Síntese geológica e geocronológica do Cráton São Luís e do Cinturão Gurupi na região do Rio Gurupi (NE-Pará / NW-Maranhão)

A região do Gurupi, na fronteira norte dos estados do Pará e Maranhão, é historicamente dividida em dois domínios geocronológicos (Rb-Sr, K-Ar): um paleoproterozóico (Cráton São Luís), outro neoproterozóico (Cinturão Gurupi). Dados geocronológicos em zircão (evaporação de Pb), recentemente disponibilizados, além de dados inéditos aqui apresentados, contemplam a maioria das unidades litoestratigráficas e litodêmicas regionais e, juntamente com poucos dados de Sm-Nd em rocha total, mostram ser o período entre 2,0 e 2,2 Ga a época de formação da quase totalidade das unidades rochosas (juvenis ou não). Apenas um granitóide possui idade de cristalização neoproterozóica (~0,55 Ga) e o Arqueano apresenta-se somente como vestígio em cristais herdados de zircão e em idades modelo Sm-Nd (T_DM) de protólitos de algumas unidades. A avaliação das características composicionais, metamórficas, estruturais, geofísicas e geocronológicas das diversas unidades sustenta a divisão prévia em dois domínios, mas mostra que o domínio (cinturão) Gurupi possui história orogênica paleoproterozóica comum ao domínio (cráton) São Luís. O domínio Gurupi possui características de orógeno colisional, enquanto que o domínio São Luís possui características acrescionárias e ambos participaram da colagem orogênica Paleoproterozóica (2,2 - 2,0 Ga), muito importante em escala global. No Neoproterozóico, a borda sul desse conjunto foi afetada por forte atividade tectônica direcional e litogênese muito limitada, como reflexo da colagem orogênica Brasiliana, definindo a configuração atual do Cinturão Gurupi e da borda sul do Cráton São Luís.

Petrografia, geocronologia e significado tectônico do Nefelina Sienito Brejinho: extremo noroeste da Província Borborema

No extremo noroeste da Província Borborema foi identificado um maciço alcalino subsaturado, o Nefelina Sienito Brejinho (NSB), alojado em gnaisses do Paleoproterozoico do Complexo Granja. As investigações envolveram mapeamento de detalhe do corpo, acompanhado de análises petrográficas e geocronológicas, que permitiram reconstruir a sua história evolutiva. Foram identificadas cinco fácies petrográficas, com a sua distribuição cartográfica, associações mineralógicas presentes e análises texturais/estruturais sugerindo a atuação de processos de cristalização fracionada, com forte controle da ação da gravidade e imiscibilidade de líquidos na história da cristalização magmática do maciço. Os estudos geocronológicos realizados pelo método Rb-Sr em rocha total revelaram valor de 554 ± 11 Ma, interpretado como a idade mínima para cristalização e emplacement do NSB, no final do Neoproterozoico. No contexto tectônico, esse magmatismo alcalino pode ser relacionado ao evento extensional responsável pela implantação do Gráben Jaibaras e seus correlatos no oeste do Ceará, assim como à granitogênese da região, cujas idades situam-se no intervalo entre 530 e 590 Ma. Situação semelhante é reconhecida na borda norte da Bacia do Amazonas, com o Complexo Alcalino-Ultramáfico-Carbonatítico Maicuru (589 Ma) alojado no embasamento gnáissico paleoproterozoico do Cráton Amazônico. A situação geológica e temporal do NSB permite situá-lo posteriormente à tectônica transcorrente representada na área pela Zona de Cisalhamento Santa Rosa, uma ramificação do Lineamento Transbrasiliano, e anterior à Bacia do Parnaíba. Disso resulta que esse magmatismo alcalino pode ser interpretado como um importante registro da fase rifte que prenunciou a instalação dessa bacia no início do Paleozoico. A sua caracterização, até então sem similar na Província Borborema, abre novas perspectivas de pesquisa em todo o embasamento da Bacia do Parnaíba, tendo em vista a importância tectônica e metalogenética desse tipo de magmatismo.

Multielemental evaluation of garbage bags by EDXRF

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Implicações sobre a natureza das fontes mantélicas envolvidas na gênese da Província Magmática do Paraná

Neste estudo, buscou-se compreender os processos e materiais que contribuíram às fontes mantélicas envolvidas na gênese da Província Magmática do Paraná (PMP). O papel que a crosta, a litosfera e o manto terrestre (raso ou profundo) desempenham na geração de assinaturas químicas e isotópicas em basaltos de derrames continentais e oceânicos tem sido um tema intensamente pesquisado e muito debatido na literatura internacional (e.g., Sheth, 2005). Embora muitos estudos já tenham sido conduzidos na PMP, existe ainda uma grande controvérsia com relação à natureza das regiões-fonte dos magmas (manto litosférico continental ou manto sublitosférico), como também acerca dos mecanismos geodinâmicos que provocam o início da fusão dessas regiões-fonte (e.g., Bellieni et al., 1984; Hawkesworth et al., 1992). Nesse contexto, uma grande quantidade de análises de elementos traço (litófilos e siderófilos), como também dados isotópicos de quatro sistemas de decaimento radioativo (Rb-Sr, Sm-Nd, U-Th-Pb e Re-Os), foram obtidos em basaltos que ocorrem no norte e sul da PMP. A regionalização geoquímica e isotópica (Sr-Nd-Pb) observada nas rochas basálticas da PMP tem sido frequentemente interpretada como resultante da fusão de manto litosférico subcontinental heterogêneo. Entretanto, dados geoquímicos de elementos altamente siderófilos e de razões isotópicas 187Os/188Os, obtidos neste estudo, indicam uma fonte mantélica homogênea, já que os basaltos baixo-TiO2 e alto- TiO2 possuem razões isotópicas de ósmio muito similares (187Os/188Osi = 0,1295±0,0018), as quais são distintas daquelas de manto litosférico subcontinental antigo (Proterozoico ou Arqueano; 187Os/188Os = 0,113). Os dados isotópicos Re-Os exibem uma tendência linear (187Re/188Os versus 187Os/188Os), definindo uma isócrona de boa qualidade (idade = 131,6 ± 2,3 Ma), consistente com as idades obtidas em trabalhos anteriores utilizando o método 40Ar/39Ar. Comumente, a PMP tem sido geneticamente ligada ao hotspot Tristão da Cunha via Rio Grande Rise e Walvis Ridge. No entanto, as razões isotópicas de ósmio determinadas nas rochas da Ilha de Tristão da Cunha são muito distintas daquelas dos basaltos da PMP, reforçando que essa pluma não deve ter contribuído com matéria na gênese dessas rochas. Desta forma, para explicar todas as características isotópicas dos basaltos da PMP é necessário o envolvimento de três componentes mantélicos (Marques et al., 1999). Um deles, empobrecido e semelhante ao DMM, estaria presente na fonte dos basaltos alto-TiO2 e baixo-TiO2, dominando as composições isotópicas de ósmio. Os outros dois componentes enriquecidos (EM-I e EM-II) seriam responsáveis pelas variações nas composições isotópicas de Sr-Nd-Pb. Na gênese dos basaltos alto-TiO2 provavelmente ocorreu envolvimento de crosta continental inferior delaminada (EM-I), enquanto nas rochas baixo-TiO2 há indicação de participação de crosta continental superior reciclada (EM-II). O componente empobrecido possui composição semelhante à de peridotitos de arco, sendo que essa assinatura isotópica pode ter sido gerada por processos metassomáticos relacionados a processos de subducção neoproterozóicas, que teriam modificado o manto astenosférico (DMM). No processo de aglutinação do Gondwana esse manto astenosférico metassomatizado pode ter sido incorporado à base da litosfera, sofrendo posterior refusão, por ocasião da atividade ígnea da PMP.

Metamorfismo ercinico di bassa-pressione: evoluzione tettonico-metamorfica del complesso di mandatoriccio (massiccio della Sila - Calabria)

Low-pressure/high-temperature (LP/HT) metamorphic belts are characterised by rocks that experienced abnormal heat flow in shallow crustal levels (T > 600 °C; P < 4 kbar) resulting in anomalous geothermal gradients (60-150 °C/km). The abnormal amount of heat has been related to crustal underplating of mantle-derived basic magmas or to thermal perturbation linked to intrusion of large volumes of granitoids in the intermediate crust. In particular, in this latter context, magmatic or aqueous fluids are able to transport relevant amounts of heat by advection, thus favouring regional LP/HT metamorphism. However, the thermal perturbation consequent to heat released by cooling magmas is responsible also for contact metamorphic effects. A first problem is that time and space relationships between regional LP/HT metamorphism and contact metamorphism are usually unclear. A second problem is related to the high temperature conditions reached at different crustal levels. These, in some cases, can completely erase the previous metamorphic history. Notwithstanding this problem is very marked in lower crustal levels, petrologic and geochronologic studies usually concentrate in these attractive portions of the crust. However, only in the intermediate/upper-crustal levels of a LP/HT metamorphic belt the tectono-metamorphic events preceding the temperature peak, usually not preserved in the lower crustal portions, can be readily unravelled. The Hercynian Orogen of Western Europe is a well-documented example of a continental collision zone with widespread LP/HT metamorphism, intense crustal anatexis and granite magmatism. Owing to the exposure of a nearly continuous cross-section of the Hercynian continental crust, the Sila massif (northern Calabria) represents a favourable area to understand large-scale relationships between granitoids and LP/HT metamorphic rocks, and to discriminate regional LP/HT metamorphic events from contact metamorphic effects. Granulite-facies rocks of the lower crust and greenschist- to amphibolite-facies rocks of the intermediate-upper crust are separated by granitoids emplaced into the intermediate level during the late stages of the Hercynian orogeny. Up to now, advanced petrologic studies have been focused mostly in understanding P-T evolution of deeper crustal levels and magmatic bodies, whereas the metamorphic history of the shallower crustal levels is poorly constrained. The Hercynian upper crust exposed in Sila has been subdivided in two different metamorphic complexes by previous authors: the low- to very low-grade Bocchigliero complex and the greenschist- to amphibolite-facies Mandatoriccio complex. The latter contains favourable mineral assemblages in order to unravel the tectono-metamorphic evolution of the Hercynian upper crust. The Mandatoriccio complex consists mainly of metapelites, meta-arenites, acid metavolcanites and metabasites with rare intercalations of marbles and orthogneisses. Siliciclastic metasediments show a static porphyroblastic growth mainly of biotite, garnet, andalusite, staurolite and muscovite, whereas cordierite and fibrolite are less common. U-Pb ages and internal features of zircons suggest that the protoliths of the Mandatoriccio complex formed in a sedimentary basin filled by Cambrian to Silurian magmatic products as well as by siliciclastic sediments derived from older igneous and metamorphic rocks. In some localities, metamorphic rocks are injected by numerous aplite/pegmatite veins. Small granite bodies are also present and are always associated to spotted schists with large porphyroblasts. They occur along a NW-SE trending transcurrent cataclastic fault zone, which represents the tectonic contact between the Bocchigliero and the Mandatoriccio complexes. This cataclastic fault zone shows evidence of activity at least from middle-Miocene to Recent, indicating that brittle deformation post-dated the Hercynian orogeny. P-T pseudosections show that micaschists and paragneisses of the Mandatoriccio complex followed a clockwise P-T path characterised by four main prograde phases: thickening, peak-pressure condition, decompression and peak-temperature condition. During the thickening phase, garnet blastesis started up with spessartine-rich syntectonic core developed within micaschists and paragneisses. Coevally (340 ± 9.6 Ma), mafic sills and dykes injected the upper crustal volcaniclastic sedimentary sequence of the Mandatoriccio complex. After reaching the peak-pressure condition (≈4 kbar), the upper crust experienced a period of deformation quiescence marked by the static overgrowths of S2 by Almandine-rich-garnet rims and by porphyroblasts of biotite and staurolite. Probably, this metamorphic phase is related to isotherms relaxation after the thickening episode recorder by the Rb/Sr isotopic system (326 ± 6 Ma isochron age). The post-collisional period was mainly characterised by decompression with increasing temperature. This stage is documented by the andalusite+biotite coronas overgrown on staurolite porphyroblasts and represents a critical point of the metamorphic history, since metamorphic rocks begin to record a significant thermal perturbation. Peak-temperature conditions (≈620 °C) were reached at the end of this stage. They are well constrained by some reaction textures and mineral assemblages observed almost exclusively within paragneisses. The later appearance of fibrolitic sillimanite documents a small excursion of the P-T path across the And-Sil boundary due to the heating. Stephanian U-Pb ages of monazite crystals from the paragneiss, can be related to this heating phase. Similar monazite U-Pb ages from the micaschist combined with the lack of fibrolitic sillimanite suggest that, during the same thermal perturbation, micaschists recorded temperatures slightly lower than those reached by paragneisses. The metamorphic history ended with the crystallisation of cordierite mainly at the expense of andalusite. Consequently, the Ms+Bt+St+And+Sill+Crd mineral assemblage observed in the paragneisses is the result of a polyphasic evolution and is characterised by the metastable persistence of the staurolite in the stability fields of the cordierite. Geologic, geochronologic and petrographic data suggest that the thermal peak recorded by the intermediate/upper crust could be strictly connected with the emplacement of large amounts of granitoid magmas in the middle crust. Probably, the lithospheric extension in the relatively heated crust favoured ascent and emplacement of granitoids and further exhumation of metamorphic rocks. After a comparison among the tectono-metamorphic evolutions of the different Hercynian crustal levels exposed in Sila, it is concluded that the intermediate/upper crustal level offers the possibility to reconstruct a more detailed tectono-metamorphic history. The P-T paths proposed for the lower crustal levels probably underestimate the amount of the decompression. Apart from these considerations, the comparative analysis indicates that P-T paths at various crustal levels in the Sila cross section are well compatible with a unique geologic scenario, characterized by post-collisional extensional tectonics and magmas ascent.

Petrographie und Geochemie des Eckfelder Maares und von Hillscheid und die Einbindung in den Hocheifelvulkanismus

Tertiäre Vulkanite aus dem Eckfelder Maar, dem Hillscheider Diatrem und dem Hillscheid Basalt (Schlot) wurden petrologisch und geochemisch untersucht. Bis auf tonige Klasten aus dem Bohrkern des Eckfelder Maares handelt es sich bei allen weiteren Proben um undifferenzierte basische Vulkanite. Die tonigen Klasten aus dem Bohrkern müssen der ehemaligen Landoberfläche vor der Eruption des Eckfelder Maares zugerechnet werden, in dessen Krater sie während der Eruption hineingefallen sind. Bis auf die Proben des Hillscheid Basaltes sind die Proben alteriert. Die Alteration zeigt sich an der Bildung von Zeolithen und Calcitmineralisationen, die primär und sekundär gebildete Hohlräume aufgefüllt haben oder an einer vertonten Grundmasse der Proben, die daneben Mineraleinschlüsse (Spinell) und kantige Fremdgesteinsbruchstücke enthalten können. Bei den Proben mit vertonter Grundmasse handelt es sich um Palagonite, Umwandlungsprodukte aus Sideromelan (basaltischem Glas). Geochemische Analysen an Grundmassepräparaten der alterierten bis vertonten Proben zeigen, dass außer den immobilen Elementen Ti, Nb, Zr, Y alle weiteren Elemente teilweise bis vollständig abgereichert worden sind. Eine Ausnahme bildet Barium (Ba), welches z.T. in beträchtlichen Mengen in Zeolithen (Harmotom) angereichert wurde. Bei den Proben aus dem Eckfelder Maar kann die Alteration bis Vertonung der Proben alleine mit der Palagonitisierung und Verwitterung erklärt werden. Es gibt keine Hinweise auf Materialzufuhr und damit für sich anschließende hydrothermale Prozesse. Die Proben des Hillscheider Diatrem sind wesentlich geringer alteriert (glasige Grundmasse). Neuste Erkenntnisse aus einer Bohrung im Sommer 1999 im vermuteten Zentrum des Hillscheider Diatrems beschränken das Diatrem maximal auf einen kleineren Bereich im Nordosten der bisherigen Lokation. Bei der Bohrung stieß man nach 20 Meter auf Anstehendes. Im Hangschutt darüber fand man Blöcke des Hillscheid Basaltes. Eine geringere Größe der Lokation zusammen mit der geringen Alteration könnten auf deren Entstehung mit einer initialen Maarphase gefolgt von Schlackentätigkeit hinweisen. Die Schlacken könnten die ersten Ablagerungen vor Verwitterung geschützt haben. Allerdings gibt es keine Funde die eine Schlackentätigkeit belegen. Beim sogenannten 'Hillscheider Diatrem' könnte es sich aber auch um Hangschutt aus der Randbreccie des Hillscheider Basaltes handeln. Zusammen mit Bruchstücken aus dem Schlot des Hillscheid Basaltes wären die Palagonite des sogenannten 'Hillscheider Diatrem' erst in jüngster Zeit im Bereich einer Uferböschung zur Ablagerung gekommen. Dies würde allerdings das sogenannte 'Hillscheider Diatrem' in seiner Existenz in Frage stellen. Vergleiche der Proben des Hillscheid Basaltes mit basischen Hocheifelvulkaniten deuten auf kogenetische Beziehung aller Proben untereinander und ordnen die Proben des Hillscheid Basaltes geochemisch dem Hocheifelvulkanismus zu. REE- und weitere Spurenelementgehalte und auch deren Elementverhältnisse weisen für alle tertiären Eifelvulkanite gemeinsam auf Mantelschmelzen aus dem Bereich eines Granatperidotits mit niedrigen Aufschmelzgraden um ein Prozent hin. Vergleich der Elementverhältnisse hochinkompatibler Elemente im Bezug auf die Bildung mafischer Schmelzen mit primitivem Mantel deuten darauf hin, dass der Mantel im Bereich der Hocheifel verarmt ist an K, Rb, Sr und angereichert an Ba und eventuell an Nb. Ursachen für diese von typisch primären Mantelzusammensetzung abweichenden Verhältnisse könnten durch Mischungen von Mantelschmelzen mit lithosphärischem Mantel (K-Anomalie) und durch Anreicherungen mit fluiden Phasen (Ba-Anomalie) oder auch Schmelzen aus einem tieferliegenden Plume (Kelberger Hoch) verursacht worden sein. Englischer Zusammenfassung:

Geochemie, Geochronologie und Isotopie der Grundgebirgsgneise und ausgewählter Marmore der zentralen Kykladen, Griechenland

Das Attisch-Kykladische Massiv ist Teil der Helleniden, einem Gebirgszug im östlichen Mittelmeerraum zwischen Alpen und Himalaya. Seit dem späten Mesozoikum unterlag diese Region dem Einfluß der alpidischen Orogenphasen. Die vorliegende Arbeit befasst sich mit den präalpidischen Grundgebirgseinheiten der Kykladen, die durch grünschieferfaziell überprägte Gneise und Marmore repräsentiert werden. Diese wurden geochemisch und isotopisch analysiert. Geochemisch stellen sie granitische und granodioritische Orthogneise subalkalinen, peraluminösen Charakters dar. Ihre Haupt- und Spurenelementsignaturen deuten auf die Existenz separater Schmelzen ähnlicher krustaler Zusammensetzung hin. Dies reflektiert auch die Rb/Sr- und Sm/Nd-Isotopie mit Errorchronenaltern von ca. 310 Ma. Die 87Sr/86Sri-Zusammensetzung besitzt einen rel. niedrigen Mittelwert von 0.7072±0.0019. Das 143Nd/144Ndi-Verhältnis von 0.51187±0.00011 korrespondiert mit leicht erhöhten eNd-Werten zwischen -8 und –7, was auf komplex zusammengesetzte Ausgangsschmelzen deutet (Hybridtyp "Hs" ). Zirkonanalysen liefern magmatische Intrusionsalter von 300 Ma, ererbte Körner bis zu 2305±1 Ma, was die Beteiligung alter Kontinentalkruste belegt. Das Tectonic Setting befindet sich im aktiven Kontinentalrand und wird als VAG klassifiziert. Ausgewählte Marmore von Naxos weisen ein relativ niedriges 87Sr/86Sr-Verhältnis von 0.707295±0.000012 auf, das mit dem des Meerwassers während des Mittleren Juras übereinstimmt und mit dem Pb/Pb-Isochronenalter von 172±17 Ma korreliert

Geocronologia u-pb del Plutone della Valle del Cervo (Biella)

I complessi intrusivi alcalini, e i loro prodotti di alterazione, costituiscono una delle principali risorse di materie prime ad alto valore economico. Il magmatismo alcalino è di solito associato a fenomeni di rifting continentale che può essere legato o ad un’incipiente divergenza di placche litosferiche (Rift dell’Africa Orientale) o ad eventi distensivi post-collisionali. La caratterizzazione petrografica, geochimica e geocronologia di questi complessi sono prerequisiti essenziali per ogni progetto di prospezione mineraria. In Italia uno dei complessi alcalini più conosciuti è il Plutone Oligocenico della Valle del Cervo (Biella). La petrografia e la composizione degli elementi maggiori e in traccia dei diversi litotipi che lo compongono sono conosciuti nella letteratura geologica. Le datazioni radiometriche disponibili, tuttavia, sono limitate a dati Rb-Sr su separati di biotite e a più recenti dati U-Pb su zircone separato dal piccolo nucleo granitico del Plutone Oligocenico della Valle del Cervo. In entrambi i casi sono state ottenute età radiometriche di circa 30 Ma. Questo lavoro di tesi si basa sull’analisi degli isotopi U-Pb di zirconi separati dalla sienite e dal monzogranito a megacristalli di ortoclasio che costituiscono gran parte del Plutone Oligocenico della Valle del Cervo. Dopo una caratterizzazione petrografica e geochimica dei campioni scelti si è proceduto alla separazione degli zirconi tramite i metodi di routine che includono separazione gravimetrica, magnetica e liquidi pesanti. I rapporti isotopici U-Pb degli zirconi sono stati determinati presso l’Istituto di Geoscienze e Georisorse, Unità di Pavia, usando la tecnica di spettrometria di massa accoppiata ad una microsonda ad ablazione laser (LA ICP-MS). Le età U-Pb medie determinate per la sienite (33 Ma) e per il monzogranito a megacristalli di ortoclasio (29 Ma) sono da considerarsi età magmatiche relative alla cristallizzazione dei fusi che diedero origine a queste rocce.

Petrology and geochronology of "muscovite age standard" B4M

Muscovite B4M, distributed in 1961 as an age standard, was ground under ethanol. Five grain size fractions were obtained and characterized by X-ray diffraction. They display a mixing trend between a phengitic (enriched in the fraction <0.2 µm) and a muscovitic component (predominant in the fraction >20 µm). High-pressure phengite is preserved as a relict in retrograde muscovite. Electron microprobe analyses of the distributed mineral separate reveal at least four white mica populations based on Si, Al, Mg, Na, Fe and F. Rb/K ratios vary by one order of magnitude. Rb–Sr analyses link the mineralogical heterogeneity to variable Rb/Sr and 87Sr/86Sr ratios. The grain size fractions define no internal isochron. Relict fine-grained phengite gives older ages than coarse-grained retrograde greenschist facies muscovite. The inverse grain size–age relationship also characterizes 39Ar/40Ar analyses. Cl/K anticorrelates with step ages: Cl-rich coarse muscovite is younger than Cl-poor fine relict phengite. Sr and Ar preserve a similar isotopic inheritance despite peak metamorphism reaching 635±20 °C. A suitable mineral standard requires that its petrological equilibrium first be demonstrated. Relicts and retrograde reaction textures are a guarantee of isotopic disequilibrium and heterogeneous ages within single crystal at the micrometre scale.

Geochronology and isotope geochemistry of Eocene dykes intruding the Ladakh Batholith

In order to determine the extent and timing of dyke formation in the Ladakh Batholith we examined about 30 mostly andesitic dykes intruding the Ladakh batholith in a ca. 50 km wide area to the west of Leh (NW India). The dykes in the east of the area trend E-NE and those in the west trend N-NW. The difference in orientation is also evident in the petrography and isotopic signatures. The eastern dykes contain corroded quartz xenocrysts and show negative ε0(Nd) and positive ε0(Sr) values, where as the western dykes do not contain quartz xenocrysts and exhibit positive ε0(Nd) and near-zero ε0(Sr) values. The variability in Sr-Nd isotopes (ε0(Nd) = 3.6 to −9.6, ε0(Sr) = 0.4 to 143) and the quartz xenocrysts can best be explained by (differing degrees of) crustal assimilation of the parent magma of the dykes. Separated minerals from five dykes were dated by 40Ar-39Ar incremental heating: amphibole ages range between 50 and 54 Ma, and one biotite dated both by Rb-Sr and by 40Ar-39Ar gave an age of 45 Ma. One dated pseudotachylyte sample attests to brittle faulting at ca. 54 Ma. The combination of structural field evidence with petrographic, isotopic and geochronological analyses demonstrates that the dykes did not form from a single, progressively differentiating magma chamber, despite having formed in the same tectonic setting around the same time, and that processes such as crustal assimilation and magma mixing/mingling also played a significant role in magma petrogenesis.

Sand geochemistry on sediment core CRP-3 from the Ross Sea, Antarctica

A total of 167 samples distubuted throughout the CRP-3 drillhole from 5.77 to 787.68 mbsf and representing fine to coarse sandstones have been analysed by X-ray fluorescence spectrometry (XRF) Bulk sample geochemistry (major and trace elements) indicates a dominant provenance of detritus from the Ferrar Supergroup in the uppermost 200 mbsf of the core. A markedly increased contribution from the Beacon sandstones is recognized below 200 mbsf and down to 600 mbsf. In the lower part of CRP-3, down to 787.68 mbsf, geochemical evidence for influxes of Ferrar materials is again recorded. On the basis of preliminary magnetostratigraphic data reported for the lower 447 mbsf of the drillhole, we tentatively evaluated the main periodicities modulating the geochemical records. Our results identify a possible influence of the precession, obliquity and long-eccentricity astronomical components (21, 41, and 400 ky frequency bands) on the deposition mechanisms of the studied glaciomarine sediments.

(Table 2) Trace element composition of seleceted clast and matrix materials from ODP Site 125-786

Shipboard examination of volcanic and sedimentary strata at Site 786 suggested that at least four types of breccias are present: flow-top breccias, associated with cooling and breakup on the upper surface of lava flows; autobreccias, formed by in-situ alteration at the base of flows; fault-gouge breccias; and true sedimentary breccias derived from weathering and erosion of underlying flows. It is virtually impossible to assess the origin of breccia matrix by textural and mineralogical analyses alone. However, it is fundamental for our understanding of breccia provenance to determine the source component of the matrix material. Whether the matrix is uniquely clastderived can be determined by geochemical fingerprinting. Trace elements that are immobile during weathering and alteration do not change their relative abundances. A contribution to the matrix from any source with an immobile trace element signature different from that of the clasts would appear as a perturbation of the trace element signature of the matrix. Trace element analysis of bulk samples from clasts and matrix material in individual breccia units was undertaken in a fashion similar to that used by Brimhall and Dietrich (1987, doi:10.1016/0016-7037(87)90070-6) in analyzing soil provenance: (1) to help distinguish between sedimentary and volcanic breccias, (2) to determine the degree of mixing and depth of erosion in sedimentary breccias, and (3) to analyze the local provenance of the individual breccia components (matrix and clasts). The following elements were analyzed by X-ray fluorescence (XRF): Rb, Sr, Ba, U, Zr, Cu, Zn, Ti, Cr, and V. Of these elements, Zr and Ti probably exhibit truly immobile behavior (Humphris and Thompson, 1978, doi:10.1016/0016-7037(78)90222-3 ). The remaining elements are useful as a reference for the extent of compositional change during the formation of matrix material (Brimhall and Dietrich, 1987, doi:10.1016/0016-7037(87)90070-6).

Geochemistry of ODP Leg 135 vitrophyric rhyolite samples

In-situ proton-microprobe analyses are presented for glasses, plagioclases, pyroxenes, olivines, and spinels in eleven samples from Sites 834-836, 839, and 841 (vitrophyric rhyolite), plus a Tongan dacite. Elements analyzed are Mn, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Pb, and Sn (in spinels only). The data are used to calculate two sets of partition coefficients, one set based on the ratio of element in mineral/element in coexisting glass. The second set of coefficients, thought to be more robust, is corrected by application of the Rayleigh fractionation equations, which requires additional use of modal data. Data are presented for phenocryst core-rim phases and microphenocryst-groundmass phases from a few samples. Comparison with published coefficients reveals an overall consistency with those presented here, but with some notable anomalies. Examples are relatively high Zr values for pyroxenes and abnormally low Mn values in olivines and clinopyroxenes from Site 839 lavas. Some anomalies may reflect kinetic effects, but interpretation of the coefficients is complicated, especially in olivines from Sites 836 and 839, by possible crystal-liquid disequilibrium resulting from mixing processes.