Crustal Assimilation Processes Research Articles

Tracking crustal assimilation processes in kimberlites from the Alto Paranaíba Igneous Province, Brazil: Petrographic and geochemical controls and the role of perovskites

Anomalous kimberlite varieties from the Cretaceous Alto Paranaíba Igneous Province (APIP, Brazil) do not follow simple classification charts, showing abundant evidence of interaction with crustal xenoliths. In this paper, we track the potential effects of crustal assimilation processes on textural, mineralogical, geochemical and isotopic signatures. Formation of reaction diopside and poikilitic phlogopite, as well as Fe/Nb variations in perovskites, records progressive increases of not only alumina and K2O contents, but also of silica activity and fO2, compatible with the assimilation of and/or interaction with the Si-rich crust. High bulk rock “contamination indexes”, decreasing Ce/Pb and Gd/Lu and slightly increasing Rb/Sr ratios are observed in the APIP kimberlite varieties and/or facies showing progressively more remarkable petrographic evidence of crustal contamination. Significant variability of 87Sr/86Sri ratios detected in whole rock, bulk perovskite concentrates (TIMS, MC-ICP-MS) and in-situ perovskite (multi-grain approach, LA-MC-ICP-MS) of the Indaiá-I/Limeira-I intrusions (0.70507–0.70539) and their satellites Indaiá-II/Limeira-II/III (0.70508–0.70592), of the different Pântano facies (0.70513–0.70551), and of the Catalão 1b (0.70495–0.706546) indicates a tendency to more radiogenic Sr isotope compositions during kimberlite open-system crystallization. Geochemical modeling indicates the important crustal contribution to the genesis of these anomalous occurrences, with an extensive crust input for the Indaiá-II and Limeira-II/III occurrences, consistent with their petrographic evidence. The potassic to ultrapotassic geochemical affinity and the similarities between contaminated APIP kimberlite and bulk kamafugite compositions can be related to the incorporation/digestion of crustal components to the parental kimberlite magma.

Hafnium isotopic constraints on crustal assimilation in response to the tectono–magmatic evolution of the Okinawa Trough

Crustal assimilation and the input of subducted sediment in a subduction zone play vital roles in generating arc and back-arc lavas along a continental margin. However, distinguishing the impacts of these two processes on lava geochemistry is not straightforward or well understood. In this paper, we present new Hf–Nd–Sr–Pb isotopic data for 20 igneous rocks ranging in composition from basalt to rhyolite, which when combined with data from the literature allow us to fingerprint the contributions of crustal and slab-derived materials along the Okinawa Trough (OT). Consistent with previous results, the basaltic lavas were generated by melting of an Indian-type mantle source metasomatised by slab-derived fluids and subducted pelagic/detrital sediments. We confirm that Hf behaves as a non-conservative element during subduction, constrained by Hf-Nd isotopic systematics. Our new data suggest that crustal assimilation processes worked independently of the extent of magma differentiation but were determined by the stage of tectonic development in individual regions. In the southern OT (seafloor spreading stage), crustal assimilation is insignificant, and the addition of subducted sediment is 0.4%–1.5%. In the middle OT (crustal rifting stage), the basaltic and rhyolitic suites do not show distinctive Hf–Nd–Sr–Pb isotopic compositions, and both record moderate crustal assimilation (<10%) and < 0.6% addition of subducted sediment. In contrast, in the southwestern OT (crustal extension stage), the basaltic and dacitic lavas record the highest involvement of 10%–40% crustal material. We conclude that the various stages of back-arc development exerted first-order controls on the extent of crustal assimilation in the OT and had marked effects on the geochemical and isotopic compositions of OT lavas.

Fingerprints of the Kerguelen Mantle Plume in Southern Tibet: Evidence from Early Cretaceous Magmatism in the Tethyan Himalaya

Early Cretaceous magmatism suggested to be related with the Kerguelen mantle plume has been reported in both eastern and western Tethyan Himalaya (TH). Coeval magmatism has been recently discovered in the central TH. This magmatism is recorded by an intrusion next to the Xiongma township. This intrusion is composed of gabbro and dolerites. The Xiongma gabbros are characterized by significant enrichment of light rare earth elements and prominent Na-Ta depletion in the primitive mantle–normalized spider diagrams. The εNd(t) ratios of the gabbros are between −4.33 and −2.20, and initial 87Sr/86Sr ratios are 0.707807–0.708557. Lead isotopes 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb of the gabbros range from 39.042 to 39.630, 15.646 to 15.709, and 18.455 to 18.833, respectively. Zircons from the gabbros and dolerites yielded three U-Pb ages of 138.5±1.6, 133.6±2.5, and 136.4±2.2 Ma, respectively. The magmatic zircons are exclusively characterized by negative εHf(t) values. Through geochemical modeling, we concluded that the Xiongma magmatic rocks have experienced combined crustal assimilation (10%–15% for the gabbros and 28%–32% for the dolerites) and fractional crystallization processes. The gabbros are derived from an Sp+Grt facies (Grt>Sp) enriched mantle source. The Xiongma magmatism is geochemically and geochronologically comparable with those in the western and eastern TH. Through comparison study between oceanic island basalt–like rocks in both the Yarlung Zangbo Suture Zone and TH, we propose a possibility that these geochemically and geochronologically comparable magmatic rocks may be genetically related with the Kerguelen mantle plume, and thus represent the dispersed fingerprints of the Kerguelen plume preserved in southern Tibet, China.

Fingerprints of the Kerguelen Mantle Plume in Southern Tibet: Evidence from Early Cretaceous Magmatism in the Tethyan Himalaya

AbstractEarly Cretaceous magmatism suggested to be related with the Kerguelen mantle plume has been reported in both the eastern and western Tethyan Himalayan terrane. Coeval magmatism (133–138 Ma) recorded by hypabyssal intrusive rocks have been recently discovered in the central Tethyan Himalaya (TH). The hypabyssal intrusions are dominated by OIB‐like basaltic rocks intruded by later porphyritic/ophitic intermediate rocks and are characterized by strongly light rare earth element enrichment and prominent Na‐Ta depletion and Pb enrichment. The basaltic rocks have low 143Nd/144Nd ratios ranging from 0.512365 to 0.512476 but relatively high 87Sr/86Sr ratios ranging from 0.708185 to 0.708966. The ∊Nd(t) ratios of the basaltic rocks are between −4.33 and −2.20 and initial 87Sr/86Sr ratios are 0.707807 to 0.708557. Geochemical data demonstrate that these rocks have experienced combined crustal assimilation and fractional crystallization processes. Magmatic zircons from the hypabyssal rocks exclusively have negative ∊Hf (t) values ranging from −0.7 to −12.7, suggestive of assimilation of crustal material. Zircons from these hypabyssal rocks have U‐Pb ages ranging from 130 to 147 Ma. Inherited zircons have U‐Pb ages from 397 to 2495 Ma. All the zircons are characterized by negative ∊Hf(t) values. The Jiding ocean island basalt (OIB)‐like magmatism is geochemically and geochronologically comparable with that in the western and eastern Tethyan Himalaya, indicating widespread OIB‐like magmatism in the northern margin of Greater India during the Cretaceous. Collectively, these rocks can be correlated with other early Cretaceous magmatism in western Australia and northern Antarctica. Considering the similarities, we suggest that the Jiding hypabyssal rocks are also genetically related to Kerguelen plume. Within the Yarlung Zangbo Suture Zone (YZSZ), there are also numerous occurrences of OIB‐like rocks derived from mantle sources different from those of N‐MORB‐like magmas. The OIB‐like magmatism in the YZSZ is nearly coeval with that in the TH, and the two are geochemically similar. We suggest that the OIB‐like magmatism in the Neo‐Tethyan ocean and the northern margin of Greater India may represent the dispersed fingerprints of the Kerguelen plume preserved in southern Tibet, China.

Petrographic, geochemical, and isotopic evidence of crustal assimilation processes in the Indaiá-II kimberlite, Alto Paranaíba Province, southeast Brazil

ABSTRACTThe Indaiá-I and Indaiá-II intrusions are hypabyssal, small-sized ultrabasic bodies belonging to the Cretaceous magmatism of the Alto Paranaiba Alkaline Province (southeast-central western Brazil). While Indaiá-I is classified as an archetypal group-I kimberlite, Indaiá-II (its satellite intrusion) presents several petrographic and chemical distinctions: (1) an ultrapotassic composition (similar to kamafugites), (2) lower volumes of olivine macrocrysts, (3) diopside as the main matrix phase (in contrast with the presence of monticellite in Indaiá-I), (4) high amounts of phlogopite, and (5) abundant felsic boudinaged and stretched microenclaves and crustal xenoliths. Disequilibrium features, such as embayment and sieve textures in olivine and clinopyroxene grains, are indicative of open-system processes in Indaiá-II. Mineral reactions observed in Indaiá-II (e.g., diopside formed at the expense of monticellite and olivine; phlogopite nearby crustal enclaves and close to olivine macrocrysts) point to an increase in the silica activity of the kimberlite magma; otherwise partially melted crustal xenoliths present kalsilite, generated by desilification reactions. The high Contamination Index (2.12–2.25) and the large amounts of crustal xenoliths (most of them totally transformed or with evidence of partial melting) indicate a high degree of crustal assimilation in the Indaiá-II intrusion. Calculated melts (after removal of olivine xenocrysts) of Indaiá-II have higher amounts of SiO2, Al2O3, K2O, slightly higher Rb/Sr ratios, lower Ce/Pb and Gd/Lu ratios, higher 87Sr/86Sr, and lower 143Nd/144Nd than those calculated for Indaiá-I. Crustal contamination models were developed considering mixing between the calculated melts of Indaiá-I and partial melts modeled from the granitoid country rocks. Mixing-model curves using major and trace elements and isotopic compositions are consistent with crustal assimilation processes with amounts of crustal contribution of ca. 30%. We conclude that (1) Indaiá-II is representative of a highly contaminated kimberlitic intrusion, (2) this contamination occurred by the assimilation of anatectic melts from the main crustal country rocks of this area, and (3) Indaiá-I and Indaiá-II could have had the same parent melt, but with different degrees of crustal contamination. Our petrological model also indicates that Indaiá-II is a satellite blind pipe linked to the main occurrence of Indaiá-I.

Assessing Origins of End‐Triassic Tholeiites From Eastern North America Using Hafnium Isotopes

AbstractThe driving processes responsible for producing the Central Atlantic Magmatic Province, the Large Igneous Province associated with end‐Triassic rifting of Pangea, remain largely debated. Because their compositions encompass most of the Central Atlantic basalt spectrum, tholeiites from southern Eastern North America are considered pivotal for identifying magma origins. New 176Hf/177Hf measurements for 201 Ma Eastern North American tholeiites dominantly record a local petrogenetic history. Their εHf ratios, corrected to an emplacement age of 201 Ma (−7.85 to +5.86), form a positive but shallowly sloped array slightly deviating from the terrestrial array on a εHf versus εNd diagram. Comparison of 176Hf/177Hf to other isotope ratios and trace elements helps to rule out several petrogenetic scenarios, particularly mixing of melts from global depleted or enriched mantle components. In contrast, partial melting of subduction‐metasomatized mantle can explain the parental magma composition for southern Eastern North America. Such metasomatism likely occurred during Paleozoic subduction around Pangea and may have been dominated by sediment‐derived fluid reactions. The observed 176Hf/177Hf versus 143Nd/144Nd array may reflect subsequent assimilation of lower continental crust, perhaps together with limited direct melting of recycled continental crust in the asthenosphere. The proposed recycling scenario does not specifically support or preclude a mantle plume origin for the Central Atlantic Magmatic Province but instead points toward the presence of a distinct local mantle source and crustal assimilation processes during magma transport. Detailed understanding of these local effects is needed in order to more accurately understand the origins of Large Igneous Provinces.

Petrological evolution of Karlıova-Varto volcanism (Eastern Turkey): Magma genesis in a transtensional triple-junction tectonic setting

A diffuse and voluminous (>1400 km3) Miocene-Quaternary volcanic activity developed around the Karlıova Triple Junction in East Anatolia as a consequence of collisional tectonics among Anatolia, Arabia and Eurasia continental plates. The volcanic rocks of this region are grouped into three phases of activity: 1) Early Phase (Solhan volcanism; ~7.3–4.4 Ma), with emplacement of alkali basalt to trachyte lava flows and pyroclastic successions; 2) Middle Phase (Turnadağ and Varto volcanism; ~3.6–2.6 Ma), mostly with products with the same compositional range plus minor dacites and rhyolites, and 3) Late Phase (Özenç volcanism; ~2.6–0.5 Ma), with emplacement of alkali basaltic, hawaiitic and mugearitic lavas and dykes.Primitive Mantle-normalized patterns of the three rock groups share an enriched LILE and depleted HFSE contents, with overall positive spikes of Pb and mildly fractionated LREE/HREE trends showing more similar affinity to global subducting sediments rather than to magmas emplaced in mid-plate settings (i.e., OIB). Initial Sr isotopic ratios of the least evolved compositions range from values lower than BSE (87Sr/86Sri = 0.7041) to radiogenic compositions (87Sr/86Sri = 0.7050). They reflect either FC-like processes, with 87Sr/86Sri up to 0.7064, or closed system fractional crystallization, with 87Sr/86Sri = 0.7046–0.7049. Initial Nd are higher than ChUR estimate for the most and the least evolved compositions (143Nd/144Ndi = 0.51267–0.51280), indicating provenance from isotopically depleted sources. Lead isotopic ratios are characterized by a remarkable homogeneous 206Pb/204Pb (18.95–19.04), with 207Pb/204Pb (15.65–15.72) and 208Pb/204Pb (38.87–39.21) slightly above the Northern Hemisphere Reference Line, pointing towards the EMII end-member.Geochemical modelling for the least evolved volcanic units indicate the likely generation from an amphibole-bearing spinel-lherzolitic source. P-T calculations for partial melting calculations gave lithospheric pressures for initial magma generation (0.8–1.3 GPa). Possible cause of melting might be related to passive upwelling of asthenosphere as a response to the local extension linked to the development of North Anatolian and East Anatolian Fault Zones. Anyhow, volcanic units from the KTJ display only limited geochemical signatures of garnet-bearing sources, or any HIMU-OIB like characteristics, as instead observed in the other portions of the Eastern Anatolia.The long-lasting complex tectonic evolution of the Eastern Anatolia is responsible for the large geochemical variability of the magmatic products. However, the general characteristics of KTJ volcanic rocks are mainly dominated by subduction-related signatures, with most of the primary magma characteristics having been heavily masked by fractionation and crustal assimilation processes.

Generation of a potassic to ultrapotassic alkaline complex in a syn-collisional setting through flat subduction: Constraints on magma sources and processes (Otjimbingwe alkaline complex, Damara orogen, Namibia)

The ~545 Ma-old syn-collisional Otjimbingwe alkaline complex is composed of pyroxene-amphibole-biotite-bearing, mildly nepheline-normative to quartz-normative rocks ranging in composition from monzogabbro to monzonite, syenite and granite. The alkaline rocks have moderate to high SiO2 (50.5–73.0 wt%) and Na2O + K2O (5.1–11.5 wt%) and moderate to low MgO (6.6–0.2 wt%) concentrations. All samples have high large ion lithophile element (LILE: Ba up to 4600 ppm) and high-field-strength element contents (HFSE; Zr: 155–1328 ppm; Nb: 16–110 ppm; Ta: 1.4–7.1 ppm and Hf: 4–24 ppm) and have strongly fractionated LREE patterns ((La/Yb)N = 14–51). The most primitive members lack significant negative Eu anomalies. Mantle-normalized multi-element diagrams show depletion in Ba, Rb, Nb (Ta), P and Ti. The alkaline rocks have moderate radiogenic initial 87Sr/86Sr ratios (0.7061–0.7087) and unradiogenic initial ɛNd values (−3.9 to −6.1). This isotope signature, associated with high LREE/HFSE ratios indicates that the parental melts were generated in enriched portions of the shallow lithospheric mantle, which was probably affected by previous subduction zone processes. In addition, correlations between Sr and Nd isotopes indicate that some of these variations result from combined crustal assimilation and fractional crystallization (AFC) processes. A new model of flat subduction is presented that explains most of the unsolved problems in the orogenic evolution of the Damara orogen, namely (i) the absence of early intrusive rocks with a clear subduction zone setting, (ii) the absence of high-pressure rocks such as blueschists and eclogites, (iii) the unusual distribution of igneous rocks with a clear predominance of granite and granodiorite and (iv) the need for a asthenospheric window during a classical subduction to explain the high T/moderate P granulite facies conditions in the overriding plate.

Mechanism of emplacement and origin of the Ildır lava dome in the Karaburun Peninsula, western Anatolia (Turkey)

The Karaburun Peninsula, which is considered to be a part of the Anatolide-Tauride Block, is situated between western Anatolia and the eastern part of the Aegean Sea, in a post-collisional extensional environment, and is located within the Izmir-Balıkesir Transfer Zone. The Paleozoic – Mesozoic basement rocks of the peninsula are cut and covered by Miocene volcanic successions, comprising mostly andesitic lavas and volcanoclastic deposits that interfinger with detrital lacustrine deposits. A NW-SE elongated andesitic exogenous composite lava dome structure (major axis is 4 km and minor axis is 1 km) was identified and dated to be 16.39 ± 0.34 Ma in age. It cuts and covers the Alaçatı andesitic pyroclastic and epiclastic deposits of the Ildır peninsula. Internal structures and igneous fabrics indicate a composite exogenous lava dome structure with short distance lava flows over a flat floor of Alaçatı ignimbrite. Emplacement of this Ildır composite lava dome was controlled by fissure-like volcanic activity along multiple fault lineaments as a result of extensional duplex – negative palm-tree lineaments which are related to the Miocene NW-SE directed sinistral strike-slip transtensional fault zone of the Karaburun peninsula. Comparison of the andesitic Ildır lava dome to all volcanic rocks of the Karaburun peninsula implies that the parental (more mafic) magma which formed the lava dome was generated by partial melting of a highly contaminated mantle source which had been modified by subduction-derived fluid/melt components. The Ildır melts were subsequently differentiated by fractional crystallization and crustal assimilation processes in the crust.

The Central Atlantic Magmatic Province (CAMP) in Morocco

Abstract The Central Atlantic Magmatic Province (CAMP) is a large igneous province (LIP) composed of basic dykes, sills, layered intrusions and lava flows emplaced before Pangea break-up and currently distributed on the four continents surrounding the Atlantic Ocean. One of the oldest, best preserved and most complete sub-provinces of the CAMP is located in Morocco. Geochemical, geochronologic, petrographic and magnetostratigraphic data obtained in previous studies allowed identification of four strato-chemical magmatic units, i.e. the Lower, Intermediate, Upper and Recurrent units. For this study, we completed a detailed sampling of the CAMP in Morocco, from the Anti Atlas in the south to the Meseta in the north. We provide a complete mineralogical, petrologic (major and trace elements on whole-rocks and minerals), geochronologic (40Ar/39Ar and U–Pb ages) and geochemical set of data (including Sr–Nd–Pb–Os isotope systematics) for basaltic and basaltic–andesitic lava flow piles and for their presumed feeder dykes and sills. Combined with field observations, these data suggest a very rapid (&lt;0·3 Ma) emplacement of over 95% of the preserved magmatic rocks. In particular, new and previously published data for the Lower to Upper unit samples yielded indistinguishable 40Ar/39Ar (mean age = 201·2 ± 0·8 Ma) and U–Pb ages (201·57 ± 0·04 Ma), suggesting emplacement coincident with the main phase of the end-Triassic biotic turnover (c.201·5 to 201·3 Ma). Eruptions are suggested to have been pulsed with rates in excess of 10 km3/year during five main volcanic pulses, each pulse possibly lasting only a few centuries. Such high eruption rates reinforce the likelihood that CAMP magmatism triggered the end-Triassic climate change and mass extinction. Only the Recurrent unit may have been younger but by no more than 1 Ma. Whole-rock and mineral geochemistry constrain the petrogenesis of the CAMP basalts. The Moroccan magmas evolved in mid-crustal reservoirs (7–20 km deep) where most of the differentiation occurred. However, a previous stage of crystallization probably occurred at even greater depths. The four units cannot be linked by closed-system fractional crystallization processes, but require distinct parental magmas and/or distinct crustal assimilation processes. EC-AFC modeling shows that limited crustal assimilation (maximum c.5–8% assimilation of e.g. Eburnean or Pan-African granites) could explain some, but not all the observed geochemical variations. Intermediate unit magmas are apparently the most contaminated and may have been derived from parental magmas similar to the Upper basalts (as attested by indistinguishable trace element contents in the augites analysed for these units). Chemical differences between Central High Atlas and Middle Atlas samples in the Intermediate unit could be explained by distinct crustal contaminants (lower crustal rocks or Pan-African granites for the former and Eburnean granites for the latter). The CAMP units in Morocco are likely derived from 5–10% melting of enriched peridotite sources. The differences observed in REE ratios for the four units are attributed to variations in both source mineralogy and melting degree. In particular, the Lower basalts require a garnet peridotite source, while the Upper basalts were probably formed from a shallower melting region straddling the garnet–spinel transition. Recurrent basalts instead are relatively shallow-level melts generated mainly from spinel peridotites. Sr–Nd–Pb–Os isotopic ratios in the CAMP units from Morocco are similar to those of other CAMP sub-provinces and suggest a significant enrichment of the mantle-source regions by subducted crustal components. The enriched signature is attributed to involvement of about 5–10% recycled crustal materials introduced into an ambient depleted or PREMA-type mantle, while involvement of mantle-plume components like those sampled by present-day Central Atlantic Ocean Island Basalts (OIB, e.g. Cape Verde and Canary Islands) is not supported by the observed compositions. Only Recurrent basalts may possibly reflect a Central Atlantic plume-like signature similar to the Common or FOZO components.

Petrogenesis of the Triassic Cuyo basin magmatism: Controls on the magmatic evolution of passive rifts basins in Western Gondwana

The rifting magmatism of the Cuyo basin (Western Argentina) is characterized by the presence of pyroclastic rocks and basaltic lavas, Triassic in age, emplaced during active faulting that occurred along an ancient terrane boundary. The pyroclastic rocks display calc-alkaline signature and were generated in shallow magma chambers with plagioclase fractionation associated with underplating processes. The basaltic lavas have alkaline to transitional composition with an OIB signature along with a negative NbTa anomaly and trace elements ratios that reflect crustal assimilation processes which are confirmed by their isotopic composition. Moreover, the NdSm isotopes suggest the involvement of a Precambrian upper crust in their genesis. The geochemistry of the basalts also reveals that the magmas generated in deep magma chambers that evolved through amphibole or garnet fractionation. Considering the geodynamic scenario, crustal assimilation processes could have been favored by the presence of highly fracturated ancient suture zones which could have promoted magma-host rock interaction during the magma ascent.

Tectonic setting and mineralization potential of the Zefreh porphyry Cu-Mo deposit, central Iran: Constraints from petrographic and geochemical data

The Zefreh porphyry Cu-Mo deposit is located in the central part of the Cenozoic Urumieh-Dokhtar Magmatic Arc (UDMA), 75 km NE of Isfahan city, central Iran. The Zefreh rocks, ranging from diorite to granodiorite in composition, formed in a subduction-related arc setting, and likely in a pre-plate collisional environment. They are enriched in Th, U, Rb, Pb, light rare earth elements (LREE), but depleted in Ti, Nb, Ta, Ba, Sr, and P. Negative to slightly positive Eu anomalies (0.5–1.1), low to moderate Sr contents, (189–567 ppm), low to moderate Sr/Y ratios (5–23), and low La/Yb ratios (3–8) in the Zefreh magmatic suite rule out considerable amounts of fractionated amphibole from a hydrous magma in the lower crust. Arc crustal thickness in the Zefreh area was probably <40 km at the time of emplacement of the granitoids, under which amphibole did not fractionate considerably from the primitive magma. Melting of garnet-free lower-crustal amphibolites, together with fractional crystallization and crustal assimilation processes may be the main source of the Zefreh granitoids. On the basis of geochemical characteristics of the granitoids (e.g., low to moderate Sr contents, moderate to high contents of Y and Yb, low to moderate ratios of Sr/Y and Dy/Yb, low La/Yb ratios, negative to slightly positive Eu anomalies, and negative anomalies of Sr) and immature arc with thin crust, the Zefreh deposit is likely to be a small, sub-economic porphyry Cu-Mo deposit.

A Geochemical Overview of Mid-Archaean Metavolcanic Rocks from Southwest Greenland

The present contribution reviews bulk-rock geochemical data for mid-Archaean (ca. 3075–2840 Ma) metavolcanic rocks from the North Atlantic Craton of southwest Greenland. The data set includes the most recent high quality major and trace element geochemical analyses for ten different supracrustal/greenstone belts in the region. When distilling the data set to only include the least altered metavolcanic rocks, by filtering out obviously altered samples, mafic/ultramafic cumulate rocks, late-stage intrusive sheets (dolerites) and migmatites, the remaining data (N = 427) reveal two fundamentally distinct geochemical suites. The contrasting trends that emerge from the filtered geochemical data set, which best represents the melt compositions for these mid-Archaean metavolcanic rocks are: (1) tholeiitic (mainly basaltic) versus (2) calc-alkaline (mainly andesitic). These two rock suites are effectively separated by their La/Sm ratios (below or above three, respectively). It is demonstrated by geochemical modelling that the two contrasting suites cannot be related by either fractional crystallization or crustal assimilation processes, despite occurring within the same metavolcanic sequences. The tholeiitic basaltic rocks were directly mantle-derived, whereas the petrogenesis of the calc-alkaline andesitic rocks involve a significant (&gt;50%) felsic component. The felsic contribution in the calc-alkaline suite could either represent slab-melt metasomatism of their mantle source, mafic-felsic magma mixing, or very large degrees of partial melting of mafic lower crust. At face value, the occurrence of andesites, and the negative Nb-Ta-Ti-anomalies of both suites, is consistent with a subduction zone setting for the origin of these metavolcanic rocks. However, the latter geochemical feature is inherent to processes involving crustal partial melts, and therefore independent lines of evidence are needed to substantiate the hypothesis that plate tectonic processes were already operating by the mid-Archaean.

Upper Cretaceous weakly to strongly silica-undersaturated alkaline dike series of the Mantiqueira Range, Serra do Mar alkaline province: Crustal assimilation processes and mantle source signatures

Three series of alkaline dikes are characterized in a restricted area of the Mantiqueira Range: a weakly silica-undersaturated series (normative ne normative ne > 13), of potassic character, consisting only of basanites and tephrites, and a strongly silica-undersaturated series (normative ne > 13), of sodic character, consisting of basanites, tephrites, phonotephrites, and phonolites. Ar-Ar data for biotite from a tephriphonolite dike yielded an average age of 85.4±0.4 Ma. All three series show similar evolution trends controlled mainly by early-fractionation of olivine, spinel and clinopyroxene and, secondly, fractionation of plagioclase and apatite. Different degrees of crustal assimilation are suggested for each series. Assimilation processes are responsible for the wide 87 Sr/ 86 Sri (0.70447-0.71172) and 143 Nd/ 144 Ndi (0.51237-0.51244) ranges, for the increased K 2 O/Na 2 O, Ba/Sr and Rb/Sr ratios, and for the reduced Ce/Pb ratios. In general, a significant contribution from partially melted host rocks is associated to the weakly silica-undersaturated potassic series. A phlogopite-bearing carbonate-metasomatized heterogeneous peridotite source accounts for the main chemical variations in the least contaminated samples of each series. Geochemical signatures of the strongly sodic alkaline series indicate a greater contribution from carbonated pyroxene-rich veins in the mantle source.

Evidence for As Contamination and the Partitioning of Pd into Pentlandite and Co + Platinum Group Elements into Pyrite in the Fazenda Mirabela Intrusion, Brazil

The Fazenda Mirabela intrusion in Brazil hosts two zones of mineralization, the Santa Rita Ni-Cu sulfide ore zone occurring predominantly in an orthopyroxenite layer, and an underlying basal platinum group element (PGE) anomaly hosted in S-poor dunite. We show that in the northern and southern (marginal) zones of the intrusion, (Pt,Pd,Ni,Cu)(Fe,Bi,Te)2 minerals are accompanied by As-bearing platinum group minerals (PGM) sperrylite (PtAs2), and irarsite ([RhIrPt]AsS). These As-bearing PGM are extremely rare in the central zone of the intrusion, suggesting that As has been introduced into the margins of the intrusion from the country rocks via crustal assimilation or synmagmatic hydrothermal processes. Other PGM and precious metal minerals (PMM) including hessite (Ag2Te), Pd-Ag tellurides, electrum, and native Au are observed in the Santa Rita ore zone and do not show any lateral variation. In the semimetal- and S-poor dunite hosting the basal PGE anomaly, PGE are predominantly retained in the base metal sulfides and only minor PGE alloys are present. In this S-poor dunite Pt, Au and semimetals partitioned into a fractionated Cu-rich liquid, which formed during the crystallization of monosulfide solid solution (MSS) and was largely removed by high-temperature magmatic fluids. Palladium was not affected by this process and significant concentrations of Pd are identified in pentlandite, suggesting that Pd preferentially partitioned into MSS as it crystallized and subsequently pentlandite as it exsolved. Pyrite in the Santa Rita ore zone is Co rich and contains higher concentrations of IPGE (Os, Ir, Ru) and Rh than pyrrhotite and pentlandite, suggesting that pyrite has not replaced and inherited the PGE concentrations of these sulfides. The Pd-poor nature of pyrite also indicates that it has not replaced Pd-rich pentlandite. It is proposed that the IPGE and Co preferentially partitioned into pyrite as it exsolved from MSS.

Genesis and evolution of the Cerro Prieto Volcanic Complex, Baja California, Mexico

The Cerro Prieto Volcanic Complex (CPVC), located in northwestern Mexico, is the only surface manifestation of the Cerro Prieto Geothermal Field, the third largest producer of geothermal energy in the world. This geothermal field and the Salton Sea in the USA sit in a pull-apart basin that belongs to the trans-tensional tectonic zone that includes the San Andreas Fault system and the Salton Trough basin to the NW and the East Pacific Rise to the SE. In spite of its strategic importance in the generation of geothermal energy, the origin of Cerro Prieto and its relationship with the geothermal reservoir were unknown. In this contribution, we discuss the origin, evolution, and mechanisms of formation of this small monogenetic volcano and the magmas that fed the system. The volcanic complex is located on top of the Cerro Prieto left lateral fault to the northwest of the Cerro Prieto Geothermal Field. The complex consists of a lava cone and a series of domes (∼0.15 km3) protruding from Tertiary sandstones and recent unconsolidated sediments of the alluvial plain of the Colorado River. The Cerro Prieto Volcanic Complex consists of seven stratigraphic units emplaced in a brief time span around 78–81 ka. Its activity began with the extrusion of a dacitic lava that came into contact with water-saturated sediments, causing brecciation of the lava. The activity continued with the emplacement of dacitic domes and a dyke that were destroyed by a phreatic explosion emplacing a lithic-rich breccia. This phreatic explosion formed a 300-m-wide and 40-m-deep circular crater. The activity then migrated ∼650 m to the SW where three dacitic lava domes were extruded and ended with the emplacement of a fissure-fed lava flow. Subsequent remobilization of the rocks in the complex has generated debris and hyperconcentrated flow deposits interbedded with fluviatile sediments in the surrounding terrain. All rocks of the CPVC are dacites with phenocrysts of plagioclase, orthopyroxene, and Fe-Ti oxides and minor quartz. Gabbro-dioritic dykes intruded in Tertiary sandstones and siltstones of the local basement contain phenocrysts of plagioclase, clinopyroxene, Fe-Ti oxides, and olivine. All CPVC rocks are chemically homogeneous suggesting discrete modifications by crustal assimilation, fractional crystallization, or magma mixing processes during upper crustal residence. Aeromagnetic results suggest that the depth of the magmatic source in the area is located below 3.5 km. CPVC rocks contain chemical anomalies typical of subduction zones, which suggests that all CPVC magmas could have been generated by partial melting of the remains of the subducted Farallon plate.

Pre-collisional, Tonian (ca. 790 Ma) continental arc magmatism in southern Mantiqueira Province, Brazil: Geochemical and isotopic constraints from the Várzea do Capivarita Complex

This paper focuses on the pre-collisional mature arc magmatism (ca. 790 Ma) recorded in orthogneisses from the Várzea do Capivarita Complex (VCC), southern Mantiqueira Province (PM), Brazil. The complex comprises ortho- and paragneisses tectonically interleaved during a transpressive high grade regime (ca. 650 Ma), possibly related to oblique collision. The VCC orthogneisses are metaluminous to peraluminous calc-alkaline rocks, with high 87Sr/86Sr(i) ratios from 0.71628 to 0.72509 and εNd(790) values from −7.19 to −10.06. The VCC magmatism is correlated with other ca. 800 Ma arc sequences from southern PM, as the Porongos Metamorphic Complex (PMC) metavolcanic rocks and the orthogneisses from Cerro Bori (CB), Uruguay. All associations show signatures typical of accretionary orogens, TDM and Meso to Paleoproteroic inheritance ages, and strong evidence of crustal assimilation/contamination. Their high K contents, and the tendency to move toward the post-collisional field in geotectonic diagrams suggest that they were generated in thick-crust, mature arc environments. In contrast, the CB sequence exhibits a less mature continental-arc character, suggestive of thinner crust or shorter distance to the active margin. VCC and CB orthogneisses, and part of the PMC metavolcanic rocks may be interpreted as part of the same magmatism, or at least as fragments of similar magmatic arcs. However, VCC magmatism is distinct from continental arc sequences in the São Gabriel Block (ca. 700–750 Ma). Isotope signatures for this younger magmatism indicate major contribution of Neoproterozoic juvenile sources, with only little amounts of reworked, old continental crust. Geochemical and Sr-Nd signatures presented in this paper suggest that at least part of the PMC metavolcanic rocks are the protoliths of the VCC orthogneisses. This, together with the isotope evidence of similarity between VCC and PMC igneous and sedimentary fractions, corroborates the hypothesis that the VCC and PMC are partly expressions of the same context, wherein the magmatic and sedimentary activity occurred in a single continental arc environment. Binary mixing models point to evolved EM II mantle reservoir as source for the VCC magmatism. A Paleoproterozoic TTG association (ca. 2.0 Ga) from the Arroio dos Ratos Complex (ARC) seems to be the main crustal contaminant of the VCC magmatism. Together with the small inheritance contribution at ca. 2.0, this suggests that the crust melted at ca. 790–800 Ma was predominantly like the ARC. Our models also indicate that in addition to crustal assimilation processes, fractional crystallization was important during the VCC magmatism evolution.

Oxygen isotope heterogeneity of arc magma recorded in plagioclase from the 2010 Merapi eruption (Central Java, Indonesia)

Chemical and isotopic compositions of magmatic crystals provide important information to distinguish between deep juvenile and crustal contributions. In this work, high-resolution multicollector secondary ion mass spectrometry data reveal strong variations of δ18O values in three plagioclase crystals (800–1700μm) from two representative basaltic andesite samples of the 2010 Merapi eruption (Central Java, Indonesia). The δ18O values (from 4.6‰ to 7.9‰) are interpreted to reflect oxygen isotope heterogeneity in the melt composition during plagioclase growth. The lowest δ18O values (4.6–6.6‰) are found in anorthite-rich cores (An82–97), whereas higher δ18O values (5.7–7.9‰) are found in anorthite-poorer zones (An33–86), typically in crystal rims. Combining these new plagioclase δ18O data with δ18O of calc-silicate crustal xenoliths erupted between 1994 and 1998, the composition of glass inclusions hosted by the anorthite-rich plagioclase (An82–92), available experimental data, and the results of thermodynamic modeling using the Magma Chamber Simulator code, we conclude that the abundant anorthite-rich cores crystallized from a mantle-derived hydrous basaltic to basaltic trachyandesite melt that recharged a deeper (200–600MPa) magma storage zone, whereas lower anorthite zones crystallized at shallower levels (100–200MPa). The oxygen isotope variations in the plagioclase are explained by a two-stage model of interaction of the hydrous, mafic mantle-derived magma (1) with old crustal rocks depleted in 18O due to high temperature alteration that yielded the low δ18O values in the anorthite-rich cores at deep levels (13–20km), and later (2) with 18O-enriched carbonate material that yielded the high δ18O values in anorthite-poorer zones at shallow levels (∼4.5–9km). Thermodynamic modeling is consistent with ∼18wt.% assimilation of crustal calc-silicate material at 925–950°C and 100–200MPa by the 2010 Merapi basaltic andesite magma prior to eruption. Timescales for plagioclase phenocryst growth and residence in the magmatic plumbing system are ⩽34years. The combined data thus reveal efficient magma recharge and crustal assimilation processes that characterize the open-system magma storage and transport systems associated with the 2010 Merapi eruption.

Role of crustal assimilation and basement compositions in the petrogenesis of differentiated intraplate volcanic rocks: a case study from the Siebengebirge Volcanic Field, Germany

The Siebengebirge Volcanic Field (SVF) in western Germany is part of the Cenozoic Central European Volcanic Province. Amongst these volcanic fields, the relatively small SVF comprises the entire range from silica-undersaturated mafic lavas to both silica-undersaturated and silica-saturated differentiated lavas. Owing to this circumstance, the SVF represents a valuable study area representative of intraplate volcanism in Europe. Compositions of the felsic lavas can shed some new light on differentiation of intraplate magmas and on the extent and composition of potential crustal assimilation processes. In this study, we provide detailed petrographic and geochemical data for various differentiated SVF lavas, including major and trace element concentrations as well as Sr–Nd–Hf–Pb isotope compositions. Samples include tephriphonolites, latites, and trachytes with SiO2 contents ranging between 53 and 66 wt%. If compared to previously published compositions of mafic SVF lavas, relatively unradiogenic 143Nd/144Nd and 176Hf/177Hf coupled with radiogenic 87Sr/86Sr and 207Pb/204Pb lead to the interpretation that the differentiated volcanic rocks have assimilated significant amounts of lower crustal mafic granulites like the ones found as xenoliths in the nearby Eifel volcanic field. These crustal contaminants should possess unradiogenic 143Nd/144Nd and 176Hf/177Hf, radiogenic 87Sr/86Sr, and highly radiogenic 207Pb/204Pb compositions requiring the presence of ancient components in the central European lower crust that are not sampled on the surface. Using energy-constrained assimilation–fractional crystallisation (EC-AFC) model calculations, differentiation of the SVF lithologies can be modelled by approximately 39–47 % fractional crystallisation and 6–15 % crustal assimilation. Notably, the transition from silica-undersaturated to silica-saturated compositions of many felsic lavas in the SVF that is difficult to account for in closed-system models is also well explained by such amounts of crustal assimilation.

Petrographic, geochemical and isotopic evidence of crustal assimilation processes in the Ponte Nova alkaline mafic–ultramafic massif, SE Brazil

Crustal assimilation plus crystal fractionation processes of different basanite magma batches control the evolution of the Ponte Nova cretaceous alkaline mafic–ultramafic massif in SE Brazil. This massif is composed of several intrusions, the main ones with a cumulate character. Disequilibrium features in the early-crystallized phases (e.g., corrosion and sieve textures in cores of clinopyroxene crystals, spongy-cellular-textured plagioclase crystals, gulf corrosion texture in olivine crystals) and classical hybridization textures (e.g., blade biotite and acicular apatite crystals) provide strong evidence of open-system behavior. All samples are olivine- and nepheline-normative rocks with basic–ultrabasic and potassic characters and variable incompatible element enrichments. The wide ranges of whole-rock 87Sr/86Sri and 143Nd/144Ndi ratios (0.70432–0.70641 and 0.512216–0.512555, respectively) are indicative of crustal contribution from the Precambrian basement host rocks. Plagioclase and apatite 87Sr/86Sr ratios (0.70422–0.70927) obtained for the most primitive samples of each intrusion indicate disequilibrium conditions from early- to principal-crystallization stages. Isotope mixing-model curves between the least contaminated alkaline basic magma and heterogeneous local crustal components indicate that each intrusion of the massif is differentiated from the others by varied degrees of crustal contribution. The primary mechanisms of crustal contribution to the Ponte Nova massif involve the assimilation of host rock xenoliths during the development of the chamber environment and the assimilation of partial melts from the surrounding host rocks. Thermodynamic models using the melts algorithm indicate that parental alkaline basic magmas can be strongly affected by contamination processes subsequently to their initial stages of crystallization when there is sufficient energy to assimilate partial melts of crustal host rocks. The assimilation processes are considered to be responsible for the increse in the K2O/Na2O, Ba/Sr and Rb/Sr ratios. This enrichment was associated with the relevant role of biotite breakdown in the assimilated host rock partial melts. The petrological model for the Ponte Nova massif is explained as repeated influxes of antecryst-laden basanite magmas that deposited most of their suspended crystals on the floor of the upper-crust magma chamber. Each intrusion is representative of relatively primitive olivine- and clinopyroxene-phyric basanites that had assimilated different degrees of partial melts of heterogeneous host rocks. This study reveals the relevant role of crustal assimilation processes in the magmatic evolution of nepheline-normative rocks, especially in upper-crust chamber environments.