High Field Strength Element Enrichment of Pliocene--Pleistocene Island Arc Basalts, Zamboanga Peninsula, Western Mindanao (Philippines)

The Pliocene—Pleistocene magmatic activity of the Zamboangaare is linked to the southward subduction of the Oligocene—Mio-ceneSulu Sea back-are basin along the Sulu Trench. The magmaticproducts include small amounts of adakites dated from 3.8 to0.7 Ma, abundant Nb-enriched basalts and basaltic andesites(NEB) dated from 2 to 1 Ma and a lone calc-alkaline potassicbasaltic andesite dated at 0.4 Ma. Three kinds of NEB are distinguished:nearly primitive Mg-rich (MG) basalts displaying positive orno Nb anomalies with respect to adjacent incompatible elementsand more evolved low-K (LK) and calc-alkaline (CA) lavas which,despite their Nb enrichment, display negative Nb anomalies.Although the role of OIB-type mantle components has been advocatedto explain the HFSE enrichment of NEB, the spatial and temporalassociation of these rocks with adakites suggests a petrogeneticlink between them. Trace element characteristics of the NEBimply that amphibole and ilmenite might be present in theirsource. We suggest that these minerals could be added metasomaticallyto the mantle through hybridization by percolating slab melts,during which Nb and Ti are preferentially extracted from theadakitic melts. In an early stage (4–3 Ma) of the subductionof the young and hot Sulu Sea basin crust beneath the Zamboangapeninsula, adakitic liquids formed at depths of 75–85km. A few of them were emplaced at the surface but most wereconsumed through slab melt-mantle metasomatic reactions. Adakiteproduction and emplacement continued later (<2 Ma), whilethe Nb-enriched mantle was brought by convection to depths thatallowed its melting and the subsequent emplacement of NEB behindthe adakitic front of the Zamboanga are. KEY WORDS: adakite; metasomatism; Mindanao; Nb-enriched basalts; subduction ^*Corresponding author. Present address: Mines and Geosciences Bureau, North Avenue, Diliman, 1100 Quezon City, Philippines     

Controls on the alluviation of oxbow lakes by bed‐material load along the Sacramento River,California

Differences in the nature and quantity of sediment filling oxbow lakes have significant implications for the evolution of meandering rivers and the development of floodplains, influencing rates of meander migration and the valley width over which migration takes place. In an effort to identify the controls on the alluviation of oxbow lakes by coarse bed material, this study examined the sedimentary records stored within oxbow lakes of the Sacramento River of California, USA, and found that the volume of gravel in storage correlated negatively with the diversion angle separating flow between the river channel and the entrance into each lake. A method was devised for estimating the original channel bathymetry of the studied lakes and for modelling the hydraulic and sediment‐transport effects of the diversion angle within channels recently abandoned by meander cut‐off. The diversion angle determines the width of a flow separation within the abandoned‐channel entrance, reducing the discharge diverted from the river channel and thus limiting the ability of the abandoned channel to transport bed material. Aggradation rates are faster within entrances to abandoned channels with high diversion angles, resulting in the rapid isolation of lakes that store only a small volume of coarse‐grained sediment. Aggradation rates are slower within channel entrances where diversion angles are low, resulting in the slow transitioning of such channels into oxbow lakes with a larger and more extensive accumulation of coarse‐grained sediment. These findings compare well with observations in other natural settings and the mechanism which is described for the control of the diversion may explain why some oxbow lakes remain as open‐water environments for centuries, whereas others are filled completely within decades of cut‐off.     

Dacite Genesis via both Slab Melting and Differentiation: Petrogenesis of La Yeguada Volcanic Complex, Panama

La Yeguada volcanioc complex (LYVC) is one of many major volcanoesthat represent the extension of the Central American arc inwestern Panama and that have resulted from current oblique subductionsouth of Panama. There are two major phases of calc-alkalinevolcanic activity at LYVC based on mapping and K-Ar radiometricdates. The first phase began at {small tilde} 13 Ma and ceasedat {small tilde} 7?5 Ma. This sequence, termed the old group,consists of basalts to rhyolites with typical arc mineralogies(OL, CPX, PL, MGT, and OPX). The samples have similar radiogenicSr and Nd values and appear to be related by fractional crystallizationwith assimilation and/or magma mixing involved in the differentiation.The parental basalts were probably derived from the metasomatizedmantle wedge via melting induced by fluids released from thesubducted lithosphere. There was an apparent period of minor volcanic activity from7–5 to 2–5 Ma (only one documented sample from thisperiod). The second phase (<2?5 Ma), termed the young group,consists only of dacites but with very different mineralogies(PL, MGT, AM, BI, with no PX) and geochemistries (e.g., highSr and low Y and HREE) compared with the old-group dacites (andandesites and rhyolites). The dacites cannot be related to theold group by various petrogenetic modeling techniques. Thesehigh-Al dacites have the characteristics of magmas derived fromthe partial melting of the subducted oceanic lithosphere witha hornblende eclogite residuum. This has been substantiatedby geochemical modeling. Samples similar to the young-group dacites in other arcs havebeen termed adakites and arc associated with the subductionof young hot crust which may explain why the slab melts. ThePanama basin has extremely high heat flow values, comparablewith those of the Galapagos ridge system. The change from normalarc volcanism to adakites suggests that the subducted oceaniccrust became hotter as time progressed. The subduction of anoceanic ridge or new ridge development along the Sandra Riftin the Panama basin can explain the change in volcanism withtime but more geophysical data are needed.     

Petrogenesis of Archaean Trondhjemites, Tonalites, and Granodiorites from Eastern Finland: Major and Trace Element Geochemistry

The grey gneisses of eastern Finland form the basement on whichthe Archaean greenstone belts were developed. They are composedof orthogneisses emplaced during two distinct magmatic episodes:2.86 Ga (Kivij?rvi gneisses) and 2.65 (Naavala gneisses). Theirmodal and chemical compositions are those of trondhjemites,tonalites and granodiorites (TTG). Both suites show low-K₂Ocalc-alkaline differentiation trends (trondhjemitic). The aim of this study is to qualify and quantify the successionof different mechanisms by which the TTG series evolved. Theyoungest process was studied first, and the arguments then appliedin order to go back in time to the older ones. For each one,quantification was arrived at with the major elements, and theseresults provided a basis for calculation with the rare earthelements (REE). Finally the whole model was tested with othertrace elements. The petrogenetic model may be summarized as follows: meltingof the upper mantle to form a tholeiitic crust; melting of thesetholeiites transformed into garnet-bearing amphibolites to yieldthe parental magma of the TTG. The residue of the melt consistedof hornblende, plagioclase, clinopyroxene, and garnet with minoramounts of ilmenite and magnetite (10 < F < 30); and fractionalcrystallization of hornblende, plagioclase, and ilmenite withoccasional allanite and/or zircon in small amounts ((1-F) <40). No matter when they were emplaced during the Archaean, all theTTG of this part of the Baltic Shield arose from similar parentalmagmas. The petrogenetic study has shown that garnet and hornblendewere necessarily residual phases during the melting of the Archaeantholeiites. This constraint is very important, as it impliesthat the Archaean geothermal gradients occurring in subduction-zoneswere much higher than in modern times, thus allowing the partialmelting of the subducted oceanic crust.