Plate Basalts Research Articles

Petrology and phase equilibria of HP/LT eclogite at Gaoqiao, western Dabie and implications for lawsonite development in continental subduction zones

Lawsonite is of great significance for understanding fluid activity, element migration and crust–mantle interactions in subduction zones. Though studies have predicted lawsonite to be present under theP–Tregime during continental subduction, no lawsonite has been documented from natural (U) HP rocks in continental orogenic belt. In this study, we work on HP–LT eclogite (GQ–1 and GQ–2) at Gaoqiao, western Dabie to explore lawsonite formation and preservation during continental subduction and exhumation. Both samples have ubiquitous polymineralic aggregates of epidote/clinozoisite + paragonite/albite ± other minerals showing distinct rectangular or rhombic shapes developed as inclusions in garnet or in the matrix. Combined with recalculated bulk compositions similar to that of ideal lawsonite, we interpret these polymineralic aggregates to be pseudomorphs after lawsonite. Phase equilibrium modelling combined with compositional isopleth thermobarometry have constrained a segment of the prograde to peak stages to evolve from 19.0 to 19.5 kbar, ∼470°C to ∼20.0 kbar, 500C–505°C, then to ∼25 kbar, 530C–555°C in lawsonite stability fields. The progradeP–Tpath shows a two-stageP–Tevolution, with the first stage following a geothermal gradient of ∼7°C/km and the second stage decreasing to ∼6°C/km. Initial exhumation was inferred to follow an isothermal decompression process leading to lawsonite breakdown to form epidote/clinozosite ± paragoniteviathe reaction lawsonite + omphacite→ epidote + glaucophane ± paragonite + H2O at ∼19 kbar, 550°C. ModeledP/T–Xpseudosections calculated atT= 550°C andp= 25 kbar show that, when H2O content in bulk composition is more than 1.1 wt%, a certain amount of lawsonite (>13 mode%) should be present in eclogite. On the other hand, in the compositional range of natural intracontinental plate basalts, variations on O (Fe3+),XMgO[MgO/(MgO+FeO)],XCaO[CaO/(CaO+MgO+FeO+MnO+Na2O)],XNa2O[(Na2O/(CaO+Na2O)] andXAl2O3[Al2O3/(Al2O3+CaO+Na2O)] in bulk compositions have little influence on lawsonite development. In combination with previous studies, we conclude that during continent subduction along low geothermal gradient (<8°C/km), lawsonite could be formed under H2O present conditions. The absence of lawsonite in natural eclogite might be ascribed to retrograde overprint during exhumation.

Petrology and amphibole-plagioclase chemistry of amphibolites in the Ilesa area, Southwestern Nigeria

Amphibolites of Itagunmodi and Igun, within the Ilesa Area were studied in order to determine their petrochemical affinities, tectonic setting and conditions of formation; ultimately for an improved understanding of the evolution of the Ilesa Schist Belt. The amphibolites are massive in texture and occur as small to large lensoid boulders associated with quartz-biotite schist, quartzite and talc-chlorite schist. They comprise magnesio-hornblende, actinolite, labrodorite, albite and accessory titanite. Average elemental composition and ratios in the amphibolites were: Ba (872 ppm), Sr (609 ppm), Ni (89 ppm), Rb (72 ppm), Y (27 ppm), Zr (168 ppm), Nb (83 ppm), Cr (200 ppm), La (34 ppm), Cr/Ni (2.30), Y/Nb (0.63), Rb/Sr (0.10), Nb/La (2.55), and Zr/Nb (4.62). The major and trace element characteristics revealed that they are ortho-amphibolites with tholeiitic and calc-alkaline affinities, plotting in Within Plate Basalt field of the Zr vs Zr/Y diagram. The amphibolites exhibit enhanced mid-ocean ridge basalt (E-MORB) elemental characteristics with LREE > HREE enrichment and slight negative Eu anomaly. The calc-alkalic affinity of some of the amphibolites is a result of crustal contamination, and not an arc-related signature as earlier proposed. Elemental abundances and ratios revealed that the amphibolites were formed in an extensional tectonic setting where mantle plume activity and lithospheric contamination occurred. The mineralogical assemblage reflects amphibolite facies metamorphism, at temperature of formation of 450–687 °C and pressure of 0.5–4.3 kbar constrained by plagioclase-hornblende thermobarometry.

Evidence for a Single Large Igneous Province at 2.11 Ga across Supercraton Superia

Abstract The Superia supercraton palaeogeographic reconstruction (c. 2.65–2.00 Ga) is predominantly based on the shared large igneous province (LIP) record of the Superior, Hearne, Wyoming, and Karelia-Kola Archaean cratons. Palaeogeographic reconstruction relies on U–Pb geochronology (i.e. magmatic barcodes), palaeolatitudes from palaeomagnetic studies, and geometry of mafic dyke swarms as part of LIPs, as well as similarities in cover stratigraphy and/or basement geology. If contemporaneous mafic units from these cratons are indeed fragments of the same LIP, then integration of their chemistry can provide insight into the overall LIP plumbing system. A geochemical evaluation of whole-rock major and trace elements, and Nd isotopes, is presented for c. 2.11 Ga mafic units from the Marathon dykes (Superior), Griffin gabbro sills and Kazan dykes (Hearne), Bear Mountain dykes (Wyoming), and Tohmajärvi-Pirtguba dykes, Misi gabbro sills, and Oravaara and Hirsimaa volcanic rocks (Karelia-Kola). These units include the Nieminen dyke, Western Karelia, for which we present a new U–Pb CA-ID-TIMS date of 2114.4 ± 3.0 Ma. Four geochemical groups are identified: Groups 1 (mid-ocean ridge basalt-like; in Karelia) and 2 (within plate basalt; in Hearne, Wyoming, and Karelia) are distinct end-members of upper mantle and plume melts, respectively; Group 3 (in Superior, Wyoming, and Karelia) is a hybrid of Groups 1 and 2; and Group 4 (in all cratons) formed by assimilation and fractional crystallisation of Groups 1, 2, and 3. We present a model of the LIP plumbing system and define three magmatic stages by combining our geochemical interpretations with existing geochronology. The early stage (2135–2130 Ma) is limited to Group 1 dykes. The middle stage (2130–2113 Ma) includes Groups 1, 3, and 4. Onset of the late stage (2113–2101 Ma) is marked by the introduction of Group 2 but Groups 1 and 4 are also abundant. Finally, with our integrated tectonic and geochemical model, we discuss the mineralisation potential and prospectivity of the entire c. 2.11 Ga magmatic system.

Magmatism associated with lithospheric thinning, mantle upwelling, and extensional tectonics: Evidence from Carboniferous-Permian dyke swarms and granitoids from Inner Mongolia, Central Asian Orogenic Belt

The east-Ujimqin complex located north of the Erenhot-Hegenshan ophiolite belt in North China is composed of extensive Carboniferous and Permian magmatic rock suites, which preserve the records of an extensional tectonic regime following collision. These rocks provide important clues to the closure of the Hegenshan ocean and intraplate processes. Here we investigate the diorite dyke swarms in association with granitic rocks from the Xing'an-Mongolian Orogenic Belt (XMOB) through field investigations, petrology, geochemistry, zircon U-Pb and Lu-Hf studies, to gain insights into the timing, magma source and tectonic setting during Late Carboniferous and Early Permian. The diorite rocks display sub-alkaline, calc-alkaline, and high-K calc-alkaline features. Their extensional setting is suggested by plots in the fields of within plate basalt and active continental margin + alkaline oceanic arc. Zircon U-Pb data on magmatic zircons indicate that the granitoids crystallized at ca. 301 Ma, whereas the diorite dykes were emplaced at 286 Ma. Xenocryst zircon grains with 320 to 300 Ma ages in the dyke samples suggest incorporation from the surrounding granites. Zircon Lu-Hf isotopic data show consistently positive εHf(t) values (5.3 to 10.8) and plot between CHUR (Chondritic Uniform Reservoir) and depleted mantle lines suggesting that depleted mantle sources contributed significantly to the magma source. The computed model ages suggest that the magma source involved Neoproterozoic juvenile components.We propose a model where the magmatism occurred within an extensional tectonic setting caused by asthenosphere upwelling and lithospheric thinning. Our study also constrains the timing of Early Permian extension which suggests that the Hegenshan Ocean, as part of the Central Asian Orogenic Belt closed prior to 286 Ma.

Petrochemistry and Sr, Nd, Pb Isotopic Characteristics of Basic Dykes of Mikir Hills, Assam

The basic dykes in Mikir Hills have gabbroic composition. The prominent trends of these dykes are E-W in Southern part of Mikir Hills and NE-SW trend in the Northern Part of Mikir Hills. The geochemical characterization of dyke rock shows that they are subalkaline to alkaline basalt in nature, medium to high potassic, high titanium, Fe enriched tholeiites. The evolved nature of magma is represented by low Mg# 39.34 to 49.76 and are characterized by relatively low abundances of MgO, Cr, Ni, Zr, Y and high Rb, Ba, Th, Nb, indicating the within plate basalt (WPB) and continental rift basalt (CRB) chemical affinity. This dyke activity may be related to the extensional tectonics related to Proterozoic rifting hence, showing characteristics of continental rift basalt (CRB) type. The nine samples of gabbro define a five-point Rb-Sr isochron age of 1200±67 Ma with 87Sr/86Sr ratio of 0.70477±0.00042 with MSWD of 0.85. The Sm-Nd model ages (TDM) varies from 1600 to 2018Ma (eNd +0.11 to −2.04) indicates that the protolith from which these gabbroid rocks are derived is mostly Palaeo- to Meso-Proterozoic in age. The isotopic evolution of initial Sr, Nd and Pb reveals crustal contamination.

Petrography, Geochemistry and Petrogensis of Pleistocene Basaltic Flow from Northwest Atarous Area, Central Jordan

Fifteen basaltic rock samples were collected from central Jordan at the Atarous volcanism basaltic flow area. The samples cover about 8 km2 from the Atarous Basalt flow (AB). The AB flow was introduced in the Miocene to Pleistocene periods. The samples analyze major and trace elements by using XRF. Petrography, Geochemistry and Petrogensis have investigation to carried out for the AB. The petrography analyses of the AB rocks show they are composed of plagioclase (labradorite and bytownite), pyroxene (augite), and olivine (forsterite); accessory minerals include apatite and secondary minerals magnetite, ilmenite, spinel and iddingsite. The AB is classified within alkaline to sub-alkaline and tholeiitic to Calc-Alkaline basalt. The Mg# range between 0.39 and 0.49 of basalt samples exhibits different degrees of fractionation with a low degree of melting < 15% as indicated from the varying concentration of incompatible trace elements Ba, Rb, Sr. Trace elements of primary magna show low variable abundances of compatible and incompatible elements, which reflecs a homogenous source. Geochemical parameters such as Mg# and high Ti contents indicate that the corresponding magmas are of primary origin. The tectonic setting of AB is explained by using discrimination diagrams, Ti-Zr-Sr and Nb-Zr-Y and Ti-Zr-Y, the AB plotted within the plate basalt, alkali basalt and Calk alkaline basalt field, respectively. The spider diagram shows the samples AB enrichment of the Ba, K, Nb and Ce, depletion of Nb and Y. The AB exhibited positive Nb, Ce and Ti anomalies, and negative anomalies of Ba, Sr, and P. It is attributed to the fractionation of feldspar for Ba and Sr and apatite for P depletion. The spider diagram showed a positive Nb peak, which conforms to the tertiary and to recent continental alkali basalt provinces and indicates that the AB is the product of lithosphere from upwelling asthenosphere mantle.

Geochemistry and Tectonic Setting of Kohe Siahvolcanoes, North Qorveh, Sanandaj, Iran

The KoheSiah Volcanoes are located in the North of Qorveh city in the west of Iran. The KoheSiah volcanoes include several craters bearing domes located in the Kordestan Province, between latitudes 35°23'13.54” to 35°21'26.63”N and longitudes 47°54'38.26”E to 47°56'01.42”E. Based on the petrographic and geochemical analysis, the volcanic rocks in the KoheSiah area are classified as basalt, Alkali basalt, trachy-andesites and mugearite. The studied samples are alkaline with sodic to potasic feature based on Na2O/K2O ratios and belonging to Shoshonite series. Most of the alkaline volcanic rocks in the study area fall in the field of within in plate basalts (WPB) and represent of a mantle metasomatism trend associated with variable degrees in the partial melting of an enriched mantle source.

Geochemistry of eclogites of the Tso Morari complex, Ladakh, NW Himalayas: Insights into trace element behavior during subduction and exhumation

Whole rock major and trace element compositions of seven eclogites from the Tso Morari ultra-high pressure (UHP) complex, Ladakh were determined with the aim of constraining the protolith origins of the subducted crust. The eclogites have major element compositions corresponding to sub-alkaline basalts. Trace element characteristics of the samples show enrichment in LILE's over HFSEs (Rb, Th, K except Ba) with LREE enrichments ((La/Lu)n = 1.28–5.96). Absence of Eu anomaly on the Primitive Mantle normalized diagram suggests the absence of plagioclase fractionation. Positive correlation between Mg# with Ni and Cr suggests olivine fractionation of mantle melts. Narrow range of (La/Yb)n (2.1–9.4) and Ce/Yb (6.2–16.2) along with Ti/Y (435–735) ratios calculated for the Tso Morari samples is consistent with generation of melts by partial melting of a garnet free mantle source within the spinel peridotite field. Ternary diagrams (viz. Ti–Zr–Y and Nb–Zr–Y) using immobile and incompatible elements show that the samples range from depleted to enriched and span from within plate basalts (WPB) to enriched MORB (E-MORB) indicating that the eclogite protoliths originated from basaltic magmas. Primitive Mantle normalized multi element plots showing significant Th and LREE enrichment marked by negative Nb anomalies are characteristic of continental flood basalts. Positive Pb, negative Nb, high Th/Ta, a narrow range of Nb/La and the observed wide variation for Ti/Y indicate that the Tso Morari samples have undergone some level of crustal contamination. Observed geochemical characteristics of the Tso Morari samples indicate tholeiitic compositions originated from enriched MORB (E-MORB) type magmas which underwent a limited magmatic evolution through the process of fractional crystallization and probably more by crustal contamination. Observed geochemical similarities (viz. Zr, Nb, La/Yb, La/Gd, La/Nb, Th/Ta ratios and REE) between Tso Morari eclogites and the Group I Panjal Traps make the trap basalt the most likely protoliths for the Tso Morari eclogites.

Geochemical Characteristics and Tectonic Setting of Amphibolites in Ifewara Area, Ife-Ilesha Schist Belt, Southwestern Nigeria

Trace and Rare Earth Elements (REEs) data are used to constrain the geochemical evolution of the amphibolites from Ifewara in the Ife-Ilesha schist belt of southwestern Nigeria. The amphibolites can be grouped into banded and sheared amphibolites. Major element data show SiO2 (48.34%), Fe2O3 (11.03-17.88%), MgO (5.76-9.90%), CaO (7.76-18.6%) and TiO2 (0.44-1.77%) contents which are similar to amphibolites in other schist belts in Nigeria. The Al2O3 (2.85-15.55%) content is varied, with the higher values suggesting alkali basalt protolith. Trace and rare earth elements composition reveal Sr (160-1077ppm), Rb (0.5-22.9ppm), Ni (4.7-10.2ppm), Co (12.2-50.9 ppm) and Cr (2-7ppm). Chondrite-normalized REE patterns show that the banded amphibolites have HREE depletion and both negative and positive Eu anomalies while the sheared variety showed slight LREE enrichment with no apparent Eu anomaly. The study amphibolites plot in the Mid Oceanic Ridge Basalts (MORB) and within plate basalt fields on the Zr/Y vs Zr discriminatory diagrams. They are further classified as volcanic arc basalt and E-type MORB on the Th- Hf/3- Ta and the Zr-Nb-Y diagrams. The amphibolites precursor is considered a tholeiitic suite that suffered crustal contamination, during emplacement in a rifted crust.

Early Neoproterozoic emplacement of the diabase sill swarms in the Liaodong Peninsula and pre-magmatic uplift of the southeastern North China Craton

Diabase sill swarms are widespread within the Neoproterozoic sedimentary rocks in the Liaodong Peninsula (named as the Dalian mafic sill swarms), eastern North China Craton (NCC). Our new zircon LA-ICP-MS U–Pb and baddeleyite SIMS Pb–Pb dating results on five diabase sill samples emplaced into the Qiaotou, Cuijiatun and Xingmincun Formations in the Liaodong Peninsula indicate their emplacement during the early Neoproterozoic (Tonian) period at 0.92–0.89Ga and pre-magmatic regional uplift prior to ca. 0.92–0.89Ga. The above results provide important constraints on the upper boundary of the Neoproterozoic strata and their macroscopic carbonaceous fossils and organic-walled microfossils in the eastern and southeastern NCC and indicate they are Tonian in age. The early Neoproterozoic Dalian diabase sills belong to the tholeiitic series and are characterized by low contents of SiO2 and K2O, high contents of TiO2, Fe2O3T and MgO, slight light REE (LREE)-enrichment and no Eu anomalies on chondrite-normalized REE patterns, enrichment of high field strength element (HFSE) and lack of negative Nb, Ta, Zr, Hf, P and Ti anomalies on primitive mantle-normalized spidergrams; and exhibit geochemical characteristics of within plate basalt on discrimination diagrams. The newly identified Dalian diabase sills, together with the previously reported Xu-Huai and Sariwon mafic sills and the Dashigou mafic dykes, constitute an early Neoproterozoic (0.92–0.89Ga) large igneous province in the NCC (Sino-Korean Craton). Formation of the early Neoproterozoic diabase sill (dyke) swarms in the NCC is probably related to a continental rifting event that have led to breakup of southeastern NCC from some other continents in the Rodinia supercontinent. Breakup of the NCC from the Rodinia supercontinent at around 0.92–0.89Ga is also supported by pre-magmatic regional uplift of the southeastern NCC prior to ca. 0.92–0.89Ga and evolution trend of the Dalian diabase sills from within plate basalt to mid-ocean ridge basalt on the Zr/Y vs. Zr discrimination diagram.

Geochemistry and Petrology of Basic Volcanic Rocks of Jabal Al Haruj Al-Aswad, Libya

The Al Haruj Intra-continental Volcanic Province is the largest part of the extensive volcanic activity in Libya which is considered to be a typical within plate basalts. The volcano-tectonics evolution of this province, as well as its origin, are still widely disputed. According to K-Ar dating previously studied, the volcanic activity started in the Late Miocene and lasted until at least the Late Pleistocene. The field may still be volcanically active. The mafic rocks of Jabal Al Haruj have been classified into six major phases or groups. These phases have been differentiated using Landsat images together with aerial photographs of different scales as well as field observations. The topographic forms of the earliest phase are highly eroded while the forms of the latest phase are usually fresh and very well preserved as regards primary features. Mafic lavas of this field consist of alkali basalts to olivine tholeiites (transitional basalt) which contain olivine as essential constituent together with clinopyroxene, plagioclase and glass. The basalt exhibits intergranular, intersertal, ophitic and subophitic relations. Amygdaloidal and glomerporphyritic textures are also observed. The basaltic rocks of different ages and from different localities are petrographically rather similar. Phenocrysts of olivine probably the result from slow cooling in crustal magma chambers prior to eruptions, suggesting that magmas ascended slowly through the crust. 109 samples have been carefully collected from various phases, some of these samples have been chosen for major and trace elements analyses, using XRF in order to determine the characteristics of the mantle source and investigate crustal interaction. The major and trace elements revealed a slightly significant chemical diversity among the phases and within each phase. The normative classification of most of these rocks shows close agreement with their modal classification. A vague correlation between MgO and most major oxides in the studied samples suggests different degrees of partial melting rather than fractional crystallization. A characteristic feature of the studied volcanic rocks is the relatively constant ratios of certain incompatible trace elements (Nb/Zr, Rb/Zr), which provides strong evidence of a common source. In addition, the rocks display similar patterns of the peaks and troughs; this strongly suggests that they have a common parent and common subsequent processes. The compatible transitional metals Ni (81 - 193 ppm) and Cr contents (238 - 361 ppm) and relatively low Mg# (Mg/(Mg + Fet)) (52 - 62) give an indication that the studied basaltic rocks have slightly to moderately fractionated olivine and/or spinel. The magmatism of this volcanic field seems to be related to reactivation of pre-existing structures during the passive rifting of the Sirt Basin that most likely produced in response to convergence between European and African plates since Jurassic until Holocene times.

An investigation on the Eocene Pushtasar basaltic lava in relation to Moghan Aulacogene

The present paper represents a petrological and geochemistrical study of Pushtasar basaltic lava in the Moghan region. This basaltic unit with interbeds of Tuff and Breccia tuff divides Upper Eocene sediments into two parts as an east-west band, the thickness of which thins from 1,300 m in the east to a few meters in the west. Therefore, petrography studies and geochemical studies were conducted on the gained samples along different parts of the rim. According to this study, the basic unit which distributes the Upper Eocene sediments into two parts is a transitional magma series with internal plate basalt geodynamic origin. Based on the previous studies, which are mainly focused on providing geological maps and oil prospecting, it seems that there was a descent in tension condition at the end of Eocene and first part of Oligocene after the compression phase in the Upper Cretacea. Probably, this tension condition made a suitable environment for the subsidence of sediments, making hydrocarbon reserves, as the processes have been done in the continental rift or Aulacogene.

Structural and petrological evidence for the continuation of the Isfahan fault system across the Urumieh-Dokhtar zone of central Iran

The Isfahan fault system, a north-trending, dextral strike-slip fault across the Sanandaj-Sirjan zone, represents the boundary between the northwestern and the southeastern parts of the Sanandaj-Sirjan zone and it terminates in the north at the southern boundary of the Urumieh-Dokhtar zone. This paper focuses on the continuation of the Isfahan fault system across the Urumieh-Dokhtar zone north of Isfahan city. The Urumieh-Dokhtar magmatic assemblage belt is located along the active margin of the Iranian plate and the Arabian plate. The Karkas fault strikes nearly north-south, has a length of about 40 km, a normal component of movement, and it truncates the Urumieh-Dokhtar zone. Due to the location of this fault and the mechanism similar to the Isfahan fault system, the Karkas fault can be considered a continuation of the Isfahan fault system that has been displaced dextrally by the southwestern bordering faults of the Urumieh-Dokhtar zone. The unique juxtaposition association of the Silurian volcanic rocks in the Urumieh-Dokhtar zone, near the Karkas fault, provides an important evidence regarding the major role of this fault in the geological evolution of the region. The Silurian volcanic rocks outcrop in two districts of the study area and generally are composed of basalts. The alkaline basalt composition is determined from mineralogy and immobile elements geochemistry. The geotectonic setting diagrams classified the Silurian volcanic rocks as the within plate basalts. Thus, an intracontinental rifting under extensional tectonic regime can be inferred as the setting that controlled formation of these volcanic rocks. They were created by an alkaline to transitional magmas generated due to low partial melting at depth. The alkaline basalts were most likely derived from an asthenosphere-dominated mantle source due to extension and partial melting. The north trending extensional faults affected thinned overlying continental lithosphere in the Paleozoic era, facilitating magma penetration and eruption.

Magma Genesis in Kabanjahe Region Continental Margin Arc of Sumatra

DOI: 10.17014/ijog.v6i2.120 Volcanic rocks in Kabanjahe region, Karo Regency, North Sumatra Province, are products of old Toba Caldera, Sibayak Volcano, and Sipiso-piso Volcano. Rhyolitic tuff is the main lithology distributed over a large area in this region. Others are basaltic, basaltic andesitic, andesitic, dacitic, and rhyolitic lavas. Data show that the rock was originated from magma of a continental origin formed at a subduction zone environment. Petrogenetic modelling suggests that the range in composition was mainly controlled by a fractional crystallization of plagioclase, clinopyroxene, hornblende, and biotite. Harker’s variation diagram of major and trace elements show a continuous range that indicates they are cognate. The lava in this area belongs to a high-K, calc-alkaline series, with particular high Nb concentrations. The composition of these high-Nb lavas is more similar to those of intra plate basalts rather than those of calc-alkaline or arc-tholeiitic basalt. The high anomaly of Nb which is accompanied by high Th, Rb, and normative corundum suggests that the source may also be enriched in incompatible elements, a characteristic feature of alkali magmatism. The similarity of the trace element of volcanic rocks to the within-plate basalts indicates that the convecting mantle wedge above subducted slabs contains variable proportions of MORB-source and OIB-source components; fluids added were derived from the subducted slab. Hence, it is interpreted that the high Nb concentration of volcanic rocks from Kabanjahe region were generated from subduction modified OIB source components. Alternatively, a deep seated faulting conduit magma from the lower mantle resulted in the alkaline enrichment of the volcanics. This article performs a petrological aspect, especially based on geochemical analysis including major elements, trace elements, and rare earth elements. The results are plotted into a general and specific classification used in petrology.

Interpretacion geodinámica de los complejos ofiolíticos de Costa Rica

The ophiolitic complexes along the Pacific coast of northwestern Costa Rica can be divided inyo three units, the Santa Elena, Matapalo, and Esperanza units. The ultramafic rocks of the Santa Elena peninsula are overthrusted as a nappe onto the Matapalo unit. The Matapalo unit contains mid-ocean ridge basalts and radiolarites. The Esperanza unit is mainly made up of volcanic arc basalts and radiolarites, but also contains mid-ocean ridge basalts. The ophiolite complexes along the Pacific coast in southern Costa Rica and Panama correspond in their lithology with the Esperanza unit. The Matapalo unit contains alkalic with in plate basalts . These are considered to be related to the "sill event" in the Caribbean in the Santonian to Campanian. The thickening and stiffening of the Caribbean crust during this event may be responsible for the formation of a subduction zone along the present Middle American landbridge.

1420 Ma diabasic intrusives from the Mesoproterozoic Singhora Group, Chhattisgarh Supergroup, India: Implications towards non-plume intrusive activity

Besides offering significant clues towards tracking the geochemical evolution of the mantle and architectural reconstruction of different ‘supercontinent’, geochronological and geochemical appraisal of igneous inputs are also important to bracket the depositional time frame of any lithopackage, particularly, the unfossiliferous sedimentary successions. The present study deals with diabasic intrusive within Mesoproterozoic Saraipalli Formation, which is an argillaceous constituent present at the basal part of nearly 400 m thick four-tiered unmetamorphosed but deformed sedimentary succession of Singhora Group, Chhattisgarh Supergroup, central India. The SE–NW trending intrusive comprises mainly of plagioclase and augite together with minor orthopyroxene, biotite and opaque minerals. Though some plagioclase laths are partially sericitized, the ophitic-to-subophitic texture of the rock is well preserved. Major and trace element geochemical data indicate that this intrusive is basalt-to-basaltic andesite in character and of subalkaline basalt affinity. Multi-element plot shows overall LILE-enrichment and enrichment of Pb and slight depletion of Nb and P, coupled with moderate La/Nb and Th/Nb ratios. Zr, Y and Nb ternary diagrams plot in the fields of within plate basalt. Selected HFSE ratios indicate a non-plume source with crustal assimilation/sediment mixing. Sm–Nd and Rb–Sr isotope data show that the intrusive has Srinitial and Ndinitial of 0.709377–0.706672 and 0.510919–0.510815, respectively. Positive ε t Nd [t = 1420 Ma] values (+0.3 to + 2.3) indicate depleted isotopic nature of their protolith. The calculated T DM age is 1.7–1.9 Ga. The mineral-whole rock isochron data (Sm–Nd systematics) of the intrusive implies an emplacement age of ca. 1420 Ma. Considering synchronous terrain boundary shear zone development in Bastar craton on the southeastern part of the Singhora basin, mafic magmatism in Eastern Ghats and large-scale basic intrusion in Sausar mobile belt, a major tectono-thermal event around 1400 Ma is surmised that affected eastern Indian craton. Moreover, geochronology of a bedded porcellanite unit (ca. 1500 Ma) at the base and a discordant basic intrusive (ca. 1420 Ma) allowed a unique opportunity to qualitatively offer an upper bound of time bracket for the deposition of Saraipalli Formation, i.e., ∼80 Ma.

High pressure, near‐liquidus phase equilibria of the Home Plate basalt Fastball and melting in the Martian mantle

Near‐liquidus phase equilibria experiments have been conducted on a synthetic Fastball basalt composition, as analyzed at Home Plate plateau of Mars (Gusev Crater), to test if it represents a primitive mantle derived melt and place constraints on the temperature of the ancient mantle and on the lithosphere‐asthenosphere boundary of Mars. The Fastball basalt is multiply saturated with olivine and orthopyroxene at ∼1.2 GPa and 1430°C. Based on melting models, we predict that the Fastball composition could be produced by 13–23% equilibrium melting of the Martian mantle, with extraction of melt from the base of ∼105 km thick lithosphere. The multiple saturation for Fastball also constrains the potential temperature of the Martian mantle to be approximately 1480–1530°C with an initial melting pressure of 4.0–4.7 GPa. This potential temperature is much lower than that of the terrestrial mantle derived from similarly ancient magmas, i.e., komatiites.

Structure and tectonics of subophiolitic mélanges in the western Hellenides (Greece): implications for ophiolite emplacement tectonics

The Jurassic Vourinos ophiolite is part of the Western Hellenide ophiolite belt in Greece and rests tectonically on the Pelagonian microcontinent. The Vourinos and coeval Pindos ophiolite to the west display suprasubduction-zone geochemical affinities, and represent remnants of oceanic lithosphere formed in a rifted incipient arc–forearc setting within the Pindos Basin. In structurally descending order, and from west to east, the subophiolitic mélange beneath the Vourinos ophiolite contains the Agios Nikolaos Formation (ANF) and a rift assemblage, both of which display ENE-vergent thrust faults, shear zones, and folds. The ANF comprises schistose mudstone with pebbles, cobbles, and boulders of arenite and wacke derived from the crystalline basement of Pelagonia. Imbricated along ENE-directed thrust faults and metamorphosed up to lower amphibolite facies, the ANF represents continental rise deposits of the rifted Pelagonian margin. The rift assemblage includes blocks of basaltic lavas, ribbon chert, micritic cherty limestone, metagabbro, dolerite dikes, and serpentinite breccia that are commonly in thrust contact with each other and are tectonically imbricated with the Pelagonian carbonates; however, primary intrusive and depositional contacts are locally well preserved. Gabbro and dolerite dikes are locally intrusive into the recrystallized carbonates and metapelitic rocks of the Pelagonian microcontinent. Lavas display mid-ocean ridge basalt within plate basalt affinities and represent Upper Triassic rift units that erupted during the separation of Pelagonia from Apulia. Gabbro, dolerite, and serpentinite breccia are the products of a magmatic rifting episode prior to the onset of seafloor spreading in the Pindos Basin. The Vourinos subophiolitic mélange thus consists of passive margin and rift assemblages that were tectonically overridden by the Vourinos ophiolite in the Middle Jurassic. The Avdella mélange beneath the Pindos ophiolite to the west includes a chaotic matrix including ophiolitic clasts and blocks, reefal to pelagic limestones, and olistostromal turbidites that range in age from Triassic to Jurassic–Cretaceous. The Avdella mélange rests tectonically on a Cretaceous–Eocene flysch unit and the Cretaceous–Eocene shelf and slope deposits of the Apulian margin along west-directed thrust faults. The evolutionary history of the Vourinos and Avdella subophiolitic mélanges indicates diachronous tectonic emplacement of the Mesohellenic oceanic slab, first in the east as a result of a trench-continent (Pelagonia) collision in the Middle Jurassic, then in the west due to the Eocene collision of Apulia with Eurasia.

Geochemistry and zircon U–Pb dating and its implications of the Yukahe HP/UHP terrane, the North Qaidam, NW China

The Yukahe eclogite-gneiss section in the western segment of the North Qaidam HP/UHP belt, NW China occurs two types of eclogites as lenses or layers in both granitic and pelitic gneisses. Zircon in situ LA-ICP-MS U–Th–Pb and trace element analyzes, combined with cathodoluminescence images yield metamorphic ages of 431 ± 4 to 436 ± 3 and 431 ± 3 to 432 ± 19 Ma for eclogites and gneisses, respectively. These nearly identical metamorphic ages suggest that the eclogites and host gneisses underwent coeval early Paleozoic HP/UHP metamorphism. A few inherited zircon cores of eclogites have magmatic characters of high REE and HREE abundance, HREE-enriched REE pattern and Th/U > 0.4, and yield protolith ages of >750 Ma. Geochemical data show that the Yukahe eclogite has alkali basaltic protoliths with high TiO 2 and K 2O + Na 2O contents and Nb/Y ratios, REE patterns and trace element contents similar to those of within plate basalts (WPB). Nd isotope analyzes give epsilon Nd ranges from −5.34 to +4.47, suggesting an enriched mantle source. These characteristics together with the geochronological results conclude that the Yukahe eclogite protoliths likely formed in a Neoproterozoic continental rift setting, and have subjected to HP/UHP metamorphism during early Paleozoic continental subduction.

제주도 섭지코지 선돌 분석구의 화산작용과 현무암

이 연구에서는 제주도 동부지역 섭지코지 일원에 분포하는 선돌 분석구와 그 주변의 현무암질 화산암류를 주 대상으로, 암석기재, 주원소 및 미량원소의 특성 등을 분석하여 선돌 분석구의 화산과정과 현무암의 암석학적 특징을 검토하였다. 선돌 분석구의 하부는 스패터 위주로, 그 상부는 적갈색 화산탄을 포함하는 화성쇄설물질이 우세하며 최상부 표층부에는 흑색의 스코리아가 다량 분포한다. 이들 화산작용은 스트롬볼리 분출상에 의해 진행되었으며 마지막에는 하와이 용암 분출상으로 용암이 분화구를 충진한 후 동북쪽으로 흘러넘치면서 분석구를 파괴하고 북쪽으로 흘러 용암 델타를 형성하였다. 분석구 중심부의 암경은 현무암으로 채워져 있다. 이 지역의 화산활동 시기는 대략 <TEX>$95\;{\pm}\;3\;ka$</TEX>에 해당한다. 연구 지역의 현무암은 전이 현무암으로 분류될 수 있으며, 감람석과 단사휘석의 정출이 분화작용에 주된 영향을 미쳤음을 보여주며, 동일마그마에서 기원하였음을 보여주고 있다. 이들은 판내부 현무암에 해당한다. The purpose of this study is to determine the petrology of basalt and the volcanic process in the Seondol cinder cone, Seobjikoji area, eastern Jeju Island. The Seondol cinder cone is mainly composed of spatters in the lower part, pyroclastic deposits including reddish brown blocks, ashes with volcanic bombs in the middle part, and dark black scoria deposits in the upper part. The volcanic sequences suggest volcanic processes that progress through Strombolian eruption and end with Hawaiian lava effusion which breached the cinder cone northwestward and extended over northwestward as lava delta and basalt emplaced as a volcanic neck in the central part of crater in the cinder cone. The age of basalt lava flows is about <TEX>$95\;{\pm}\;3\;ka$</TEX>. The basalts belong to transitional basalt and show products of fractional crystallization of clinopyroxene and olivine from a parental basalt magma on the basis of variation diagram of major, trace and rare earth elements. Basalts in the region of this study are plotted at the region of within plate basalt.