An Example of Island-Arc Petrogenesis: Geochemistry and Petrology of the Southern Luzon Arc, Philippines

Volcanism throughout the Luzon arc is associated with eastwardsubduction of the South China Sea floor along the Manila Trench.The southern section of the arc, the focus of this study, extendsfrom the Lingayen-Dingalan fault to the small islands just southof Luzon. Two segments appear to exist along this section ofthe arc the northern Bataan and southern Mindoro segments whichare separated by the Macolod Corridor. The volcanic rocks have typical arc phenocryst mineralogies:olivine, clinopyroxene, plagioclase, and titanomagnetite inthe most mafic rocks and clinopyroxene, plagioclase, orthopyroxene,titanomagnetite, ? amphibole in the more felsic samples. Complexzoning, sieve textures, and decoupling of incompatible traceelements suggest that processes such as assimilation have takenplace. The rocks from the study area range from basalts to rhyolitesand show typical calc-alkaline features. The rocks of the MacolodCorridor and Mindoro segment are particularly enriched in largeion lithophile elements (LILE), light rare earth elements (LREE),and radiogenic Sr compared with the Bataan segment. The datafall within the mantle array on Sr-Nd isotopic diagrams andgrade toward higher Sr and lower Nd isotopic values from northto south. A likely source for the volcanics of this study is either amid-ocean ridge basalt (MORB)-type mantle that undergoes higherdegrees of partial melting than regions involved in MORB generationor a previously depleted source. We suggest that the high fieldstrength element (HFSE) anomalies have been derived throughdifferential element partitioning during fluid transport fromthe subducted lithosphere to the mantle wedge. Continental crustal material seems to play a significant roleparticularly in the Macolod Corridor and the Mindoro segment,based on the high LILE, La/Sm ratios, radiogenic Sr isotopes,and ¹⁸O values. The Macolod Corridor and the Mindoro segmenthave undergone source contamination by crustal material fromthe North Palawan-Mindoro crustal block either during the collisionof this block with the Manila Trench or by subduction of sedimentsrich in this crustal material. A similar component has alsobeen detected in the Bataan segment but in minor amounts. Thetrace element and isotopic differences between the northernand southern sections of the arc are interpreted in terms ofvariable composition (i. e., variable amounts of a crustal componentintroduced from the Palawan-Mindoro crustal terrain) of themetasomatic fluids released into the source.     

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     

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.     

Na Metasomatism in the Island-Arc Mantle by Slab Melt--Peridotite Interaction: Evidence from Mantle Xenoliths in the North Kamchatka Arc

The Pliocene (7 Ma) Nb-enriched arc basalts of the ValovayamVolcanic Field (VVF) in the northern segment of the Kamchatkaarc, Russia, host abundant mantle xenoliths, including spinelIherzolites. Textural and microstructural evidence for high-temperature,multi-stage, creep-related deformations in spinel Iherzolitessupports a sub-arc mantle derivation. Pyroxene chemistry indicatesthe existence of two compositional suites: (1) a Cr-diopsidesuite with low-Tt, moderate-Al clinopyroxene compositions, and(2) an Al-augite suite with high Al and Tt, and low Cr concentrationsin clinopyroxene. Some spinel lherzolite xenoliths contain metasomaticAl-augite-type clinopyroxene, Al-Tt spinel, and felsic veinssimilar to trondhjemite melt. The Al-augite series xenolithstypically contain high-Na plagioclase, Cr-poor, Al-Fe-Mg andAl-Tt-Fe spinels, with occasional almandine-grossularite garnetand high-Al and -Na pargasitic amphibole. Pyroxene and spinel compositional trends suggest that the Crdiopsideseries xenoliths from the VVF Nb-enriched arc basalts representan island-arc mantle affected by a metasomatic event. Occurrenceof high-Na plagioclase and trondhjemitic veins favors the additionof a metasomatic component with high Na, Al and Si to the northernKamchatka arc mantle. Trondhjemitic veins, representing siliceousslab melts, compositionally exemplify the metasomatic component.Na metasomatism by peridotite-slab melt interaction is an importantmantle hybridization process responsible for arc-related alkalinemagma generation from a veined sub-arc mantle. KEY WORDS: metasomatism; island arc; mantle xenoliths; Kamchatka; mantle     

埃达克岩：关于其成因的一些不同观点

埃达克岩的概念是十多年前提出来的,指由俯冲的年轻洋壳熔融形成的火成岩。自从最初在现代岛弧近十几个地方报道埃达克岩以来,新近又在几个地方发现有埃达克岩（如日本西南部,外墨西哥火山岩带,等等）。但是,过去十 多年的研究也表明,埃达克岩可以由俯冲期间的其它过程产生（例如,沿俯冲板片的撕裂边,留在上地幔中的板片残余等）。另外,埃达克岩似乎与一些岩石呈共生组合,这些岩石包括高镁安山岩、富Nb的弦玄武岩（NEAB）,还可能有玻安岩（几个研究者已在玻安岩中发现有埃达克岩的组分）。高镇安山岩不是来自埃达克与地幔的相互作用（Adak-type),就是来自此相互作用期间地幔的熔融（Piip-type);富Nb的弦玄武岩,据认为是来自一种被埃达我岩广泛交代的地幔的部分熔融。作为一个新的岩套,埃达克岩交代火山岩系列已被建议用来解释各种岩石组合。此外,大量的富Pb弧玄武岩也已被发现包含有超镁铁质的地幔包体,而这些包体有高亏损地幔与埃达克反应的明显证据。关于主要与下地壳熔而不是冲板片有关的埃达克岩的起源已提出几种假说,一个模型认为,下地壳熔融出现在玄武质岩浆底侵下地壳时。但是,有许多理由似乎可以排除这种模式。另一种模型认为,在大陆地壳很厚的区域,下地壳可能变成榴辉岩,从而拆离并下沉到地幔中（拆沉）。这个拆沉过程将导致下地壳下中拆沉的下地壳的上部与相对热的地幔接触,进而可引起下地壳熔融和埃达克岩的形成。这使我们认为,在中国东部发现的与俯冲作用无关的白垩纪埃达克可能是下地壳熔融与拆沉作用的产物。我们 还要强调,如果下地壳熔融与拆沉作用真能形成埃达克岩,那么埃达克岩这一术语不应该仅仅局限于与板片熔融有关的过程,而应包括那些与下地壳熔融有关的过程。太古宙的大陆地壳主要由奥长花岗岩、英云闪长岩和英安岩（TTD）组成。这种大陆地壳是来自板片熔融还是下地壳熔融仍是有争议的。然而,我们认为,太古宙期间地幔的较高温度会导致较多的洋中脊的形成,从而产生比今天“更多”的年轻洋壳的俯冲。据此,我们认为,太古宙TTD大陆地宙主要由板片熔融形成。我们也注意到,太古宙是广泛金矿化的时期。有些研究者还发现,金和铜的矿化与埃达克质交代火山岩系列有关。因此,该火山岩系列可能会寻找金属矿床的一个重要标志。