Petrology, geochemistry and geodynamic implications of Jurassic island arc magmatism as revealed by mafic volcanic rocks in the Mesozoic low-grade sequence, eastern Rhodope, Bulgaria

In the eastern Bulgarian Rhodope, mafic extrusive rocks and underlying greenschists are found in the Mesozoic low-grade unit, which represents the northern extension of similar sequences including the Evros ophiolites in Thrace (Greece). Both rock types define a suite of low-Ti tholeiitic basalts to transitional boninitic basaltic andesites and andesites and associated metapyroclastites (greenschists), intruded at its base by diorite dikes of a boninitic affinity. Mafic lavas and greenschists display large ion lithophile element (LILE) enrichment relative to high-field strength elements (HFSE), flat REE patterns of a slight light REE depletion, a strong island arc tholeiite (IAT) and weak MORB-like signature. All these rocks are characterized by negative Nb anomalies ascribed to arc lavas. They have positive epsilon Nd(i) values in the range of +4.87 to +6.09, approaching the lower limit of MORB-like source, and relatively high ((207)Pb/(204)Pb)(i) (15.57-15.663) at low ((206)Pb/(204)Pb)(i) (18.13-18.54) ratios. The Nd isotopic compositions coupled with trace element data imply a dominantly depleted MORB-like mantle source and a contribution of subduction modified LILE-enriched component derived from the mantle wedge. The diorite dike has a low eNdi value of -2.61 and is slightly more Pb radiogenic ((207)Pb/(204)Pb)(i) (15.64) and ((206)Pb/(204)Pb)(i) (18.56), respectively, reflecting crustal contamination. Petrologic and geochemical data indicate that the greenschists and mafic extrusive rocks represent a magmatic assemblage formed in an island arc setting. The magmatic suite is interpreted as representing an island arc-accretionary complex related to the southward subduction of the Meliata-Maliac ocean under the supra-subduction back-arc Vardar ocean/island arc system. Magmatic activity appears to have initiated in the north during the inception of the island arc system by the Early-Middle Jurassic time in the eastern Rhodope that most likely graded to back-arc spreading southwards as represented by the Late Jurassic MORB-type Samothraki Island ophiolites. This tectonic scenario is further constrained by paleotectonic reconstructions. The arc-trench system collided with the Rhodope in the Late Jurassic times. (c) 2007 Elsevier B.V. All rights reserved.

A geochemical and petrological study of volcanic rocks in the Beardmore-Geraldton Archaean Greenstone Belt, Northwestern Ontario

Three repetitive sequences of northward youngIng, east striking, linear, volcano-sedimentary units are found in the late Archaean BeardmoreGeraldton greenstone belt, situated within the Wabigoon subprovince of the Superior Province of northwestern Ontario. The volcanic components are characterised by basaltic flows that are pillowed at the top and underlain by variably deformed massive flows which may In part be intrusive. Petrographic examination of the volcanic units indicates regional metamorphism up to greenschist facies (T=3250 C - 4500 C, P=2kbars) overprinted by a lower amphibolite facies thermal event (T=5750 C, P=2kbars) confined to the south-eastern portion of the belt. Chemical element results suggest olivine, plagioclase and pyroxene are the main fractionating mineral phases. Mobility studies on the varIOUS chemical elements indicate that K, Ca, Na and Sr are relatively mobile, while P, Zr, Ti, Fet (total iron = Fe203) and Mg are relatively immobile. Discriminant diagrams employing immobile element suggests that the majority of the samples are of oceanic affinity with a minor proportion displaying an island arc affinity. Such a transitional tectonic setting IS also refle.cted in REE data where two groups of volcanic samples are recognised. Oceanic tholeiites are LREE depleted with [La/Sm] N = 0.65 and a relatively flat HREE profile with [Sm/Yb] N = 1.2. Island arc type basalts (calc-alkaline) are LREE enriched, with a [La/Sm] N = 1.6, and a relatively higher fractionated HREE profile with [Sm/Yb] N = 1.9. Petrogenetic modelling performed on oceanIC tholeiites suggests derivation from a depleted spinel lherzolite source which undergoes 20% partial melting. Island arc type basalts can be derived by 10% partial melting of a hypothetical amphibolitised oceanic tholeiite source. The majority of the volcanic rocks in the Beardmore-Geraldton Belt are interpreted to represent fragments of oceanic crust trapped at a consuming plate margin. Subsequent post accretionary intrusion of gabbroic rocks (sensu lato) with calc-alkaline affinity is considered to result in the apparent hybrid tectonic setting recognized for the BGB.

Petrography, Petrochemistry and petrogenesis of Huronian volcanic rocks of the Elliot Lake region, Ontario

In the Elliot lake region of northern Ontario, Yolcanlc lava piles represent the lowermost units of the Huronian SUpergroup. These rocks general1y trend east-west and belong to the Elliot lake Group. They are s1tuated on the north and south limbs or the QuIrke lake Syncline. The volcanIc rocks of this study contain a secondary minerai assemblage consisting of actinolite, biotite, chlorIte, eptdote/cllnozoislte tttanomagnettte and calcite characteristic of greenschist metamorphism. Compilation of data suggests that metamorphism of the volcanic rocks proceeded between 325- and 425-C and between 2.4 and 4.7 kb. Geochemtcally these lavas represent tholeiitic and calc-alkaline assemblages. The tholeiites are character1sttcally enriched tn Fe and Tt and consist mainly of basalts, basaltic andesites and andesites. These rocks are believed to have formed by the partIal melting of a peridottte source at low P-T. In contrast, the calc-alkaline rocks are depleted in Fe and TI, but show a signIficant enrichment In 51 and Zr; andesIte Is the major rock type for thIs assemblage. I·t Is postUlated that the calc-alkalIne sU1te of rocks was the result of eIther the partIal meltIng of abasaltic·magma at shallow depth, or the melttng of s1al1c crustal materIal due to the added we1ght of tholeiitIc material on an unstable crust and to downwarplng processes Inttlated by convection cells.

Gamma-ray spectrometry signature of main volcanic rocks of Serra Geral formation: preliminary results

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Petrogenesis of Cenozoic volcanic rocks in the NW sector of the Gharyan volcanic field, Libya

The north-western sector of the Gharyan volcanic field (northern Libya) consists of trachytic-phonolitic domes emplaced between similar to 41 and 38 Ma, and small-volume mafic alkaline volcanic centres (basanites, tephrites. alkali basalts. hawaiites and rare benmoreites) of Middle Miocene-Pliocene age (similar to 12-2 Ma). Two types of trachytes and phonolites have been recognized on the basis of petrography, mineralogy and geochemistry. Type-1 trachytes and phonolites display a smooth spoon-shaped REE pattern without negative Europium anomalies. Type-2 trachytes and phonolites show a remarkable Eu negative anomaly, higher concentration in HFSE (Nb-Ta-Zr-Hf), REE and Ti than Type-1 rocks. The origin of Type-1 trachytes and phonolites is compatible with removal of clinopyroxene, plagioclase, alkali feldspar, amphibole. magnetite and titanite starting from benmoreitic magmas. found in the same outcrops. Type-2 trachytes and phonolites could be the result of extensive fractional crystallization starting from mafic alkaline magma, without removal of titanite. In primitive mantle-normalized diagrams, the mafic rocks (Mg#= 62-68, Cr up to 514 ppm, Ni up to 425 ppm) show peaks at Nb and Ta and troughs at K. These characteristics, coupled with low Sr-87/Sr-86(i) (0.7033-0.7038) and positive epsilon(Nd) (from +4.2 to + 5.3) features typical of the mafic anorogenic magmas of the northern African plate and of HIMU-OIB-like magma in general. The origin of the mafic rocks is compatible from a derivation from low degree partial melting (3-9%) shallow mantle sources in the spinel/gamet facies. placed just below the rigid plate in the uppermost low-velocity zone. The origin of the igneous activity is considered linked to passive lithospheric thinning related to the development of continental rifts like those of Sicily Channel (e.g.. Pantelleria and Linosa) and Sardinia (e.g., Campidano Graben) in the Central-Western Mediterranean Sea. (C) 2012 Elsevier B.V. All rights reserved.

Geochronology, geochemistry and isotopic compositions of the volcanic rocks from oceanic (Hawaii) and continental (Eifel) intra-plate environments

The Eifel volcanism is part of the Central European Volcanic Province (CEVP) and is located in the Rhenish Massif, close to the Rhine and Leine Grabens. The Quaternary Eifel volcanism appears to be related to a mantle plume activity. However, the causes of the Tertiary Hocheifel volcanism remain debated. We present geochronological, geochemical and isotope data to assess the geotectonic settings of the Tertiary Eifel volcanism. Based on 40Ar/39Ar dating, we were able to identify two periods in the Hocheifel activity: from 43.6 to 39.0 Ma and from 37.5 to 35.0 Ma. We also show that the pre-rifting volcanism in the northernmost Upper Rhine Graben (59 to 47 Ma) closely precede the Hocheifel volcanic activity. In addition, the volcanism propagates from south to north within the older phase of the Hocheifel activity. At the time of Hocheifel volcanism, the tectonic activity in the Hocheifel was controlled by stress field conditions identical to those of the Upper Rhine Graben. Therefore, magma generation in the Hocheifel appears to be caused by decompression due to Middle to Late Eocene extension. Our geochemical data indicate that the Hocheifel magmas were produced by partial melting of a garnet peridotite at 75-90 km depth. We also show that crustal contamination is minor although the magmas erupted through a relatively thick continental lithosphere. Sr, Nd and Pb isotopic compositions suggest that the source of the Hocheifel magmas is a mixing between depleted FOZO or HIMU-like material and enriched EM2-like material. The Tertiary Hocheifel and the Quaternary Eifel lavas appear to have a common enriched end-member. However, the other sources are likely to be distinct. In addition, the Hocheifel lavas share a depleted component with the other Tertiary CEVP lavas. Although the Tertiary Hocheifel and the Quaternary Eifel lavas appear to originate from different sources, the potential involvement of a FOZO-like component would indicate the contribution of deep mantle material. Thus, on the basis of the geochemical and isotope data, we cannot rule out the involvement of plume-type material in the Hocheifel magmas. The Ko’olau Scientific Drilling Project (KSDP) has been initiated in order to evaluate the long-term evolution of Ko’olau volcano and obtain information about the Hawaiian mantle plume. High precision Pb triple spike data, as well as Sr and Nd isotope data on KSDP lavas and Honolulu Volcanics (HVS) reveal compositional source variations during Ko’olau growth. Pb isotopic compositions indicate that, at least, three Pb end-members are present in Ko’olau lavas. Changes in the contributions of each component are recorded in the Pb, Sr and Nd isotopes stratigraphy. The radiogenic component is present, at variable proportion, in all three stages of Ko’olau growth. It shows affinities with the least radiogenic “Kea-lo8” lavas present in Mauna Kea. The first unradiogenic component was present in the main-shield stage of Ko’olau growth but its contribution decreased with time. It has EM1 type characteristics and corresponds to the “Ko’olau” component of Hawaiian mantle plume. The second unradiogenic end-member, so far only sampled by Honololu lavas, has isotopic characteristics similar to those of a depleted mantle. However, they are different from those of the recent Pacific lithosphere (EPR MORB) indicating that the HVS are not derived from MORB-related source. We suggest, instead, that the HVS result from melting of a plume material. Thus the evolution of a single Hawaiian volcano records the geochemical and isotopic changes within the Hawaiian plume.

Sedimentology, Depositional Age, and Provenance of Sedimentary and Volcanic Rocks Exposed Along Willow Creek, Eastern Susitna Basin, South-Central Alaska: Implications for Modification of a Forearc Basin by Spreading Ridge Subduction

This is the first detailed study of the westernmost portion of the outcrop belt, which extends along the western flank of the Talkeetna Mountains and includes thick, well-exposed outcrops along Willow Creek in the eastern Susitna basin. New sedimentologic, compositional, and geochronologic data were obtained from stratigraphic sections within Arkose Ridge Formation strata at Willow Creek. This data combined with new geologic mapping and geochronologic data from Willow Bench and Kashwitna River Bluff (north of Willow Creek), and from the Government Peak area (east of Willow Creek), help constrain depositional processes and source terranes that provided detritus to the westernmost Arkose Ridge Formation strata.

A Microscopic Study of some Volcanic Rocks in the Vicinity of Butte

The rhyolite rocks in the vicinity of Big Butte present a very interesting, though complicated study. It seems that no extensive or conclusive work has ever been done upon these extrusives. I have found that the rhyolites present a very interesting problem, especially microscopically, and that no doubt an intensive study along these lines will solve the problems concerning age relations of different flows and origin of the magma that produced the material for emanations.

Timing of Tertiary Extension in the Railroad Valley Pioche Transect, Nevada: Constraints from Ar-40/Ar-39 Ages of Volcanic Rocks

Time-space relations of extension and volcanism place critical constraints on models of Basin and Range extensional processes. This paper addresses such relations in a 130-km-wide transect in the eastern Great Basin, bounded on the east by the Ely Springs Range and on the west by the Grant and Quinn Canyon ranges. Stratigraphic and structural data, combined with 40Ar/39Ar isotopic ages of volcanic rocks, document a protracted but distinctly episodic extensional history. Field relations indicate four periods of faulting. Only one of these periods was synchronous with nearby volcanic activity, which implies that volcanism and faulting need not be associated closely in space and time. Based on published dates and the analyses reported here, the periods of extension were (1) prevolcanic (pre-32 Ma), (2) early synvolcanic (30 to 27 Ma), (3) immediately postvolcanic (about 16 to 14 Ma), and (4) Pliocene to Quaternary. The break between the second and third periods is distinct. The minimum gap between the first two periods is 2 Ma, but the separation may be much larger. Temporal separation of the last two periods is only suggested by the stratigraphic record and cannot be rigorously demonstrated with present data. The three younger periods of faulting apparently occurred across the entire transect. The oldest period is recognized only at the eastern end of the transect, but appears to correlate about 150 km northward along strike with extension in the Northern Snake Range-Kern Mountains area. Therefore the oldest period also is regional in extent, but affected a different area than that affected by younger periods. This relation suggests that distinct extensional structures and master detachment faults were active at different times. The correlation of deformation periods of a few million years duration across the Railroad Valley-Pioche transect suggests that the scale of active extensional domains in the Great Basin may be greater than 100 km across strike.

Major and trace element analyses of ODP Leg 120 volcanic rocks (Table 1)

Widespread Lower Cretaceous magmatism occurred along the Indian-Australian/Antarctic margins, and in the juvenile Indian Ocean, during the rifting of eastern Gondwana. The formation of this magmatic province probably began around 120-130 Ma with the eruption of basalts on the Naturaliste Plateau and at Bunbury, western Australia. On the northeast margin of India, activity began around 117 Ma with the Rajmahal continental basalts and associated lamprophyre intrusions. The formation of the Kerguelen Plateau in the Indian Ocean began no later than 114 Ma. Ultramafic lamprophyres (alnoites) were emplaced in the Prince Charles Mountains near the Antarctic continental margin at ~ 110 Ma. These events are considered to be related to a major mantle plume, the remnant of which is situated beneath the region of Kerguelen and Heard islands at the present day. Geochemical data are presented for each of these volcanic suites and are indicative of complex interactions between asthenosphere-derived magmas and the continental lithosphere. Kerguelen Plateau basalts have Sr and Nd isotopic compositions lying outside the field for Indian Ocean mid-ocean ridge basalts (MORB) but, with the exception of Site 738 at the southern end of the plateau, within the range of more recent hotspot basalts from Kerguelen and Heard Islands. However, a number of the plateau tholeiites are characterized by lower 206Pb/204Pb ratios than are basalts from Kerguelen Island, and many also have anomalously high La/Nb ratios. These features suggest that the source of the Kerguelen Plateau basalts suffered contamination by components derived from the Gondwana continental lithosphere. An extreme expression of this lithospheric signature is shown by a tholeiite from Site 738, suggesting that the southernmost part of the Kerguelen Plateau may be underlain by continental crust. The Rajmahal tholeiites mostly fall into two distinct geochemical groups. Some Group I tholeiites have Sr and Nd isotopic compositions and incompatible element abundances, similar to Kerguelen Plateau tholeiites from Sites 749 and 750, indicating that the Kerguelen-Heard mantle plume may have directly furnished Rajmahal volcanism. However, their elevated 207Pb/204Pb ratios indicate that these magmas did not totally escape contamination by continental lithosphere. In contrast to the Group I tholeiites, significant contamination is suggested for Group II Rajmahal tholeiites, on the basis of incompatible element abundances and isotopic compositions. The Naturaliste Plateau and the Bunbury Basalt samples show varying degrees of enrichment in incompatible elements over normal MORB. The Naturaliste Plateau samples (and Bunbury Basalt) have high La/Nb ratios, a feature not inconsistent with the notion that the plateau may consist of stretched continental lithosphere, near the ocean-continent divide.

(Table 1) Isotopic composition of Sm and Nd in mantle restites of the Central Atlantic and in associated plutonic and volcanic rocks

Ultrabasic rock samples collected from two areas of the crustal zone of the Mid-Atlantic Ridge (MAR): (1) 13-17°N (near the intersection of the ridge axis with the 15°20'N prime fracture zone), and (2) 33°40'N prime (the western intersection of the MAR crest with the Heis fracture zone) were objects of this study. Samples of peridotite and of plutonic and volcanic rocks associated with it were used to measure their Sm/Nd, 143Nd/144Nd, and 147Sm/144Nd ratios, which allowed to test time and genetic relationships between evolution of mantle material under the ridge crest and products of its magmatic activity. Results of this work proved ubiquitous discrepancy between melting degree values of extremely depleted mantle peridotites in the MAR area between 14°N and 16°N, obtained using petrologic and geochemical methods. This discrepancy suggests large-scale interaction between mantle material and magmatic melts and fluids enriched in incompatible elements or fluids. The results obtained suggest that repeated melting of the mantle under the axial MAR zone is an universal characteristic of magmatism in low-velocity spreading centers. The results of this study also proved the crestal MAR zone in the Central Atlantic region show distinct indications of isotope-geochemical segmentation of the mantle. It is suggested that the geochemically anomalous MAR mantle peridotite in the zone of the MAR intersection with the 15°20'N prime fracture zone can be interpreted as fragments of mantle substrate, foreign for the Atlantic mantle north of the equator.