Peter A. Floyd

Geochemical discrimination of different magma series and their differentiation products using immobile elements

Abstract The abundance and distribution of selected minor and trace elements (Ti, Zr, Y, Nb, Ce, Ga and Sc) in fresh volcanic rocks can be used to classify the differentiation products of subalkaline and alkaline magma series in a similar manner to methods using normative or major-element indices. A number of variation diagrams may be used to distinguish common volcanic rock types in terms of the above elements. As these elements are immobile during post-consolidation alteration and metamorphic processes, this method of rock-type classification may, when applied to metavolcanic rocks, prove more reliable than the commonly used methods that utilize major elements, some of which are known to be mobile.

Tectonic environment of the Devonian Gramscatho basin, south Cornwall: framework mode and geochemical evidence from turbiditic sandstones

The Portscatho Formation, within the allochthonous unit of the Middle and Upper Devonian Gramscatho Group, is a thick sequence of deep-water sandstones and interbedded slates deposited by southerly-derived turbidity currents into the Gramscatho basin of south Cornwall. Throughout an approximately 3.5 km thick sequence, the Portscatho Formation is petrographically and chemically coherent, except that the upper section shows a higher proportion of metamorphic clasts, high, but variable Cr, and low, uniform Zr abundances. Complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantly acidic composition, similar to average upper continental crust, but with an admixture of minor intermediate/basic material. Flysch deposition took place in a fore-arc setting. The presence of an arc to the south of Cornwall during the Devonian implies that there was subduction at the margin of the Gramscatho basin, whose ultimate closure was accommodated by the northward stacking of flysch–ophiolite nappes.

Identification and discrimination of altered and metamorphosed volcanic rocks using immobile elements

Abstract Altered and metamorphosed volcanic suites can be characterized in terms of magma series (alkaline, sub-alkaline) and degree of differentiation (basaltic, andesitic, etc.) using elements that are immobile (Ti, Zr, Y, Nb, Ce, Ga, Sc) during secondary alteration processes. Geochemical data on meta-volcanic suites in the greenschist, amphibolite and granulite facies have been plotted on a series of immobile-element diagrams to illustrate the method of discrimination and classification. Extrusive and high-level intrusive volcanic rocks that have undergone subsequent alteration or metamorphism, such as spilites, keratophyres, tuffs, greenstones, greenschists, ortho-amphibolites and ortho-gneisses, can be plotted. Evidence for the volcanogenic nature of the metamorphic rocks must be obtained before the diagrams (which are based on fresh volcanic rocks) can be used in a meaningful way.

Geochemistry and provenance of Rhenohercynian synorogenic sandstones: implications for tectonic environment discrimination

Abstract The provenance of synorogenic greywackes from Devonian flysch successions occupying structurally similar positions at opposite ends of the Rhenohercynian zone in SW England and Germany is evaluated. Greywackes from both regions are petrographically and chemically similar, although minor differences are seen in the relative proportions of lithics; the Gramscatho group being richer in volcanic and metavolcanic clasts, whereas the Giessen group is richer in metasedimentary clasts. Absolute abundances of Ni-Cr-V and Zr-Hf-Y vary to a limited extent in each group and reflect variable mafic detritus and heavy mineral inputs respectively. Framework mode parameters and chemical data indicate the Rhenohercynian greywackes were derived mainly from a calc-alkali, acidic, dissected continental arc source, with minor MORB-like and argillaceous metasedimentary components. Upper continental crust-normalized multi-element patterns for the greywackes are characteristic of the continental arc/active margin tectonic environment. However, ‘Mid-Proterozoic’ model Nd ages for the Gramscatho greywackes suggest that an active Devonian arc source is unlikely. Instead the range of chemical and isotopic composition displayed mainly reflects mixing between acidic arc terranes of Proterozoic age and Devonian (Lizard-type) oceanic crust. Petrographic and geochemical discrimination diagrams alone cannot resolve the temporal decoupling between source and basin and may lead to an erroneous interpretation of tectonic setting.

Geochemistry and petrogenesis of intrusive and extrusive ophiolitic plagiogranites, Central Anatolian Crystalline Complex, Turkey

Plagiogranites associated with the Sarikaraman ophiolite of the Central Anatolian Crystalline Complex, Turkey, closely resemble other plagiogranites from supra-subduction zone-type ophiolites of Neotethys. The ophiolite is remarkable in displaying a higher proportion of the plagiogranite suite (ca. 10% by volume) than is usually associated with such bodies. The Sarikaraman plagiogranites are represented by intrusive sheets and netvein trondhjemites largely developed at the top of the upper gabbros and as multiphase dykes within the sheeted dyke complex. The plagiogranite dykes are considered to feed extrusive silicified rhyolites associated with the basaltic lavas in the volcanic section of the ophiolite. Field relations suggest that the trondhjemites were probably generated from the roof section of a dynamic and evolving gabbroic magma chamber. Both the deep-seated trondhjemites and the volcanic rhyolites constitute the Sarikaraman plagiogranite suite. Geochemically there is complete overlap between the intrusive trondhjemites and extrusive rhyolites, which are characterised by (MORB-normalized) low HFS element contents with small negative NbTa anomalies and variably enhanced LIL element abundances. Unlike other plagiogranites, however, the Sarikaraman suite is not characterized by consistently low K2O contents; a feature that reflects the variable mobilization of the LIL elements under lower greenschist facies conditions. The REE are uniformly enriched relative to the basic components of the complex, but have similar normalized patterns exhibiting mild light REE depletion. In terms of their origin, the initial or most primitive plagiogranite melts could have been generated by either fractional crystallization (70–85% of clinopyroxene-feldspar ± amphibole) or partial melting (5–15% batch melting) of a gabbroic ‘source material’, although only the first process can produce most of the range of the plagiogranite compositions. As a group the plagiogranites exhibit some degree of internal variation which can be generated by further fractionation largely dominated by feldspar with minor apatite and amphibole.

Neogene tectonics and plate convergence in the eastern Mediterranean: New data from southern Turkey

Data reported here demonstrate that the Misis Complex of southern Turkey comprises a major olistostromic body, formed prior to the early Miocene and tectonically emplaced upon a structurally imbricated pile of deep and shallow marine sediments of early Miocene to Pliocene age. Magmatic rocks forming olistoliths in the Misis Complex have a back-arc geochemical signature, and variations in the facies, paleoenvironments, and paleotransport vectors within the associated sediments provide a record of Miocene collisional events in southeast Turkey. The pseudo-accretionary imbrication and structural style of the Misis Complex are attributed to continued collisional over-thrusting, but there is evidence within the imbricated stack of late sinistral strike-slip faulting that relates to post-Miocene inception of motion along the East Anatolian fault system. The new data favor these geodynamic models proposed for the Neogene evolution of the northeastern Mediterranean that involve oblique convergence and associated strike-slip adjustments.

Geochemical Character and Tectonic Environment of Neotethyan Ophiolitic Fragments and Metabasites in the Central Anatolian Crystalline Complex, Turkey

Abstract The Central Anatolian Crystalline Complex (CACC) or Kırşehir Block is part of the metamorphosed leading edge of the Tauride-Anatolide Carbonate Platform. It contains oceanic remnants derived from the Neotethys Ocean (İzmir-Ankara-Erzincan branch) which separate it from the Sakarya microcontinent. Two tectonic units are distinguished: an amphibolite facies Mesozoic ‘basement’, dominated by platform marbles, over which is thrust a younger fragmented Upper Cretaceous ophiolite sequence. Three metabasite horizons were sampled to reconstruct the development of the oceanic components: (1) fragmented Upper Cretaceous (90-85 Ma) stratiform ophiolitic members comprising gabbros, sheeted dykes, basalt lavas and pelagic sediments thrust over all other units; (2) a tectonised admixture of basite, ultramafic and felsic blocks in an ophiolitic mélange (Upper Cretaceous matrix) thrust over the basement metamorphic rocks; and (3) amphibolites concordant with ‘basement’ marbles and minor pelagics of the largely (?)Triassic Kaleboynu Formation in the lower part of the carbonate platform. Metabasalts and metagabbros from isolated fragments of the stratiform ophiolites form geochemically coherent groups and indicate the influence of a subduction component during their development. It is considered that the suprasubduction zone ophiolites record the association of a tholeiitic arc and an adjacent back-arc basin with more mid-ocean ridge basalt (MORB)-like compositions. Metabasite blocks within the tectonised ophiolitic mélange slice are MORB like, together with minor ocean island basalt (OIB) and island arc basalts, and may be tectonically related to ophiolitic units within the accretionary wedge of the Ankara Mélange. Concordant amphibolites of the Kaleboynu Formation are largely OIB types and reflect an early ensialic rifting stage of the Tauride-Anatolide Carbonate Platform. Small ocean basins also developed at this time, as recorded by the presence of MORB and associated pelagics. The CACC block, together with parts of the Ankara Mélange, are considered to represent oceanic lithosphere (comprising both early spreading centre and latter subduction-influenced crust) and continental carbonate platform that were subsequently ejected from an accretionary-subduction complex on collision with the Sakarya microcontinent.

Geochemical discrimination and petrogenesis of alkalic basalt sequences in part of the Ankara melange, central Turkey

East of Ankara, Turkey, within the major thrust-bound tectonic units of the Ankara melange (Karakaya and Anatolian Nappes), are numerous blocks of low-grade metamorphosed submarine basaltic lavas. Stable incompatible element geochemistry indicates that the majority of these basalts are alkaline and exhibit a range of characteristics typical of many within-plate oceanic islands (OIB). There is a broad chemical overlap between alkaline basalts from within the Triassic Karakaya Nappes (metamorphic and limestone block melanges) and the Cretaceous Anatolian Nappe (ophiolitic melange). A complete sequence of submarine alkaline basalt lavas was sampled within the ophiolitic melange (the 500 m Kiliclar section) to illustrate the variation and petrogenesis of a small seamount-related crustal segment, prior to tectonic dismemberment. The basalts can be divided into three chemical groups that relate to discrete petrographic segments of the lava pile and probably reflect variable degrees of partial melting of a garnet-bearing enriched source. The chemistry of alkaline basalts within the ophiolitic melange, together with their sedimentary associations, suggests an initial submarine environment adjacent to the volcaniclastite debris-filled moats of oceanic islands or seamounts and their volcanically-active arches. Dismemberment of the seamount-related basalts and pelagic sediments took place during subduction of the oceanic crust and their eventual accretion within a forearc platform.

Review of geochemical variation in Lower Palaeozoic metabasites from the NE Bohemian Massif: intracratonic rifting and plume-ridge interaction

Abstract During early Palaeozoic time the Cadomian basement of the northern margin of Gondwana underwent extensive rifting with the formation of various crustal blocks that eventually became separated by seaways. Attenuation of the continental lithosphere was accompanied by the emplacement of anatectic granites and extensive mafic-dominated bimodal magmatism, often featuring basalts with an ocean crust chemistry. Intrusive metabasites in deep crustal segments (associated with granitic orthogneisses) or extrusive submarine lavas at higher levels (associated with pelagic and carbonate basinal sediments) show a wide range of chemical characteristics dominated by variably enriched tholeiites. Most crustal blocks show the presence of three main chemical groups of metabasites: Low-Titholeiitic metabasalts, Main Series tholeiitic metabasalts and alkalic metabasalt series. They differ in the degree of incompatible element enrichment (depleted to highly enriched normalized patterns), in selected large ion lithophile (LIL) to high field strength element (HFSE) ratios, and abundances of HFSE and their ratios. Both the metatholeiite groups are characterized by a common enrichment of light REE-Th-Nb-Ta. High Th values (or Th/Ta ratios) and associated low εNd values (especially in the Low-Ti tholeiitic metabasalts) reflect sediment contamination in the mantle source rather than at crustal levels, although this latter feature cannot be ruled out entirely. The range of chemical variation exhibited is a consequence of the melting of (a) a lithospheric source contaminated by a sediment component (which generated the Low-Ti tholeiites), and (b) a high-level asthenospheric mid-ocean ridge basalt (MORB)-type source that mixed with a plume component (which generated the range of enriched Main Series tholeiites and the alkali basalts). It is considered that a plume played an important role in the generation of both early granites and the enriched MORB-type compositions in the metabasites. Its significance for the initial fragmentation of Gondwana is unknown, but its presence may have facilitated deep continental crust melting and the fracturing into small crustal blocks. The early-mid-Jurassic plume-instigated break-up of the southern Gondwana supercontinent is considered to be a possible tectonic and chemical analogue for Early Palaeozoic Sudetic rifting and its magmatic products.

Geochemistry and tectonic environment of basaltic rocks from the Misis ophiolitic mélange, south Turkey

Abstract The Miocene Misis tectonosedimentary melange was formed during the closing stages of the Neotethys in what is now the Adana basin of southern Turkey. A major part is characterized by large disorientated limestone blocks set in a predominantly volcanogenic matrix of bedded and chaotic units comprising basaltic pillow lavas, associated hyaloclastites, serpentinites and various acidic tuffs. All the basaltic rocks are mildly metamorphosed (zeolite facies), variably vesicular and differentially altered to smectite, chlorite, Fe-oxides, zeolites and carbonate. Petrographic and chemical data on the least altered pillow lavas indicate they were originally sparsely olivine-plagioclase-clinopyroxene-phyric tholeiites and minor amphibole-phyric basaltic andesites. The basaltic clasts in the melange matrix are genetically related to the pillow lava sequences, with the observed variation throughout the suite being governed by low-pressure fractional crystallization. As a group, the basalts are characterized by low Zr/Y (2–4), relatively high Th/Yb (0.5–0.7) and La/Nb (1.5–2.5) ratios, and moderate, chondrite-normalized, light REE enrichment [ ( La Yb ) N = 3 ] patterns. The basalts have a mildly enriched, subduction-related chemistry and were possibly formed initially in a back-arc basin setting. The supra-subduction zone characteristics of the Misis basaltic units are similar to other Tethyan basalts in eastern Mediterranean ophiolites.