Ultrapotassic Rocks Research Articles

Post-collisional crustal thickening and plateau uplift of southern Tibet: Insights from Cenozoic magmatism in the Wuyu area of the eastern Lhasa block

AbstractThe nature and timing of post-collisional crustal thickening and its link to surface uplift in the eastern Lhasa block of the southern Tibetan plateau remain controversial. Here we report on Cenozoic magmatism in the Wuyu area of the eastern Lhasa block. The Eocene (ca. 46 Ma) trachyandesites and trachydacites show slight fractionation of rare earth elements (REE), slightly negative Eu and Sr anomalies, and relatively homogeneous Sr-Nd and zircon Hf isotopes (87Sr/86Sr(i) = 0.7050–0.7063, εNd(t) = −0.92 to −0.03, εHf(t) = +2.6 to +4.8). Previous studies have suggested Neo-Tethys oceanic slab break-off at 50–45 Ma; thus, the Wuyu Eocene magmatism could represent a magmatic response to this slab break-off and originate from relatively juvenile Lhasa crust. The Miocene (ca. 15–12 Ma) dacites and rhyolites have adakitic affinities, e.g., high Sr (average 588 ppm), Sr/Y (29–136), and La/Yb (30–76) values, low Y (4–12 ppm) and Yb (0.4–0.9 ppm) contents, and variable Sr-Nd and zircon Hf isotopes (87Sr/86Sr(i) = 0.7064–0.7142, εNd(t) = −11.7 to −3.7, εHf(t) = −3.2 to +4.5). Their more enriched Sr-Nd-Hf isotopes relative to the Eocene lavas indicate that they should be derived from mixed Lhasa lower crust comprising juvenile crust, ultrapotassic rocks, and probably Indian lower crust-derived rocks. This study has also revealed the transformation from Eocene juvenile and thin crust with a thickness of <40 km to Miocene mixed and thickened crust with a thickness of >50 km. Combined with published tectonic data, we suggest that both lithospheric shortening and magma underplating contributed to eastern Lhasa block post-collisional crustal thickening. Given the spatial-temporal distribution of eastern Lhasa block magmatism and regional geology, we invoke a post-collisional tectonic model of steep subduction of the Indian plate and subsequent westward-propagating plate break-off beneath the eastern Lhasa block, which caused the surface uplift.

Middle Miocene ultrapotassic magmatism in the Himalaya: A response to mantle unrooting process beneath the orogen

AbstractPost‐collisional ultrapotassic magmatism received special attention because it is an important indicator for mantle geodynamics in a collisional orogen. So far, no mantle‐derived magmatism has been identified within the Himalayan belt, hindering our understanding of the architecture and thermal state of the deep‐seated lithosphere in this area during the orogenesis. Here, we report the first discovery of mantle‐derived ultrapotassic rock in the Himalayan region. The middle Miocene (ca. 13 Ma) lamprophyre dike intruded the Tethyan Himalaya sedimentary cover of the Ramba dome on the shoulder of the Yadong‐Gulu rift. Geochemical analyses suggest that the ultrapotassic melt was derived from the subcontinental lithospheric mantle of the Indian plate, within a relatively shallow depth in the spinel stability field. The ultrapotassic magmatism indicates asthenospheric upwelling in the middle Miocene, a response to the mantle unrooting process beneath the Himalayan orogen.

Geochemistry, risk assessment, and Pb isotopic evidence for sources of heavy metals in stream sediments around the Ulukışla Basin, Niğde, southern Turkey

Concentrations of selected elements (Al, Fe, Mn, Mo, As, Cd, Cu, Cr, Ni, Co, Pb, Sb, and Zn) and Pb isotope ratios were determined in 53 sediments from Alihoca, Gümüş, Horoz, and Çakıt streams around the south-central Taurides (Ulukışla Basin), Niğde, which is a known mining province in Turkey. Several pollution and risk assessment indices were used to assess possible heavy metal pollution in the stream sediments and the associated potential ecological risks. Concentrations of As, Cd, Cu, Cr, Ni, Co, Pb, Sb, and Zn were elevated in the streams located near ancient mines, active mines, and slag piles in the area, suggesting an influence from mining activities. The pollution assessment indices indicated that the sediments were significantly polluted by As, Cd, Sb, Zn, and Pb and moderately polluted by Cu, Ni, Cr, and Co. The sediments show very high potential ecological risk with As, Cd, Sb, and Pb as the principal contributors. Ni, Cr, As, Pb, Zn, and Cd exceeded the probable effect concentrations in most of the samples implying that their concentrations may frequently affect sediment-dwelling organisms. Multivariate statistical analyses indicate that the accumulation of heavy metals in the stream sediments is due to an interplay of anthropogenic activities (mining and agrochemical application) and geogenic processes (weathering of bedrocks and supergene alteration of base metal-rich mineralization). Pb isotopic tracing indicates that total Pb in the sediments (206Pb/207Pb = 1.09–1.29) is primarily from weathering and dissolution of ultrapotassic rocks (206Pb/207Pb up to 1.20) and galena (206Pb/207Pb up to 1.21) from the Pb-Zn-Au deposits in the area with some anthropogenic input from mine slag piles (206Pb/207Pb = 1.10).

Periodic Mixing of Magmas Recorded by Oscillatory Zoning of the Clinopyroxene Macrocrysts from an Ultrapotassic Lamprophyre Dyke

Abstract Ultrapotassic rocks are volumetrically minor, but widely distributed in different geological settings. Extensive studies have concerned mantle melting processes that generated these rocks. However, crustal processes that they may have involved are poorly known. In this paper, we describe complex oscillatory zoning patterns of clinopyroxene (Cpx) macrocrysts from an ultrapotassic lamprophyre dyke in the Kyrgyz North Tianshan orogen. These macrocrysts commonly have a corroded or patchy-zoned core surrounded by a mantle with distinct oscillatory zoning, which is, in turn, surrounded by a euhedral rim. The oscillatory zoning of the mantle is composed of alternating coarse and fine layers with a clear resorption surface, or closely packed layers with a straight or wavy boundary in back-scattered electron images. High-amplitude oscillation of Mg#, Ti, Al, Cr and Sr across the layers of the mantle is attributed to magma mixing. Low-amplitude, high-frequency oscillation of Mg# across the closely packed layers was probably developed as a result of kinetic effects or crystal movement under thermal and chemical gradients. In addition, cryptic sector zoning of some macrocrysts clearly shows a Si- and Mg-rich hourglass sector and an Al- and Ti-rich prism sector. The sector zoning indicates crystallization of these macrocrysts under low degrees of undercooling, and the presence of concentric Cr-rich and Cr-poor layers within the same grain indicates that the growth process was disrupted by multiple magma recharging events. The cores of the macrocrysts have Mg# with three distinctive ranges: <84–90 (Core I), 74–84 (Core II) and 60–70 (Core III). The mantles have Mg# ranging from 64 to 90 without a distinct gap. The rims have a narrow range of Mg# from 76 to 80. The cores and mantles with high Mg# (≥85) have variable La/Yb from 1·8 to 5·0 and Dy/Yb from 2·3 to 4·6. The macrocrysts overall have variable 87Sr/86Sr from 0·7072 to 0·7084. Highly variable trace elements and 87Sr/86Sr within a single grain indicate that both primary and evolved magmas with different compositions were periodically recharged into the crustal magma reservoirs. Modelling results reveal that the melts in equilibrium with the Cpx macrocrysts may have been derived from the magma reservoirs at three different depths equivalent to crystallization pressures of ∼5·4, ∼3·3 and ∼1·6 kbar, respectively, making up a transcrustal magmatic system. The Cpx-laden melts in deep magma reservoirs may have been frequently transported to shallower reservoirs. Magma mixing in the shallower reservoirs led to heterogeneous magmas with different cooling rates and chemical compositions. Early crystallized Cpx crystals were overprinted with diverse zoning patterns during overgrowth and accumulation. Thus, the complex zoning patterns and compositions of the Cpx macrocrysts have important implications for a transcrustal magmatic system in the formation of ultrapotassic rocks.

Miocene Olivine Leucitites in the Hoh Xil Basin, Northern Tibet: Implications for Intracontinental Lithosphere Melting and Surface Uplift of the Tibetan Plateau

Abstract The generation of Miocene–Pliocene post-collisional magmatic rocks in northern Tibet was coeval with surface uplift, meaning that understanding the petrogenesis of these rocks should provide clues to the mechanism of uplift of the Tibetan Plateau. However, the nature of the source(s) of Miocene–Pliocene post-collisional rocks is unresolved, especially for potassic–ultrapotassic rocks. This study focuses on 16 Ma olivine leucitites in the Hoh Xil Basin of northern Tibet, which display the lowest SiO2 (43·4–48·8 wt%) contents of all Miocene–Pliocene magmatic rocks in northern Tibet and have high MgO (4·85–8·57 wt%) contents and high K2O/Na2O (>1) ratios. Whole-rock geochemical compositions suggest that the olivine leucitites did not undergo significant fractional crystallization or crustal assimilation. All samples are enriched in large ion lithophile elements relative to high field strength elements, and they exhibit uniform whole-rock Sr–Nd isotope [(87Sr/86Sr)i = 0·7071–0·7077 and εNd(t) = −3·1 to −3·9] and olivine O isotope (5·8–6·6 ‰, mean of 6·2 ± 0·2 ‰, n = 21) compositions. We propose that the olivine leucitites were derived by low-degree partial melting of phlogopite-lherzolite in garnet-facies lithospheric mantle. Given the tectonic evolution of the Hoh Xil Basin and adjacent areas, we suggest that southward subduction of Asian (Qaidam block) lithosphere after India–Asia collision transferred potassium and other incompatible elements into the lithospheric mantle, forming the K-enriched mantle source of the Miocene–Pliocene potassic–ultrapotassic rocks. Removal of lower lithospheric mantle subsequently induced voluminous Miocene–Pliocene magmatism and generated >1 km surface uplift in the Hoh Xil Basin.

Interpretation of the origin of analcimes with mineralogical, microtextural, and geochemical investigations: a case study from Aktepe region (NE of Kalecik, Ankara, Central Anatolia, Turkey)

The presence of analcime minerals in volcanic rocks is a topic requiring attention during classification of rocks. In fact, when transformation from leucite to analcime is present, the original chemistry of the rock will change and this situation will cause falsification in identification of the original character of the rock. As a result, it is important to determine the origin of the analcime in rocks containing analcime. This study focuses on the investigation of the origin of analcimes occurring in alkaline volcanic rocks (tephrite, phonotephrite) of the Aktepe region, located in the northeastern part of Ankara (Central Anatolia, Turkey). These volcanic rocks consist of abundant clinopyroxene, analcime, Fe-Ti oxides, minor olivine pseudomorphs, and scarce microcrystals of feldspar. The analcime phenocrysts are typically euhedral to anhedral (nearly rounded) in shape with up to 1 cm diameter and there are no other hydrous minerals in the samples. Leucite-like crystals are rarely preserved in the cores of larger analcime crystals and twin lamellas observed in them could demonstrate this preservation. Even partially transformed leucites are rare but not impossible to find in nature because of the rapid kinetics of the transformation of leucite to analcime. Examination of microprobe and EDS spectra, corona-type texture observed around the analcime–leucite interface suggest that some leucite crystals did not transform entirely to analcime crystals and a progressive transformation of leucite into analcime occurred locally in the region. Analcime and preserved leucite-like crystal existence could also be seen from the obtained X-ray diffractograms. The fresh fracture surfaces of separated large analcime crystals have microporous texture indicating a characteristic feature of analcimes that formed by an ion-exchange process from leucite. Geochemically, calculations of gains and losses of Na and K contents from the original bulk-rock chemical composition reveal that Na content increase is counterbalanced by a decrease in K content. This could also be strongly explained by an analcimization process and/or progressive transformation of leucite into analcime. It is concluded that analcimes formed secondarily from leucite by an ion-exchange process after cooling of magma via graded infiltration of Na-bearing fluids into the Aktepe ultrapotassic alkaline volcanic rocks.

Ordovician shoshonitic to ultrapotassic volcanism in the central Norwegian Caledonides: The result of sediment subduction, mantle metasomatism and mantle partial melting

Shoshonitic to ultrapotassic, mantle-derived volcanic rocks found within certain accretionary and collisional settings have trace element patterns comparable to those of common arc-related rocks, but with extreme enrichments in highly incompatible elements. Such rocks, previously unknown in the Scandinavian Caledonides, have been discovered in the Oppdal area in the Trondheim Nappe Complex, central Norway. The volcanic rocks are part of the Skarvatnet unit, which consists of (1) the Kinna volcanic succession, (2) the Storgruvpiken rhyolite, and (3) the Skaret conglomeratic succession. The Kinna volcanic succession is interpreted as consisting mainly of submarine pyroclastic flows. A trachytic bed from within this succession is dated to 474 ± 1 Ma. The Storgruvpiken rhyolite is interpreted as a shallow, subvolcanic intrusion or volcanic dome, and is dated to 470 ± 1 Ma. The Skaret conglomerates were deposited in a shallow-marine, tectonically active setting post-dating the Storgruvpiken rhyolite. The Kinna volcanic succession is highly enriched in Th, U, Pb and LREE, with trace element signatures remarkably similar to high-K to ultrapotassic rocks of the Alpine-Himalayan and other orogenic belts. By analogy with these recent examples, the Kinna volcanic succession is interpreted as the result of a two-stage process: (1) subduction and partial melting of continent-derived material caused extensive metasomatism of the overlying mantle wedge, (2) partial melting of this heterogeneous mantle source produced the highly enriched mantle melts. The Storgruvpiken rhyolite, particularly enriched in beryllium, is interpreted as the result of partial melting of Kinna-type rocks at depth. The Skarvatnet unit is interpreted as the result of a complex tectonic evolution at the margin of the Iapetus Ocean, involving the following stages: (1) intra-oceanic subduction, producing the Løkken-Vassfjellet-Bymarka ophiolite at 487–480 Ma, followed by (2) the arrival of the Laurentian margin or an associated micro-continent at ca. 480 Ma, leading to the subduction of large amounts of continent-derived material and subsequent metasomatism of the overlying mantle wedge, (3) arc-continent-collision and slab retreat/slab break-off, causing an intermittent phase of mantle-derived, highly enriched magmatism preserved as the Kinna volcanic succession and the Storgruvpiken rhyolite at 475–470 Ma.

In situ U-Pb geochronology, Lu-Hf and Sm-Nd isotope systematics of the Hoidas Lake REE deposit, northern Saskatchewan, Canada

The diopside-allanite veins and apatite breccia veins of the Hoidas Lake LREE deposit cross-cut the deformed Archaean and Paleoproterozoic granitoid gneisses of the Rae Subprovince, in northern Saskatchewan, Canada. The timing and source of this unusual and complex REE deposit are not well-constrained. Here we report on the results of LA-ICP-MS U-Pb geochronology of zircon and titanite from diopside-allanite veins and green apatite and monazite from apatite breccia veins, Hf isotopic data for zircon, and Sm-Nd isotopic data for titanite, apatite and monazite, in an attempt to constrain the age and duration of crystallization of REE-bearing phases, and provide implications on the source of the mineralization. Geological relationships and in situ U-Pb data indicate that the REE veins formed after peak metamorphism at ca. 1.9 Ga. However, there are zircon crystals in the diopside-allanite veins with concordant U-Pb dates as old as ~2350 Ma, the oldest of which may have been inherited from Arrowsmith Orogen granitoids. Zircon rims show a concordant cluster of U-Pb dates around 1911 Ma, which are interpreted to represent new zircon growth, based on textural features and their distinct initial ɛHf values (26.9 and 34.3), compared to the inherited zircon cores, which range from −9.5 to −2.0. Younger U- Pb Concordia ages of titanite (ca. 1860 Ma and 1830 Ma) and monazite (ca. 1843 Ma), and discordant apatite 207Pb/206Pb dates (ca. 1840 Ma), are considered to be the best constraints on the initiation of REE mineralization. The initial ɛNd values of titanite, green apatite and monazite (−15.2 to −10.7) are comparable to, but lower than the ɛNd values for the Martin Group alkali basalts (Beaverlodge Domain) and the ultrapotassic rocks of the Christopher Island Formation (Baker Lake Basin), both of which also yielded similar U-Pb dates to those of the youngest Hoidas Lake REE-bearing minerals. These regionally occurring alkali units originated from a similar source, most probably an ancient enriched lithospheric mantle reservoir. The melts and fluids, from which the Hoidas Lake deposit formed, then interacted with Rae craton crust as they utilized structures while they were transported from the source region to the depositional site.

Two-Stage Origin of K-Enrichment in Ultrapotassic Magmatism Simulated by Melting of Experimentally Metasomatized Mantle

The generation of strongly potassic melts in the mantle requires the presence of phlogopite in the melting assemblage, while isotopic and trace element analyses of ultrapotassic rocks frequently indicate the involvement of subducted crustal lithologies in the source. However, phlogopite-free experiments that focus on melting of sedimentary rocks and subsequent hybridization with mantle rocks at pressures of 1–3 GPa have not successfully produced melts with K2O >5 wt%–6 wt%, while ultrapotassic igneous rocks reach up to 12 wt% K2O. Accordingly, a two-stage process that enriches K2O and increases K/Na in intermediary assemblages in the source prior to ultrapotassic magmatism seems likely. Here, we simulate this two-stage formation of ultrapotassic magmas using an experimental approach that involves re-melting of parts of an experimental product in a second experiment. In the first stage, reaction experiments containing layered sediment and dunite produced a modally metasomatized reaction zone at the border of a depleted peridotite. For the second-stage experiment, the metasomatized dunite was separated from the residue of the sedimentary rock and transferred to a smaller capsule, and melts were produced with 8 wt%–8.5 wt% K2O and K/Na of 6–7. This is the first time that extremely K-enriched ultrapotassic melts have been generated experimentally from sediments at low pressure applicable to a post-collisional setting.

Pervasive Miocene melting of thickened crust from the Lhasa terrane to Himalaya, southern Tibet and its constraint on generation of Himalayan leucogranite

The Oligocene to Miocene period is one of the most important stages during the formation and evolution of the Tibetan Plateau which generated the tectonic framework similar to the present. The rise of southern Tibet at this time was accompanied by widespread and intense tectonomagmatic activities. The Himalayan region was dominated by leucogranites considered as pure metasediments derived S-type granites, while the Lhasa terrane developed coeval thickened crust derived high Sr/Y calcalkaline rocks and metasomatized mantle derived ultrapotassic rocks. It is puzzling why the adjacent amalgamated regions show contrasting magmatic and dynamic process.This contribution presents a systematic study of newly found various dikes, including minette, aplite, granodiorite porphyry and granite porphyry, which are widespread from the southern Lhasa terrane, across the Yarlung Tsangpo suture zone (YTSZ) and into the Tethyan Himalaya. These dikes yield 16–11 Ma U-Th-Pb ages from zircon, titanite and monazite. The aplite and granodiorite porphyry dikes (high Sr/Y calcalkaline) and minette dikes (ultrapotassic) previal from the Lhasa terrane to the YTSZ. They are identical to the coeval high Sr/Y and/or ultrapotassic rocks from the Lhasa terrane, indicating a similar melting process of thickened lower crust and metasomatized lithospheric mantle. To the south of YTSZ, the middle Miocene granite porphyry in the Tethyan Himalaya exhibit high Sr/Y ratios (50–138), relatively low initial 87Sr/86Sr ratio (0.7064–0.7098), high εNd(t) value (−8.31 to −1.91) and positive εHf(t) value (+1.55 to +4.33), in contrast to the high 87Sr/86Sr and low εNd(t) Himalayan leucogranites. They were likely generated by partial melting of thickened lower crust dominated by amphibolite with significant contributions from juvenile magma. The two-mica granite of Bendui pluton from the Tethyan Himalaya is similar to the granite porphyry in element composition, while the highly evolved muscovite granite there resembles the Himalayan leucogranite. Together, the granite porphyry, Bendui two-mica granite and leucogranite from the Tethyan Himalaya show a transition trend in elemental and Sr-Nd isotopic compositions, implying that the high Sr/Y granite porphyry dikes may represent the more primitive magma of Himalayan leucogranites. In this case, the Himalayan leucogranites, at least in part, represent highly evolved I-type granites with extensive assimilation of ancient crustal materials instead of pure metasediment-derived partial melts. Therefore, we suggest that the thickened lower crust in southern Tibet underwent pervasive partial melting during Oligocene to Miocene as a result of the removal of subducting Indian plate.

Geology and petrology of the potassic and ultrapotassic rocks from the northern part of Senirkent (Isparta-SW Turkey): evidence of magma–carbonate wall-rock interactions

Potassic, ultrapotassic lava domes, flows, and volcano-sedimentary deposits including leucite-bearing and leucite-free grains crop out to the north of Senirkent (Isparta), at the apex of the Isparta Angle (southwestern Turkey). Ultrapotassic rocks are nepheline-foidolite, italite, leucitite, melilite leucitite, melilite-gehlenite leucitite, diopside leucitite, and leucitophyre with lowest SiO2 content (38.71–44.72 wt%, but the leucitophyre has 51.4 wt%), higher K2O (4.36–10.19 wt%), MgO (3.25–6.98 wt%), and K2O/Na2O ratio (2.65–32.8). Potassic rocks are phonolitic leucitites, sanidine-phyric phonolite, and trachytes with SiO2 (49.35–64.46 wt%), K2O (8.28–11.38 wt%), MgO (0.17–1.39 wt%), and K2O/Na2O ratio (2.25–11.8). Some leucitites, diopside leucitites, and phonolitic leucitites enclose partly carbonatite occurences or associated with carbonate-rich part including carbonatite globules, phlogopite, clinopyroxene, and/or leucite, melanite crystals embedded in glassy volcanic matrix and/or carbonate matrix. Primitive mantle-normalized incompatible element patterns display strong enrichments in LILEs (Cs, Rb, Ba, U, Th, Sr) relative to HFSEs (Nb, Ti) with distinct negative Nb–Ti anomalies, suggesting subduction-related lavas. Chondrite-normalized REE patterns are characterized by light REE enrichment (251–493 times chondrite for La) and flat heavy REE patterns (10–27 times chondrite for Yb). All the leucite-bearing and leucite-free rocks display similar, subparallel chondrite-normalized and primitive mantle-normalized trace element patterns, suggesting cogenetic origin. Mineralogical, petrological, and geochemical data suggest magma–carbonate wall-rock interaction and tapping of different parts of the melts within the magma chamber along the fault lines formed during neotectonic period and extension-related block faulting.

Geochronology and geochemistry of the Tongcheng ultrapotassic hypabyssal intrusions at the south‐western margin of the Ordos Block, China: Constraints on the geodynamic processes beneath the North China Craton

The Tongcheng ultrapotassic rocks intrude in Mesoproterozoic limestones in Chongxin County, Gansu Province, China, at the south‐western margin of the Ordos Block. They are characterized by high K2O and Al2O3 but low SiO2, FeOT, MgO, and CaO. The ultrapotassic rocks are relatively depleted in Nb, Ta, Ti, Zr, and Hf and enriched in the large‐ion lithophile elements Rb, Ba, and Th. Zircons from the ultrapotassic rocks yield an Early Cretaceous U–Pb age of 120 Ma. The slightly enriched whole‐rock Sr and Nd isotopic compositions ((87Sr/86Sr)i = 0.705–0.706; εNd = −4.9 to −5.2) indicate an enriched lithospheric mantle source. Combined with regional tectonic data, the Tongcheng ultrapotassic rocks were generated in an extensional environment, which were caused by the upwelling of asthenospheric mantle. Subduction of the palaeo‐Pacific Plate in Early Cretaceous resulted in large‐scale lithospheric thinning in the eastern North China Craton (NCC), which may have also occurred in the western part of the NCC, but the scale of thinning was smaller compared with that of the eastern NCC.

98Mo/95Mo and 238U/235U in lamproites, shoshonites, and high-K calc-alkaline rocks from Western Alps: inferences on their genesis

Oligocene ultrapotassic and associated potassic rocks from Western Alps derived from the crystallisation of magmas originated from partial melting of mantle sources extremely enriched in recycled crustal components. These rocks are well suited for studying the effects of subduction-related, sediment-derived metasomatism on Molybdenum and Uranium isotopes. This paper reports the first 98Mo/95Mo and 238U/235U data on potassic and ultrapotassic rocks at destructive plate margins. Both 98Mo/95Mo and 238U/235U ratios show large variations in the samples from Western Alps. U isotope compositions are consistent with an increasing role of metasomatising melts from recycled sediments, which explains the variable enrichment in potassium and incompatible trace elements passing from high-K calc-alkaline to lamproitic magmas through shoshonitic ones. The variation of Mo isotope compositions is more complex due to the extreme depletion in Mo observed and since their values exceed the range observed for volcanic arcs. These features were investigated considering several possible processes such as secondary weathering, hosting of Mo related to residual mineral phases during sediment melting or physical removal during subduction. The results were also discussed in the framework of the complex processes responsible for the genesis and geochemical characteristics of the Tethyan Realm Lamproites, particularly in relationship with the exotic SALATHO geochemical component.

First Identification of Mafic Igneous Enclaves in Miocene Lavas of Southern Tibet With Implications for Indian Continental Subduction

AbstractThe history of Indian continental subduction beneath Asian plate remains unclear. Miocene ultrapotassic rocks in southern Tibet, with extremely enriched isotopes, have often been used to trace mantle metasomatism and geodynamic processes associated with Indian continental subduction. These rocks, however, may have been contaminated by Lhasa ancient crust. Uncertainties on primary ultrapotassic magmas obscure their mantle sources. Here we report on first mafic igneous enclaves in Cenozoic lavas of southern Tibet. They consist principally of clinopyroxene, phlogopite, and sanidine and have a zircon U‐Pb age of 21.5 ± 0.3 Ma. Mineral and bulk‐rock geochemical characteristics indicate their crystallization from primary ultrapotassic magmas. Bulk‐rock Sr‐Nd, clinopyroxene Sr, and zircon O isotopes demonstrate their isotopically enriched character. Combined with precollisional basalts, we suggest that the mantle source of the enclaves was enriched by the subducted Indian continent. This study implies that the Indian continent had subducted beneath central Lhasa no later than early Miocene.

Geogenic versus anthropogenic behaviour and geochemical footprint of Al, Na, K and P in the Campania region (Southern Italy) soils through compositional data analysis and enrichment factor

Geochemical studies that focus on environmental applications tend to approach the chemical elements as individual entities and may therefore offer only partial and sometimes biased interpretations of their distributions and behaviour. A potential alternative approach is to consider a compositional data analysis, where every element is part of a whole. In this study, an integrated methodology, which included compositional data analysis, multifractal data transformations and interpolation, as well as enrichment factor analysis, was applied to a geochemical dataset for the Campania region, in the south of Italy, focusing in particular on the behaviour, footprints and sources of a smaller pool of elements: Al, Na, K and P. The initial dataset included 3669 topsoil samples, collected at an average sampling density of 1 site per 2.3 km2, and analyzed (after an aqua regia extraction) by a combination of ICP-AES and ICP-MS for 53 elements. Frequency based methods (Clr biplot, Enrichment Factor computation) and frequency spatial-method (fractal and multifractal plots) allowed identifying the relationships between the elements and their possible source patterns in Campania soils in relation to a natural occurring concentrations in geogenic material (rocks, soils and sediments) or human input. Results showed how the interpretation of concentration and behaviour of Al, Na, K and P was enhanced thanks to the application of data log-ratio transformation in univariate and multivariate analysis compared to the use of raw or log-normal data. Multivariate analyses with compositional biplot allowed the identification of four element associations and their potential association with the underling geology and/or human activities. When focusing on the smaller pool of elements (Al, P, K and Na), these relationships with the unique geology of the region, were largely confirmed by multifractal interpolated maps. However, when the local background was used for the calculation of the enrichment factor, the resulting interpolated maps allowed to identify smaller areas where the greater concentrations of P could not be possibly associated to a mineralisation (e.g., ultrapotassic rocks) but were more likely to be associated to anthropogenic input such as agriculture activities with potentially extensive use of phosphate fertilizers. The integrated approach of this study allowed a more robust qualitative and quantitative evaluation of elemental concentration, providing in particular new and vital information on the distribution and patterns of P in soils of the Campania region, but also a viable, more robust, methodological approach to regional environmental geochemistry studies.

Petrology and geochemistry of the Mesoproterozoic Vattikod lamproites, Eastern Dharwar Craton, southern India: evidence for multiple enrichment of sub-continental lithospheric mantle and links with amalgamation and break-up of the Columbia supercontinent

Numerous lamproite dykes are hosted by the Eastern Dharwar Craton, southern India, particularly towards the northwestern margin of the Cuddapah Basin. We present here a comprehensive mineralogical and geochemical (including Sr and Nd isotopic) study on the lamproites from the Vattikod Field, exposed in the vicinity of the well-studied Ramadugu lamproite field. The Vattikod lamproites trend WNW–ESE to NW–SE and reveal effects of low-temperature post-magmatic alteration. The studied lamproites show porphyritic texture with carbonated and serpentinized olivine, diopside, fluorine-rich phlogopite, amphibole, apatite, chromite, allanite, and calcite. The trace-element geochemistry (elevated Sr and HFSE) reveals their mixed affinity to orogenic as well as anorogenic lamproites. Higher fluorine content of the hydrous phases coupled with higher whole-rock K2O highlights the role of metasomatic phlogopite and apatite in the mantle source regions. Trace-element ratios such as Zr/Hf and Ti/Eu reveal carbonate metasomatism of mantle previously enriched by ancient subduction processes. The initial 87Sr/86Sr-isotopic ratios (calculated for an assumed emplacement age of 1350 Ma) vary from 0.7037 to 0.7087 and ɛNd range from − 10.6 to − 9.3, consistent with data on global lamproites and ultrapotassic rocks. We attribute the mixed orogenic–anorogenic character for the lamproites under study to multi-stage metasomatism. We relate the (1) earlier subduction-related enrichment to the Paleoproterozoic amalgamation of the Columbia supercontinent and the (2) second episode of carbonate metasomatism to the Mesoproterozoic rift-related asthenospheric upwelling associated with the Columbia breakup. This study highlights the association of lamproites with supercontinent amalgamation and fragmentation in the Earth history.

Ultrapotassic Volcanism of the Valagin Ridge, Kamchatka

New isotopic-geochemical data are reported on the Late Cretaceous–Paleocene ultrapotassic volcanic rocks of the alkaline–ultrabasic complex of the Valagin Ridge, Eastern Kamchatka. The high Mg, low Ca and Al contents at high K/Na ratios in these rocks make them similar to the Mediterranean-type lamproites and ultrapotassic rocks. The low contents of high-field strength (HFSE) and heavy rare-earth (HREE) elements relative to the MORB composition, and the low Sr and high Nd isotopic ratios indicate the formation of their primary melts from a depleted mantle source. The enrichment of the ultrapotassic rocks in the large-ion lithophile elements (LILE) can be explained by the fluid influx in melts during melting of subsided oceanic crust.

Cenozoic mantle composition evolution of southern Tibet indicated by Paleocene (~ 64 Ma) pseudoleucite phonolitic rocks in central Lhasa terrane

The question of whether continental subduction processes in collisional orogenic belts can trigger wide-spread mantle metesomatism and crustal material recycling remains unresolved. Miocene (25–8Ma) ultrapotassic rocks in southern Tibet are the only mantle-derived magmatic rocks emplaced after the collision between India and Asia and they have been linked to the onset of east-west extensional stresses as the surface uplift of the Tibetan Plateau reached near-maximum elevation. However, their petrogenesis remains highly controversial, particularly the issue of whether their extremely enriched Sr-Nd isotopic characteristics were related to metasomatism derived from subducted Indian continental materials during the Cenozoic. Here we report on a Paleocene silicate-unsaturated, pseudoleucite phonolitic dike, in the Rongniduo area of central Lhasa terrane. In-situ SIMS (secondary ion mass spectrometry) apatite U-Pb age indicates the dike was generated at 64.1±4.2Ma, which slightly predates the age of initial India and Asia collision (about 55–50Ma). This is the oldest age yet reported for ultrapotassic rocks in southern Tibet. Samples from this dike have distinctly more depleted Sr-Nd (whole rock: (87Sr/86Sr)i=0.7064 to 0.7062, εNd(t)=−1.5 to 0.4; in situ apitite: (87Sr/86Sr)i=0.7059 to 0.7060, εNd(t)=−2.0 to 0.4) isotopic compositions, than those of Miocene (25–8Ma) ultrapotassic rocks in the central Lhasa terrane ((87Sr/86Sr)i=0.7106 to 0.7399, εNd(t)=−10.6 to −18.5). Our new data provides important constraints on pre-collisional mantle characteristics beneath the Lhasa terrane. We suggest that these ~64Ma pseudoleucite phonolitic rocks were derived from the enriched lithospheric mantle metasomatized by subducted Tethyan oceanic materials in response to Neo-Tethyan slab roll-back. As a consequence, the younger Miocene ultrapotassic rocks, which display different geochemical compositions from the pre-collisional ultrapotassic rocks, were most probably derived from a mantle source metasomatized by subducted Indian continental materials after ~64Ma. Our results indicate that the addition of subducted continental components plays an important role in changing mantle constituents beneath collisional orogenic belts.

Structural and chemical variations in phlogopite from lamproitic rocks of the Central Mediterranean region

Micas from mafic ultrapotassic rocks with lamproitic affinity from several localities of the Central Mediterranean region were studied through single-crystal X-ray diffraction (SC-XRD), electron microprobe analysis (EMPA) and Secondary Ion Mass Spectrometry (SIMS); Mössbauer Spectroscopy (MöS), when feasible, was also applied to minimise the number of unknown variables and uncertainties. Analysed lamproitic samples cover the most important Central Mediterranean type localities, from Plan d'Albard (Western Alps) to Sisco (Corsica), Montecatini Val di Cecina and Orciatico (Tuscany, Italy) and Torre Alfina (Northern Latium, Italy). The studied crystals show distinctive chemical and structural features; all of them belong to the phlogopite-annite join and crystallise in the 1M polytype, except for micas from Torre Alfina, where both 1M and 2M1 polytypes were found. Studied micas have variable but generally high F and Ti contents, with Mg/(Mg+Fe) ranging from ~0.5 to ~0.9; 2M1 crystals from Torre Alfina radically differ in chemical composition, showing high contents of Ti and Fe as well as of Al in both tetrahedra and octahedra, leading to distinctive structural distortions, especially in tetrahedral sites.SIMS data indicate that studied micas are generally dehydrogenated with OH contents ranging from ~0.2 apfu (atoms per formula unit) for Orciatico and Torre Alfina to ~1.4 for Plan d'Albard crystals; this feature is also testified by the length of the c parameter, which decreases with the loss of hydrogen and/or the increase of the F→OH substitution. Chemical and structural data suggest that the entry of high charge octahedral cations is mainly balanced by an oxy mechanism and, to a lesser extent, by a M3+,4+-Tschermak substitution. Our data confirm that Ti preferentially partitions into the M2 site and that different Ti and F contents, as well as different K/Al values, are both dependant upon fH2O and the composition of magma rather than controlled by P and T crystallisation conditions.The obtained data help to discriminate among lamproite-like rocks formed within a complex geodynamic framework but still related to a destructive tectonic margin and evidence different trends for micas from the youngest Torre Alfina (Northern Latium) lamproites, referred to the Apennine orogeny and those of the older lamproites from Orciatico, Montecatini Val di Cecina (Tuscany), Western Alps, and Corsica, the latter referred to the Alpine orogeny. Phlogopite crystals from the older lamproites fall within the compositional and structural field of worldwide phlogopites from both within-plate and subduction-related settings. Phlogopite from the Plio-Pleistocene lamproite-like occurrence in Tuscany and Northern Latium, despite crystals with low Mg# of the Torre Alfina rock plot well within the general field of the other crystals in less evolved samples, follows a different evolution trend similar to that of shoshonites from Tuscany and Northern Latium. On this basis, we argue that the observed differences are inherited by slight differences in the magma compositions that are related to different genetic and evolution pathways.

Reply to the comments on “Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust”

Stepanov et al. (Contrib Mineral Petrol, 2017) question our conclusion that the UPVs in southern Tibet were derived by partial melting of an old, metasomatized subcontinental lithospheric mantle (SCLM) of the subducted Indian plate. Instead, they propose that these ultrapotassic volcanic rocks (UPVs) are shoshonitic and were generated in two steps: direct melting of crustal rocks first, and then the melts interacted with mantle peridotite. However, the trace element, isotopic, thermal, structural, and seismic evidence is consistent with the xenolith evidence (Wang et al in Contrib Mineral Petrol 172:62, 2016) for hybridisation of ascending Indian subcontinental lithospheric mantle-derived UPV magmas with the deep, isotopically unevolved, Tibetan crust. This necessitates a model whereby partial melting of subducting Indian SCLM generates the UPV suite of southern Tibet.