K-rich Volcanism Research Articles

The Jurassic–Cretaceous K-rich Volcanism of the Nercha Depression in Eastern Transbaikalia and Its Geodynamic Nature

The Jurassic–Cretaceous K-rich Volcanism of the Nercha Depression in Eastern Transbaikalia and Its Geodynamic Nature

Tectonic transition revealed by upper mantle heterogeneities and anisotropy of the SE margin of the Tibetan Plateau: Insights into the Cenozoic intraplate volcanisms

The SE margin of the Tibetan Plateau is a key channel for understanding the plateau's evolution. However, the links between mantle dynamics, surface deformation, and associated intraplate volcanism remain elusive. Here, 3D radially anisotropic seismic structures of the upper mantle beneath the SE margin of the Tibetan Plateau are presented based on adjoint waveform tomography using dense arrays. The seismic image shows a prominent tectonic transition along 26°N, characterized by strong heterogeneities. In the north of 26°N, lithospheric delamination is revealed by high-velocity anomalies between the Tibetan Plateau and the Yangtze Craton at the 150–250 km depth, which may drive the early Cenozoic potassic volcanism. In the south of 26°N, asthenospheric flow is suggested by significantly low velocity and positive radial anisotropy at the uppermost mantle, which probably accounts for the late Cenozoic sodic volcanism. We propose that such upper mantle dynamics shape the surface expression of plateau uplift and intraplate volcanism. Plain language summaryThe mechanism controlling the growth and volcanism of the Tibetan Plateau remains ambiguous. We present clues from subsurface structures using earthquake seismology. We observe strong heterogeneities of velocity and anisotropy in the upper mantle of the SE margin of the Tibetan Plateau. The results suggest that lithospheric delamination and asthenospheric flow control the generation of the early Cenozoic K-rich volcanism and late Cenozoic Na-rich volcanism, respectively. Our study indicates that tectonic transition might exist in the subsurface of the SE margin of the Tibetan Plateau.

Late Miocene K-rich volcanism in the Eslamieh Peninsula (Saray), NW Iran: Implications for geodynamic evolution of the Turkish–Iranian High Plateau

Post-collisional volcanism in northwestern Iran is represented by the Saray high-K rocks including leucite-bearing under-saturated and leucite-free silica saturated rocks. We report Ar–Ar age data which constrain the age as ca. 11Ma (late Miocene). Most of clinopyroxene phenocrysts from the volcanic rocks have complex oscillatory zoning, with high Ti and Al cores, low Ti and high Al mantled clinopyroxenes, grading into low Ti and Al outer rims. All the rocks are highly enriched in incompatible trace elements and have identical Sr–Nd–Pb isotopes. Enrichment in incompatible elements and other geochemical features for the Saray lavas suggest a metasomatized sub-continental lithospheric mantle (SCLM) as the magma source. The negative Nb–Ta–Ti anomalies for the Saray lavas compare with the features of subduction-related magmatism with negligible contamination with ancient crustal components. The highly radiogenic 87Sr/86Sr and 207Pb/204Pb isotopic values of the Saray lavas imply the involvement of slab terrigenous sediments and/or a continental lithosphere. Isotopically, the volcanic rocks define a binary trend, representing 5–8% mixing between the primary mantle and sediment melts. Our melting models suggest residual garnet in the source and are incompatible with partial melting of amphibole and/or phlogopite bearing lherzolites, although the complex geochemical features might indicate the result of mixing between melts produced by different sources or a homogenous melt passing through a compositionally-zoned mantle during multiple stages of partial melting and melt migration. The geochronological, geochemical and isotopic data for the Saray rocks suggest that these Late Miocene magmas were derived from a small degree of partial melting of subduction-metasomatized (subcontinental) lithospheric mantle source in a post-collisional setting.

Mantle source characteristics and melting models for the early-middle Miocene mafic volcanism in Western Anatolia: Implications for enrichment processes of mantle lithosphere and origin of K-rich volcanism in post-collisional settings

In Western Anatolia, early-middle Miocene post-collisional volcanism is represented by K-rich volcanic rocks; including calc-alkaline, high-K calc-alkaline, shoshonitic and ultrapotassic products. Low-SiO 2 and relatively high-MgO mafic volcanic rocks in the region are represented by high-K volcanic rocks (HKVR) in the west, where strike–slip tectonics related to the İzmir–Balıkesir Transfer Zone dominates, while shoshonitic and ultrapotassic volcanic rocks (SHVR and UKVR) occur in the eastern part of the region, where mid-crustal units were exhumed along detachment faults during orogenic collapse. All three rock groups have nearly identical Sr–Nd isotope ratios, but the SHVR and UKVR are more enriched in incompatible trace elements relative to the HKVR. Comparison of the geochemical characteristics of the most primitive lavas (SiO 2 < 55 wt.%, Mg# > 65 and MgO > 6 wt.%) provides an important tool in discussing the origin and evolution of the K-rich magmatic activity in this post-orogenic setting. Geochemical features of the three rock groups require they were all derived from a primitive mantle-like source that, on the basis of Sr–Nd isotope mixing models, was supplemented by addition of 7–15% sediment components, including both sediment fluid and sediment melt. Low-degree partial melting (~ 5–10%) of this modified mantle source can account for the incompatible trace element budgets of the HKVR, but an additional trace element enrichment process is required to explain the compositions of the SHVR and UKVR. Numerical modeling suggests that this enrichment developed by multi-stage melting and melt percolation processes in a thicker metasomatized mantle lithosphere than that which produced the HKVR. This petrogenetic model accords with tectonic observations of lithospheric thickening from west to east in the area (where strike–slip deformation occurred to the west and regional uplift and exhumation occurred to the east). Hence, the first enrichment stage of the mantle source (HKVR plus SHVR and UKVR) is interpreted to be the result of subduction-related processes during southward retreat of the Aegean subduction system, with the second-stage of enrichment (SHVR and UKVR) developing in the mantle lithosphere during lithospheric extension.

Coexisting calc-alkaline and ultrapotassic magmatism at Monti Ernici, Mid Latina Valley (Latium, central Italy)

New major and trace element data, and Sr-Nd-Pb-O isotopic ratios for volcanic mafic rocks outcropping at Monti Ernici in the Mid Latina Valley (southern Latium) are reported, with the aim of investigating the nature and evolution of Plio-Quaternary K-rich volcanism in Central Italy. Petrographical and geochemical studies allow us to identify mafic rocks ranging from ultrapotassic (HKS) to shoshonitic (SHO), and calc-alkaline (CA), these last ones being identified for the first time. The CA rocks exhibit the most primitive signatures for Sr, Nd, and Pb isotopes ( 87 Sr/ 86 Sr =0.706326–0.706654; 143 Nd/ 144 Nd = 0.512388–0.512361; 206 Pb/ 204 Pb = 18.944–18.940). The δ 18 O values are variable (δ 18 O cpx from +5.75 to +7.08 ‰; and δ 18 O ol from +5.50 to +6.23 ‰), suggesting interaction with carbonate wall rocks. Radiogenic isotope ratios and incompatible elements distribution have several characteristic in common with equivalent rocks from Pontine Islands (Ventotene), Campania and Aeolian arc volcanoes. Conversely, the HKS rocks closely resemble the ultrapotassic rocks from the Roman Province ( 87 Sr/ 86 Sr = 0.709679–0.711102; δ 18 O cpx from +6.27 to +7.08 ‰). The high ratios of LILE (Large Ion Lithophile Elements: Rb, Cs, Th, U, K, LREE) and HFSE (High Field Strength Elements: Ta, Nb, Zr, Hf, Ti), and radiogenic isotope compositions of CA to HKS rocks indicate that all suites contain subduction-related components, and suggest a N-MORB-type mantle source variably contaminated by hydrous fluids and/or melts released by undergoing slabs, possibly during two distinct stages of metasomatism. The coexistence of ultra-alkaline and sub-alkaline orogenic magmatism, combined with tectonic, geophysical and geological evidence, support the possibility that the mantle beneath central-southern Italy (Ernici-Roccamonfina Province) was vertically zoned and produced different magma suites during time.

Magma source components in an arc-continent collision zone: the Flores-Lembata sector, Sunda arc, Indonesia

Major, trace-element, and Sr-, Nd-and Pbisotope data are presented for volcanics from 12 active or recently active volcanoes from the islands of Flores, Adonara, Lembata and Batu Tara in the eastern Sunda are. The volcanics vary in composition from low-K tholeiite, through medium-and high-K calcalkaline types to the K-rich leucite basanites of Batu Tara. From the tholeiites to the leucite basanites there are marked increases in the concentrations of LILE (K, Rb, Ba, Sr), LREE and La/Yb, and all the volcanics have high Ba/ Nb, La/Nb and Ba/La compared with mid-ocean ridge and intraplate eruptives. K/Cs values are generally lower than OIB values, and overlap those of other arc volcanics and northeast Indian Ocean sediments. The volcanics exhibit a broad range of 87Sr/86Sr (0.70468–0.70706), 143Nd/144Nd (0.512946–0.512447), and a moderate range in 206Pb/204Pb (18.825–19.143), 207Pb/ 204Pb (15.643–15.760) and 208Pb/204Pb (38.97–39.51). Trace-element and isotopic data suggest that the mantle beneath the eastern Sunda arc is a complex heterogeneous mixture of 3 or 4 major source components: MORB-source or depleted MORB-source, OIB-source and subducted Indian Ocean sediment. The low-K tholeiites were probably formed by relatively large degrees of melting of depleted MORB-source mantle, modified by subduction-related fluids, whereas the trace-element and isotopic characteristics of the K-rich volcanics suggest that they were derived from an OIB source which and been modified by a subduction-related melt component. The source components of the medium-to high-K calcalkaline rocks are more difficult to determine, and probably include mixtures of MORB-source or OIB-source, and melt/fluid derived from subducted oceanic sediment. Minor-and trace-element modelling calculations indicate substantial difficulties in producing the relatively low Ti-contents of arc volcanics by melting OIB-source mantle. Where OIB mantle is considered to be an important component of arc magmas it is suggested that the HFSE are buffered to relatively low concentration by a residual Ti-rich accessory phase.

U-Th-Ta fractionation in magma sources of the italian K-rich volcanism. Constraints from distribution coefficients and Th-U disequilibrium studies

U-Th-Ta fractionation in magma sources of the italian K-rich volcanism. Constraints from distribution coefficients and Th-U disequilibrium studies

Roman comagmatic province (central Italy): Evidence for subduction-related magma genesis

Geochemical data on mafic volcanics show that important affinities exist between the Roman and the calc-alkaline rocks from the Aeolian arc (south Tyrrhenian Sea). These affinities, together with the close association of calc-alkaline and K-rich volcanics in the Aeolian arc and in the Naples area, the continuity in the variation of abundances of incompatible elements from calc-alkaline to potassic suites, and the similarity in terms of major-element geochemistry, support a genetic relationship of the Roman magmatism and the subduction processes that affected the Apennines in Tertiary time and are still active under the Aeolian arc. In the genetic model presented here, both calc-alkaline and K-rich magmas were generated within a mantle heterogenously enriched in LIL elements. Composition of the mantle was modified by addition of material, probably sediments, dragged down by the undergoing slab. The geochemical and petrological differences displayed by the calc-alkaline and K-rich volcanics are accounted for by the different conditions of melting as well as by chemical and isotopic heterogeneities of the source. 26 references, 3 figures, 1 table.

Genesis, evolution and tectonic significance of K-rich volcanics from the Alban Hills (Roman comagmatic region) as inferred from trace element geochemistry

Trace element data are reported in 21 lava samples from the Alban Hills, one of the most important volcanic complexes of the Roman comagmatic region. The samples consist mostly of tephritic leucitites with minor phonolitic tephrites and tephritic phonolites emplaced during two distinct phases of activity, separated by a caldera collapse.

Trace element studies of the origin of igneous rocks

TRACE ELEMENT STUDIES are increasingly important in igneous petrogenetic investigations in as much as they place constraints on possible modes of origin of igneous rocks and also give evidence of the nature and composition of the mantle/crust systems within which magmas are generated and modified. The conference saw trace element studies applied to a wide range of igneous rock compositions, occurring within a wide range of environments. Four papers were concerned with areas in which basalt lavas predominate, i.e. Mull (Beckinsale et al.), the islands associated with the axial trough of the Red Sea (Henderson &amp; Parry), the East Scotia Sea (Saunders &amp; Tarney) where basalts are generated within an intra-oceanic marginal basin and the South Shetland Islands and Antarctic Peninsula (Weaver et al.) where different periods of magma genesis have been related to subduction and extension. Other papers dealt with metabasalts from ophiolite complexes (Pearce), K-rich volcanics (Parker), diorites and granodiorites (Brown et al), granites and charnockites (Petersen) and the ultramafic to granitic rocks of the Fongen-Hyllingen Gabbro Complex (Esbensen). An important recent development in geochemistry has been to extend the range of material which can be usefully analysed. For example, important petrogenetic information is now being obtained from altered and metamorphosed igneous rocks using ratios based on trace elements such as Ti, Zr, Y, Nb, P, Ce, Sm, Yb, abundances of which appear to be unchanged by secondary processes (Pearce). Volcanic rock geochemistry, which until recently was virtually synonymous with lava geochemistry, has now expanded its scope to