Ian A. Nicholls

Plate and intraplate processes of Hercynian Europe during the Late Paleozoic

Abstract It is speculated that until Late Carboniferous time the region of Hercynian Europe was occupied by an elongated island arc system underlain by a segment of continental crust. In the Upper Carboniferous, two subduction zones are assumed to have extended from the north and south beneath Hercynian Europe. An extensive zone of hot, partially molten upper mantle lay above and between these, and diapiric uprise of portions of this material led to separation of mafic magmas, widespread partial melting in the lower and middle crust, high temperature-low pressure metamorphism in crustal rocks, and regional uplift and extension of the crust, as indicated by intermontane troughs and their associated volcanic rocks. In Visean to Westphalian time Hercynian Europe collided with both the large neighbouring plates North America-Europe and Africa. During these diachronous collisions and owing to reduced rigidity of the relatively hot island arc crust, the irregular continental margins of the larger and thicker continental plates induced oroclinal bending of Hercynian Europe. After the collision processes had been terminated, processes of upper mantle activity continued, causing further crustal uplift and even, enhanced crustal extension for several tens of million years into the Lower Permian. Decline of the upper mantle activity beneath Hercynian Europe is indicated by crustal subsidence and formation of a peneplain in Permian time followed by the Upper Permian transgression of both the Zechstein sea and the Tethys sea which mark the end of the Hercynian geodynamic cycle.

Geochemical investigations of microgranitoid enclaves in the S-type Cowra Granodiorite, Lachlan Fold Belt, SE Australia ☆

The Cowra Granodiorite is a relatively mafic, enclave-rich, S-type pluton in the Lachlan Fold Belt, which has been cited as a type example of a restitic origin for all varieties of enclaves in Lachlan Fold Belt S-type granites. Microgranitoid enclaves from the pluton are subordinate to metasedimentary varieties and can be subdivided into two groups according to their mafic mineral assemblage: pyroxene microtonalites and biotite microgranites. No geochemical or isotopic distinction can be made between the two varieties. Petrographic evidence (acicular apatites, xenocrysts from the host granite) suggests an origin as mingled, more mafic, magmas, which have been variably contaminated by the more felsic host magma. This is supported by the fact that the microgranitoid enclaves have isotopic compositions (87Sr/86Sr(i)=0.7095 to 0.7144, eNd(i)=−9.2 to −6.9) that are generally more primitive than, or similar to, those of the host granite (87Sr/86Sr(i)=0.7142, eNd(i)=−8.8). The spread in isotopic compositions, like their trace element compositions, is considered to be the consequence of variable degrees of diffusive exchange between the felsic and more mafic magmas during slow cooling. Several studied metasedimentary enclaves are not in isotopic equilibrium with their host granite and therefore cannot represent pristine samples of the bulk source region of the granite. Instead, they represent portions of a lithologically and compositionally diverse source terrane or accidental xenoliths entrapped during emplacement.

Multistage evolution of Australian subcontinental mantle: Re-Os isotopic constraints from Victorian mantle xenoliths

Geochemical and Re-Os isotopic data for refractory spinel peridotite xenoliths from the Newer Volcanics Province of western Victoria demonstrate that parts of the subcontinental lithospheric mantle (SCLM) beneath southeastern Australia underwent melt depletion during the Late Proterozoic as well as the early Paleozoic. Late Proterozoic Re-Os model ages (as old as 900 Ma) for refractory harzburgites are significantly older than the exposed Cambrian crust. In addition, our Re-Os melt-depletion mantle model ages ( T MA ) are consistent with Sm-Nd depleted-mantle model ages ( T DM ) and older zircon U-Pb ages for I-type granites from southeastern Australia, suggesting that this region is underlain by Proterozoic continental lithosphere. In contrast, a fertile spinel lherzolite xenolith has a radiogenic Os isotopic composition (γ Os > +8) consistent with derivation from an “enriched-mantle” volume within the SCLM. Ancient subduction of Re-rich oceanic crust or melt metasomatism of the lithospheric mantle may have generated enriched-mantle volumes similar to those sampled by some ocean-island basalts (OIB) and komatiites.

Chemical modification of enclave magma by post-emplacement crystal fractionation, diffusion and metasomatism

Hornblende-bearing microgranitoid enclaves from the Swifts Creek Pluton (SCP), SE-Australia display mineralogical and textural variations from their margins to their centers. Margins are fine grained and display quench textures and are enriched in amphibole, biotite and in some cases magnetite relative to their coarser grained centers. Enclaves of this type and their adjacent granitoid host rocks have been sectioned into 0.3 to 1 cm thick slabs in order to determine the chemical variations associated with these mineralogical changes. The fine grained margins are variably enriched in Ti, Al, Mg, Fe, Mg, K, Rb, Ba, Nb, Y, Sc, V, Ni and the REE up to a factor of three relative to the enclave centers. This enrichment is compensated by a depletion of Si and Zr. Elements like Ca, Na and Sr show less coherent variation from margin to center. Host rocks in immediate contact with these enclaves are depleted in Mg, Na, K, Rb and Ba relative to host rocks that are not exposed to enclaves. No one single process can account for all the variations. Instead, we propose that the chemical variations are related to a combination of in situ crystal fractionation of isolated magma globules, mass transfer by diffusion and metasomatic exchange.

The Nd- and Sr-isotopic composition of I-type microgranitoid enclaves and their host rocks from the Swifts Creek Pluton, southeast Australia

Abstract Initial Nd- and Sr-isotopic compositions have been determined for microgranitoid enclaves and their host rocks from the Siluro-Devonian Swifts Creek Pluton within the Lachlan Fold Belt of southeastern Australia. Enclaves are of quartz dioritic to tonalitic composition and their textures support an igneous origin. Host rocks range from tonalite to granite. The isotopic data form an array in ϵ Nd  87 Sr 86 Sr space with enclaves systematically displaced towards higher ϵNd- and lower 87 Sr 86 Sr values. Enclaves range in ϵNd (t=405 Ma) from −7.3 to −3.1 with a narrow range in 87 Sr 86 Sr from 0.70613 to 0.70718 whereas host rocks range from −9.0 to −6.4 and 0.70761 to 0.70946 in ϵNd and 87 Sr 86 Sr , respectively. The isotopic relations rule out simple crystal-liquid fractionation or restite unmixing alone as the major genetic link between enclaves and host rocks. The data are also incompatible with an origin of the enclaves by crystal accumulation or liquid immiscibility. Instead, mixing between a mafic, mantle-derived and a crustal-derived end-member to generate the host rock-enclave spectrum is compatible with the data. Because the initial isotopic ratios of enclaves and host rocks are well within the range of crustal rocks, we favour the alternative view that both enclave and host-rock magmas are of crustal origin. Consequently, we avoid extrapolations to hypothetical end-members. We suggest that the enclaves from the Swifts Creek Pluton are derived by partial melting of a mafic lower crust and mixed at or near the site of magma generation with a chemically less primitive and isotopically more evolved component to generate the enclave and host-rock spectrum. The isotopic heterogeneities within the enclave and host rocks survived the superimposed effects of crystal-liquid fractionation. Model ages (tDM) for enclaves and host rocks are almost identical averaging 1.7 and 1.6 Ga, respectively. These model ages indicate that both the enclave and host-rock protoliths were derived from a mantle reservoir during the Proterozoic.

Variation in the geochemistry of mantle sources for tholeiitic and calc-alkaline mafic magmas, Western Sunda volcanic arc, Indonesia

Abstract Quaternary lavas of the normal island-arc basalt—andesite—dacite association in the islands of Java and Bali range from those belonging to tholeiitic series over Benioff-zone depths of ∼ 150 km to high-K calc-alkaline series over Benioff-zone depths of 250 km. More abundant and diverse calc-alkaline lavas are found over intermediate Benioff-zone depths. On average, basaltic lavas become slightly more alkaline (largely due to increased K contents) with increasing depth to the Benioff zone. Levels of incompatible minor and trace elements (K, Rb, Cs, Ba, Nb, U, Th, light REE) show a corresponding increase of almost an order of magnitude. Low average Mg-numbers (∼ 0.52) and Ni and Cr abundances (15–25 and 35–60 ppm, respectively) of basaltic lavas suggest that few lavas representing primary mantle-derived magma compositions are present. Calculated primary basaltic magma compositions for most tholeiitic and calc-alkaline volcanic centres are olivine tholeiites with 15–30% ol. The single high-K calc-alkaline centre considered yielded transitional alkali olivine basalt—basanite primary magma compositions. These calculated magma compositions suggest that the percentage of mantle melting decreases with increasing depth to the Benioff zone (from >25 to Calculation of REE patterns for basaltic magmas on the basis of peridotitic mantle sources with spinel lherzolite, amphibole lherzolite or garnet lherzolite mineralogy, and model REE levels of twice chondritic abundances, indicates that change in the conditions of magma genesis alone cannot explain the observed change in light-REE abundances of basaltic lavas with increasing depth to the Benioff zone. Complementary calculations of the REE levels of mantle sources required to yield the average tholeiitic, calc-alkaline and high-K calc-alkaline basaltic magma indicate that light-REE abundances must increase from 2–3 to 7–8 times chondrites with increasing depth to the Benioff zone. The percentages of mantle melting favoured on REE evidence are lower than those indicated by major-element considerations. The observed variation in incompatible element geochemistry of mantle magma sources is thought to be related directly or indirectly to dehydration and partial-melting processes affecting subducted oceanic crust. The possible nature of this relationship is discussed.

Laser ablation-inductively coupled plasma-mass spectrometry: an investigation of elemental responses and matrix effects in the analysis of geostandard materials

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) has been recently developed as a technique for rapid analysis of solid materials with only a minimum of sample preparation required. This investigation focused upon the utility of a very simple experimental and analytical protocol for samples of a geochemical nature. A selection of mainly igneous geostandards were pressed into pellets and analysed using LA-ICP-MS. initially using a basaltic standard, BHVO-l. The method was found to be effective for semi-quantitative analysis, but it was discovered that the accuracy of the concentration estimates for a suite of trace elements from each significant geochemical classification (high field strength, light and heavy rare-earth, large ion lithophile and chalco/siderophile elements) was variable both with element and with the nature of the analyte rock. Investigation of the elemental responses (intensity per unit concentration of element) for each different rock type indicated that, under constant experimental conditions, there was a variability in the total response for each element amongst the geostandards examined. It was suggested that this variability could be related to the geochemical and physicochemical characteristics of the materials sampled by the laser and some degree of matrix matching might therefore be required to improve the quality of the analysis.

Sr and Nd isotopic investigations towards the origin of feldspar megacrysts in microgranular enclaves in two I-type plutons of the Lachlan Fold Belt, southeast Australia

New Sr and Nd isotopic data are presented for several large feldspar crystals occurring in microgranular enclaves in the Swifts Creek and Bridle Track plutons, along with analyses of their host enclave groundmass and adjacent granitoid. In the Swifts Creek Pluton several previous studies have concluded that the microgranular enclaves represent admixed, more mafic and more primitive magmas, and new data presented here confirm that. Feldspar megacrysts in the microgranular enclaves have Sr and Nd isotopic signatures that are distinct from the surrounding enclave groundmass and from other enclaves in the pluton and therefore cannot have crystallised in situ. Isotopic compositions of these feldspars are more consistent with their having crystallised in a reservoir similar in composition to the most primitive granitoid analyses. The crystals were then physically transferred from the granitoid magma into the enclave while the latter was still partially liquid, thus invalidating arguments for a porphyroblastic origin. Field, petrographic and geochemical data are consistent with microgranular enclaves in the Bridle Track pluton also originating as admixed, more mafic magmas. However, Sr isotopic compositions of the enclaves are identical, within error, to the host granite and indicate that significant Sr isotopic equilibration has occurred. Nd isotopic compositions of the enclaves extend to slightly higher 143Nd/144Nd(i) and are consistent with a mingled magma origin followed by major isotopic equilibration. Feldspar phenocrysts in the studied enclave have isotopic compositions indistinguishable from both the enclave groundmass and host granite, preventing an interpretation of their origin using isotopic evidence alone.

The nature of primary rhyolitic magmas involved in crustal evolution: Evidence from an experimental study of cummingtonite-bearing rhyolites, Taupo Volcanic Zone, New Zealand☆

Abstract Many of the rhyolites of the Taupo Volcanic Zone (TVZ), North Island, New Zealand, have been interpreted as representing primary magmas from the melting of crustal sources, especially the greywackes of the TVZ Mesozoic basement. High-silica (~75% SiO 2 ) rhyolites of the Okataina caldera complex which carry cummingtonite phenocrysts have been considered as products of low-temperature (~750°C) crustal magmas formed under conditions close to saturation in water ( a H 2 O ~ 1). Experimental crystallization studies on contrasted metaluminous (2.6% Di) and peraluminous (1.5% C) Okataina rhyolites confirm that cummingtonite could have crystallized only at pressures ≤ 3 kb (depths ≤ 10 km), probably in the roof zone of a shallow crustal magma chamber and that these rhyolites are more likely to represent fractionated magmas. This view is supported by comparison of their major element compositions with those of experimental silicic liquids from the melting of model metaluminous (TVZ dacite) and peraluminous (TVZ “Western Basement” greywacke) crustal compositions. The Okataina rhyolites and also less silicic average TVZ rhyolite compositions, are notably poorer in the An-component of normative feldspar than the experimental melts, particularly those produced under conditions of a H 2 O ~ 1 (which contain up to 18% An). This suggests that they were derived from more primitive magmas (probably the end products of complex processes involving magma sources in both the crust and mantle) by crystal fractionation or assimilation-fractional crystallization (AFC) dominated by plagioclase-rich extract assemblages. In assessing the nature of magmatic processes involved in internal differentiation of the continental crust, it should be noted that primary crustal magmas are likely to be appreciably richer in plagioclase, more mafic and less silicic, than those represented by voluminous rhyolitic extrusive rocks. Normative An and PI contents may be used as an indicator of primitive character for rhyolitic crustal melts in the same way as normative Fo and Ol for picritic-basaltic melts derived from the mantle.

The petrogenesis of island arc basalts from Gunung Slamet volcano, Indonesia: Trace element and 87Sr86Sr contraints

Abstract Selected major and trace elements, rare earth element (REE) and 87 Sr 86 Sr data are presented for arc basalts from Gunung Slamet volcano, Java, Indonesia. On the basis of stratigraphy, trace element content, Zr Nb , and 87 Sr 86 Sr ratios, Slamet basalts can be broadly categorized into high abundance magma (HAM) and low abundance magma (LAM) types. Provided the quantities of ‘immobile’ trace elements (in aqueous systems) such as Nb, Hf and Zr in the mantle wedge and ensuing magmas are unaffected by additions from subducted lithosphere or overlying arc crust, a model may be developed whereby LAM are generated by higher degrees of melting in the mantle wedge (13%) compared to HAM (7%). Hf Nb or Zr Nb ratio systematics indicate that prior to metasomatism by the underlying lithosphere, the Slamet mantle wedge was similar in chemical character to transitional-MORB source mantle. Conversely, examination of immobile/mobile incompatible trace element ratios (IMITER) provide clues to the nature of the metasomatizing agent, most likely derived from the subducted slab (basalts and sediments). HAM have constant IMITER (e.g. Nb U , Zr K ), whereas LAM show a negative correlation between IMITER and 87 Sr 86 Sr . Metasomatism of the mantle wedge was modelled by interaction with either a slab-derived-melt or -aqueous fluid. Yb Sr and 87 Sr 86 Sr ratios from Slamet basalts and oceanic sediments suggest that ‘bulk’ mixing of the latter into the mantle wedge is unlikely. Instead, sediments probably interact with overlying mantle in the same way that subducted basalts do-either as melts or fluids. In the case of slab-derived melts mixing with ‘pristine’ mantle, good agreement with back-calculated values for HAM and LAM sources can be achieved only if a residual phase such as rutile persists in the subducting lithosphere. In the case of fluids, excellent agreement with back-calculated values is obtained for all elements except heavy REE. It is tentatively suggested that aqueous slab-derived fluids, relatively rich in mobile incompatible elements, are the probable metasomatizing agent responsible for the chemical characteristics, particularly low IMITER, of Slamet and other island arc basalts (IAB). Because the mobilities/solubilities of Sr in high pressure and temperature fluids are poorly known, the modelled subduction fluids are not necessarily efficient at raising 87 Sr 86 Sr in the overlying mantle wedge. As a result, positive correlations between e.g. Ba La vs. 87 Sr 86 Sr need not be observed in arc suites, especially if the relative mobilities of Sr, Ba, and La are dependent upon intensive parameters during metasomatism. Assimilation of arc crust by uprising magmas (up to ~14% of crustal Sr) can account for the range of 87 Sr 86 Sr in HAM. However, calculating the amounts of arc crustal assimilation by uprising magmas is poorly constrained since such modelling is highly dependent upon previous estimates of the degree of metasomatism undergone by the mantle wedge.