排序方式： 共有13条查询结果，搜索用时 0 毫秒
11.
The Mineralogy and Petrology of Mount Suswa, Kenya 总被引：1，自引：0，他引：1
Mount Suswa, a Quaternary volcano in the Rift Valley of Kenya,is composed of sodalite-trachytes, sodalite-phonolites, andphonolites, the majority of which are mildly peralkaline. Thelavas are predominantly feldspathic with phenocrysts of alkalifeldspar and, less commonly, slightly sodic augite and fayaliticolivine. The groundmass typically contains alkali feldspar,augite, titanomagnetite, and may in addition contain sodalite,nepheline, alkali amphibole, aenigmatite, and glass. The lavasof four stratigraphically distinct episodes can be distinguishedon the basis of mineralogy and chemical composition. These lavasare the products of at least three parental magmas, none ofwhich appears to be a derivative of the other. Each ‘magmatype’ represents an independent episode of magma generation,emplacement, and eruption. The order of eruption in the finalepisode corresponds to increasing peralkalinity and undersaturationwith respect to silica, and indicates that these lavas weregenerated via the tapping of a differentiating magma, with thefirst lavas being the least differentiated. Utilizing coexistingfeldspar, residual glass, and bulk rock compositions, the derivationof peralkaline phonolitic residual liquids from a trachyticparent is shown to be a process controlled by feldspar fractionation. 相似文献
12.
13.
The Micho?ch-Guanajuato Volcanic Field (MGVF) of central Mexicocontains 900 cinder and lava coes but lacks the large activecomposite volcanoes found in other portions of the Mexican VolcanicBelt (MVB). Scoriae and lavas from these cinder cones are primarilyolivine-basalts and olivine-andesites containing phenocrystsof olivine (plus Cr-rich spinel inclusions), plagioclase, and,less frequently, augite; pyroxene- and hornblende-andesitesare subordinate. Most samples are calcalkaline; however, alkalineand transitional rocks are also found. Compositional variationat individual cones is usually less than 5 per cent SiO2 andat Volc?n Paricutin (1943–1952) and Volc?n Jorullo (1759–1774),lava compositions have become more silica-rich with time. Alkaline cinder cones are generally older, but in the late Quaternary,both calc-alkaline and alkaline magmas erupted in the southernpart of the MGVF. Positive correlations between K, Zr, and Baand distance from the Middle America trench are distinct forevolved lavas; no correlations are found for less differentiatedlavas. In contrast, a correlation between decreasing Mg, Ni,and Cr and distance from the trench is found. In comparison to composite volcanoes in the MVB, the cinder-conelava are typically more basic. Four samples have mg-numbersand Ni contents which indicate possible mantle source regions.These samples include calc-alkaline, transitional and alkalinelavas, but all contain phenocrysts and/or microphenocrysts ofolivine, augite, and plagioclase; in these high-Mg lavas, spinelinclusions in olivine are Cr-rich. Those high-Mg lavas withsmall amounts of coexisting olivine, augite, and plagioclasephenocrysts plot close to a high-pressure (8 kb ? H2O) 0l-Aug-Plcotectic. Others project between this high-pressure clusterand the 1 atm. cotectic, indicating polybaric fractionation.Low-Mg lavas in the northern part of the MGVF result from fractionationat relatively shallow depths. Estimated olivine equilibrium temperatures decrease from about1200?C with increasing FeO/FeO + MgO, which is also accompaniedby an increase in H2O. Relative oxygen fugacities (relativeto NNO) calculated for lavas with Fe2O3+FeO show that NNO increasessystematically during an eruption, and this is well displayedat both Paricutin and Jorullo. The more oxidized lavas may containhornblende, and do so at Colima. The calc-alkaline lavas fromthroughout the MGVF only span the redox state of the Jorulloeruption, and all these continental magmas are 2–3 ordersof magnitude more oxidized than their submarine counterparts. Petrographic and mineralogical evidence supports the absenceof long-lived shallow magma reservoirs, consistent with theobserved small magma output rate in the MGVF. 相似文献