FY12a are not completely homogeneous. The mean values of ${}^{40}\text{Ar}{}^1{}^{40}\text{K}$ when referenced to the previously determined value for Hb3gr (turner et al., 1971) are:

     

3.

Strontium isotopic equilibration: A solution to a paradox     

J.C. Roddick  W. Compston 《Earth and Planetary Science Letters》1977,34(2):238-246

     

4.

The evolution of excess argon in alpine biotites — A⁴⁰Ar-³⁹Ar analysis     

J.C. Roddick  R.A. Cliff  D.C. Rex 《Earth and Planetary Science Letters》1980,48(1):185-208

Alpine biotites containing excess⁴⁰Ar have been analysed by step-heating argon analysis of both neutron irradiated and unirradiated samples. In addition to age spectra the data are discussed in terms of the thermal release of⁴⁰Ar,³⁹Ar,³⁷Ar and³⁶Ar and also displayed on a correlation plot of³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar which is used to interpret the data and present a model of isotopic evolution during metamorphic cooling. This diagram overcomes misleading complications of isochron plots. The samples exhibit the following argon systematics: (1) flat age spectra for 80–90%³⁹Ar release with anomalously old ages but early gas fractions that approximate the accepted cooling ages; (2) each sample shows decreasing³⁶Ar/⁴⁰Ar with increasing temperature of heating step with three samples having a negative correlation of³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar and one a positive correlation; (3) there appear to be two³⁶Ar components, one released at high temperatures and correlated with radiogenic⁴⁰Ar and one released at low temperatures which is not correlated with radiogenic⁴⁰Ar; and (4) there is no significant effect of neutron irradiation on the release of⁴⁰Ar and³⁶Ar.Interpretation suggests that these biotites contain a record of the evolution and isotopic composition of ambient argon retained within the metamorphic host rocks during cooling. After incorporation of argon of high⁴⁰Ar/³⁶Ar another argon component, of atmospheric composition, was retained at lower temperature and argon partial pressures.     

5.

Emplacement and metamorphism of Archaean mafic volcanics at Kambalda,Western Australia—geochemical and isotopic constraints     

J.C Roddick 《Geochimica et cosmochimica acta》1984,48(6):1305-1318

Extensive exploration and mining at Kambalda, Western Australia has revealed a conformable mafic-ultramafic succession consisting of a sulphide ore bearing ultramafic layer located between metabasalt units. Geochemical and Rb-Sr, U-Pb isotopic analyses have been carried out on two 70–140 m sections of drill core from the metabasalts and U-Pb isotopic analyses have been made on sulphide ore samples. Greenschist metamorphism of the metabasalts is dated at 2610 ± 30 m.y. and was approximately isochemical except for addition of H₂O and CO₂ and a differential mobilization of K and Rb within the units. The addition of H₂O and CO₂ is believed to have taken place shortly after extrusion of the metabasalts in a submarine environment resulting in an early greenschist metamorphism. The later 2600 m.y. event is correlated with a high thermal regime also detected at Kalgoorlie 50 km to the north. A Pb-Pb isochron age of 2720 ± 105 m.y. may represent the metamorphic event or the time of emplacement of the metabasalts. New Sm-Nd data (Chauvelet al.; claqué--Longet al.) suggest the mafic volcanics are 3200 m.y. old. If true large scale homogenization of Pb isotopes is required. Alternatively the mafic sequence may be only slightly older than 2800 m.y. and the Sm-Nd data does not have time significance.     

6.

Radiometric evidence for the age of emplacement and cooling of the Murrumbidgee Batholith     

J. C. Roddick  W. Compston 《Australian Journal of Earth Sciences》2013,60(3):223-233

Most of the rocks of the Murrumbidgee Batholith have a Rb‐Sr age of 424 ± 2 m.y. This is considered to be the time of emplacement. A small difference in the ages (4 ± 2 m.y.) between the northern and southern parts of the batholith is attributed to thermal effects caused by a slightly later time of emplacement of some of the intrusions or to a short cooling interval. Final intrusive activity ended by 414 ± 4 m.y. Younger mineral ages for some intrusions are related to later local meta‐morphic effects.     

7.

Petrology and 40Ar/39Ar geochronology of some hellenic sub-ophiolite metamorphic rocks     

J. G. Spray  J. C. Roddick 《Contributions to Mineralogy and Petrology》1980,72(1):43-55

Whole rock, electron microprobe analyses and ⁴⁰Ar/³⁹Ar geochronology of certain ophioliterelated metamorphic rocks from beneath the Pindos, Vourinos, Othris and Euboea ophiolites of Greece show that they were formed mainly from ocean-type basalts, in part under P-T conditions of the upper mantle and that they have ages between 170–180 m.y. The evidence presented is inconsistent with the view that these sub-ophiolite metamorphic rocks were produced by the obduction of ocean-type crust onto a continental margin, or that they are remnant slices of Palaeozoic ‘basement’, but is consistent with their formation by thrusting and related metamorphism occurring within ocean lithosphere during the Lower to Middle Jurassic. It is proposed that this intraoceanic metamorphism was caused by the inception of a fault zone which subsequently became the transport surface for the main phase of ophiolite emplacement that occurred in the Hellenides from the Late Jurassic to Early Cretaceous.     

8.

Trace element and isotope studies on veined,metasomatic and “MARID” xenoliths from Bultfontein,South Africa.     

J.D. Kramers  J.C.M. Roddick  J.B. Dawson 《Earth and Planetary Science Letters》1983,65(1):90-106

     

9.

Evidence for upper cretaceous transform fault metamorphism in West Cyprus     

J.G. Spray  J.C. Roddick 《Earth and Planetary Science Letters》1981,55(2):273-291

Metamorphic rocks of low-pressure/medium-temperature facies occur in West Cyprus as blocks and slivers with mafic and ultramafic screens in high-angle, serpentinite-filled fault zones. A satisfactory explanation for the origin of the metamorphic rocks has previously remained a subject of controversy. The evidence presented here, based on a study of their bulk chemistry, mineralogy and⁴⁰Ar/³⁹Ar geochronology, indicates they were produced by greenschist to amphibolite facies dynamothermal metamorphism of alkalic and tholeiitic mafic rocks and associated sediments at between 83 and 90 m.y. Their field relations show similarities with present-day oceanic fracture zones suggesting that metamorphism occurred within strike-slip faults, some of which were probably extensions of the Arakapas transform. We propose that hot crust generated at an oceanic spreading centre provided the heat for metamorphism when juxtaposed against older, cooler rocks during ridge-ridge transform movements. In addition, shear heating may have been facilitated by the strike-slip faulting and contributed to the total heat available. These interpretations are compatible with many aspects of the broader regional geology of Cyprus, provide new constraints on the early evolution of the Troodos Complex and form the basis of a model for transform fault metamorphism.     

10.

The application of isochron diagrams in⁴⁰Ar-³⁹Ar dating: A discussion     

J.C. Roddick 《Earth and Planetary Science Letters》1978,41(2):233-244

The presence of a non-radiogenic argon component within rocks and minerals at the time of their crystallization (termed initial argon) is now widely recognized. Present data indicate that the³⁶Ar concentration of this initial argon is in the order of 10^?9 cm³/g STP in a wide range of geological materials. In⁴⁰Ar-³⁹Ar analyses the preference for small samples and division of the gas evolved into a number of temperature steps means that the amount of initial³⁶Ar analyzed is often less than the argon extraction line blank. Thus if blank corrections are not made an isochron plot will represent mixing lines between atmospheric argon and the argon derived from the sample. In this case the isochron parameters will probably be in error. Secondary alteration of samples adds atmospheric argon not related to the initial argon present in a rock at the time of its formation and must be eliminated or avoided if the isochron technique is to be used. The inability to obtain isochrons for samples with excess radiogenic argon may be related to significant extraction system contamination. At present the application of the two-error regression technique is being misapplied in determining the quality of fit and in some cases underestimating the error limits assigned to the isochron parameters.     

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Hb3gr	hbld	$\text{.08504 ± .05\%}$(±lσ)	1072. m.y.
MMHb-1	hbld	$\text{.03493 ± 05\%}$	518.9 m.y.
LP-6	biot	$\text{.007735 ± .13\%}$	128.5 m.y.
FY12a	hbld	$\text{.02858 ± .25\%}$	435.0 m.y.

The evolution of excess argon in alpine biotites — A40Ar-39Ar analysis

Alpine biotites containing excess⁴⁰Ar have been analysed by step-heating argon analysis of both neutron irradiated and unirradiated samples. In addition to age spectra the data are discussed in terms of the thermal release of⁴⁰Ar,³⁹Ar,³⁷Ar and³⁶Ar and also displayed on a correlation plot of³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar which is used to interpret the data and present a model of isotopic evolution during metamorphic cooling. This diagram overcomes misleading complications of isochron plots. The samples exhibit the following argon systematics: (1) flat age spectra for 80–90%³⁹Ar release with anomalously old ages but early gas fractions that approximate the accepted cooling ages; (2) each sample shows decreasing³⁶Ar/⁴⁰Ar with increasing temperature of heating step with three samples having a negative correlation of³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar and one a positive correlation; (3) there appear to be two³⁶Ar components, one released at high temperatures and correlated with radiogenic⁴⁰Ar and one released at low temperatures which is not correlated with radiogenic⁴⁰Ar; and (4) there is no significant effect of neutron irradiation on the release of⁴⁰Ar and³⁶Ar.Interpretation suggests that these biotites contain a record of the evolution and isotopic composition of ambient argon retained within the metamorphic host rocks during cooling. After incorporation of argon of high⁴⁰Ar/³⁶Ar another argon component, of atmospheric composition, was retained at lower temperature and argon partial pressures.     

Emplacement and metamorphism of Archaean mafic volcanics at Kambalda,Western Australia—geochemical and isotopic constraints

Extensive exploration and mining at Kambalda, Western Australia has revealed a conformable mafic-ultramafic succession consisting of a sulphide ore bearing ultramafic layer located between metabasalt units. Geochemical and Rb-Sr, U-Pb isotopic analyses have been carried out on two 70–140 m sections of drill core from the metabasalts and U-Pb isotopic analyses have been made on sulphide ore samples. Greenschist metamorphism of the metabasalts is dated at 2610 ± 30 m.y. and was approximately isochemical except for addition of H₂O and CO₂ and a differential mobilization of K and Rb within the units. The addition of H₂O and CO₂ is believed to have taken place shortly after extrusion of the metabasalts in a submarine environment resulting in an early greenschist metamorphism. The later 2600 m.y. event is correlated with a high thermal regime also detected at Kalgoorlie 50 km to the north. A Pb-Pb isochron age of 2720 ± 105 m.y. may represent the metamorphic event or the time of emplacement of the metabasalts. New Sm-Nd data (Chauvelet al.; claqué--Longet al.) suggest the mafic volcanics are 3200 m.y. old. If true large scale homogenization of Pb isotopes is required. Alternatively the mafic sequence may be only slightly older than 2800 m.y. and the Sm-Nd data does not have time significance.     

Radiometric evidence for the age of emplacement and cooling of the Murrumbidgee Batholith

Most of the rocks of the Murrumbidgee Batholith have a Rb‐Sr age of 424 ± 2 m.y. This is considered to be the time of emplacement. A small difference in the ages (4 ± 2 m.y.) between the northern and southern parts of the batholith is attributed to thermal effects caused by a slightly later time of emplacement of some of the intrusions or to a short cooling interval. Final intrusive activity ended by 414 ± 4 m.y. Younger mineral ages for some intrusions are related to later local meta‐morphic effects.     

Petrology and 40Ar/39Ar geochronology of some hellenic sub-ophiolite metamorphic rocks

Whole rock, electron microprobe analyses and ⁴⁰Ar/³⁹Ar geochronology of certain ophioliterelated metamorphic rocks from beneath the Pindos, Vourinos, Othris and Euboea ophiolites of Greece show that they were formed mainly from ocean-type basalts, in part under P-T conditions of the upper mantle and that they have ages between 170–180 m.y. The evidence presented is inconsistent with the view that these sub-ophiolite metamorphic rocks were produced by the obduction of ocean-type crust onto a continental margin, or that they are remnant slices of Palaeozoic ‘basement’, but is consistent with their formation by thrusting and related metamorphism occurring within ocean lithosphere during the Lower to Middle Jurassic. It is proposed that this intraoceanic metamorphism was caused by the inception of a fault zone which subsequently became the transport surface for the main phase of ophiolite emplacement that occurred in the Hellenides from the Late Jurassic to Early Cretaceous.     

Evidence for upper cretaceous transform fault metamorphism in West Cyprus

Metamorphic rocks of low-pressure/medium-temperature facies occur in West Cyprus as blocks and slivers with mafic and ultramafic screens in high-angle, serpentinite-filled fault zones. A satisfactory explanation for the origin of the metamorphic rocks has previously remained a subject of controversy. The evidence presented here, based on a study of their bulk chemistry, mineralogy and⁴⁰Ar/³⁹Ar geochronology, indicates they were produced by greenschist to amphibolite facies dynamothermal metamorphism of alkalic and tholeiitic mafic rocks and associated sediments at between 83 and 90 m.y. Their field relations show similarities with present-day oceanic fracture zones suggesting that metamorphism occurred within strike-slip faults, some of which were probably extensions of the Arakapas transform. We propose that hot crust generated at an oceanic spreading centre provided the heat for metamorphism when juxtaposed against older, cooler rocks during ridge-ridge transform movements. In addition, shear heating may have been facilitated by the strike-slip faulting and contributed to the total heat available. These interpretations are compatible with many aspects of the broader regional geology of Cyprus, provide new constraints on the early evolution of the Troodos Complex and form the basis of a model for transform fault metamorphism.     

The application of isochron diagrams in40Ar-39Ar dating: A discussion

The presence of a non-radiogenic argon component within rocks and minerals at the time of their crystallization (termed initial argon) is now widely recognized. Present data indicate that the³⁶Ar concentration of this initial argon is in the order of 10^?9 cm³/g STP in a wide range of geological materials. In⁴⁰Ar-³⁹Ar analyses the preference for small samples and division of the gas evolved into a number of temperature steps means that the amount of initial³⁶Ar analyzed is often less than the argon extraction line blank. Thus if blank corrections are not made an isochron plot will represent mixing lines between atmospheric argon and the argon derived from the sample. In this case the isochron parameters will probably be in error. Secondary alteration of samples adds atmospheric argon not related to the initial argon present in a rock at the time of its formation and must be eliminated or avoided if the isochron technique is to be used. The inability to obtain isochrons for samples with excess radiogenic argon may be related to significant extraction system contamination. At present the application of the two-error regression technique is being misapplied in determining the quality of fit and in some cases underestimating the error limits assigned to the isochron parameters.