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1.
浙江新昌复合式火成杂岩的岩石学、地球化学及其地质意义 总被引：3，自引：0，他引：3
浙江新昌复合式火成杂岩包括复合火山岩流和复合侵入杂岩。复合火山岩流由玄武岩和流纹岩/流纹质熔结凝灰岩组成,有的含少量安山质-英安质岩流和岩石包体;复合侵入杂岩由辉绿岩和花岗岩复合而成,含闪长质-石英闪长质岩石包体。它们在空间上紧密伴生,同位素年龄为96~113 Ma;地球化学上,它们统属钙碱性岩系,具有轻稀土元素和大离子亲石元素富集、高场强元素亏损等特点,稀土元素、微量元素配分型式基本相同,Sr=0.7069~0.7079,εNd (t ) = -2.3~-5.3,指示它们的微量元素和同位素发生过充分的交换。这些特点与浙闽沿海其他复合岩流和复合侵入杂岩的特点基本一致。它们形成在伸展构造背景,由起源于受消减作用影响的岩石圈地幔部分熔融而产生的玄武岩岩浆底侵,并与深熔的壳源花岗岩浆发生不同程度的岩浆混合而形成。新昌复合式火成杂岩的研究,为更深入研究中国东南部沿海地区晚中生代地球动力学环境变化和构造-岩浆作用提供了一个典型实例。 相似文献
2.
浙江沿海晚中生代拉斑玄武岩浆侵位深度的讨论 总被引：3，自引：0，他引：3
浙江沿海晚中生代复合岩流中拉斑玄武岩高度富集大离子亲石元素,反映除了源区富集作用外还有地壳混染的贡献,Rb、Nb的丰度特征指示玄武岩浆主要与中、上地壳岩石发生了相互作用。用岩石化学成分计算的这种拉斑玄武岩浆熔体的平均密度为ρm = 2.678 g/cm3 , 根据岩浆与周围地壳的压力平衡关系,推算得来自地幔的玄武岩浆一直侵位到距地表约16.3km左右的中地壳时才滞留下来,并被相应的地壳物质所混染。 相似文献
3.
河北寿王坟花岗闪长岩中暗色微粒岩石包体的成因 总被引：10，自引：1，他引：10
暗色微粒岩石包体可以提供花岗岩浆的起源和演化、壳—幔物质及能量交换、岩浆混合等重要信息。河北兴隆寿王坟闪长质暗色微粒岩石包体及其寄主花岗闪长岩的岩石学、岩石地球化学特征显示了岩浆的混合成因,并具有以下特征:(1)在基性岩浆中丰度大于酸性岩浆中的不相容元素在与SiO2的相关图中呈一向上凸的弧形型式;(2)在基性岩浆中丰度小于酸性岩浆中的不相容元素在与SiO2的相关图中呈正相关的线性趋势;(3)相容元素在与SiO2的相关图中呈一负相关的线性趋势;(4)在w(La) w(La)/w(Sm)图上微粒包体与其寄主花岗闪长岩为一下凹的型式。基性岩浆与酸性岩浆的两端元岩浆的简单混合模型不能解释以上特征,可以用三阶段混合模型来解释,即第一阶段为基性岩浆注入酸性岩浆中,第二阶段为基性岩浆演化并分离结晶形成微粒包体,第三阶段为演化了的基性岩浆与酸性岩浆混合,形成寄主花岗闪长岩。 相似文献
4.
浆混花岗岩专题填图方法初探——以东昆仑加鲁河地区为例 总被引：1，自引：1，他引：0
东昆仑造山带花岗岩中广泛发育暗色微粒包体,含有丰富的壳幔岩浆混合作用的证据,被认为是研究岩浆混合作用的天然场所。适逢近阶段同源花岗岩谱系填图方案在造山带岩浆混合(浆混)花岗岩图区实践时深受质疑,本研究以东昆仑加鲁河地区浆混花岗岩为例,开展浆混花岗岩区专题填图试点工作,旨在探索一套适合浆混花岗岩填图的岩石单位划分方案。从野外地质、岩相学、岩石和矿物化学等不同角度论证了加鲁河花岗闪长岩及其内部包体形成于开放体系下的壳幔岩浆混合作用。在填图工作中,将图区内的岩浆岩划分为浆混花岗岩和非浆混花岗岩2个超单元。以岩浆混合作用为理论依据,将浆混花岗岩超单元划分为基性端元、酸性端元和浆混产物3个二级单位,对于2个端元岩石单位按照其矿物组成、结构构造等方面的差异(岩浆演化导致)再次划分最基本岩石单位——侵入体,对于浆混产物单位,建议可按照岩浆混合程度差异或者内部包体变化规律灵活划分基本岩石单位——浆混体。由此建立了一套可与同源花岗岩谱系单位相兼容的浆混花岗岩谱系单位划分方案,为岩浆混合花岗岩区开展填图工作提供了初步探索方案。 相似文献
5.
对我国港边岩浆混合杂岩体进行了系统的地质学、岩石学和矿物学研究，并将其分解为偏酸性端元岩浆、基性端元岩浆、混合岩浆和非岩浆混合物４个单元．从不同的角度探讨了岩浆混合和分离结晶作用同时发生对岩浆混合岩成分的制约，提出存在化学和机械两种岩浆混合方式，证明了它们与基性岩浆的侵入一起构成一个连续的岩浆作用序列．最后，在野外地质研究的基础上，结合端元岩浆起源和混合岩浆结晶ｐ－Ｔ条件的估算及端元岩浆形成构造背景的判别综合提出港边岩浆混合杂岩体的构造－岩浆模型 相似文献
6.
对我国港边岩浆混合杂岩体进行系统的地质学、岩石学和矿物学研究，并将其分解为偏酸性端夺浆、基性端元岩浆、混合岩浆和非岩浆混合物４个单元，从不同的角度探讨了岩事和分离结晶作用同时发生对岩浆混合岩成分的制约，提出存在化学和机械两种岩交混合方式，证明了它们与基性岩侵放一起构成一个连续的岩交作用序列，最天野外造背景的判别综合提出港边港混合杂岩体的构造－岩浆模型。 相似文献
7.
微粒包体作为基性岩浆和重熔酸性岩浆I型或S型之间岩浆混合或/和混合期存在的标志,其成因和意义可被重新评价。基于含微粒包体花岗岩套中所观察到的地球化学特征和露头关系,已经提出了用于研究微粒包体和寄主岩组合的一种模式。该模式由三个阶段组成。第一阶段:注入作用;第二阶段:演化作用;第三阶段:混合作用,这是在基性岩浆注入到重熔的地壳环境中发生的。在第一阶段,比酸性岩浆温度更高、粘度更低、几乎完全为液态的基性岩浆通过一次或多次注入作用侵入到酸性岩浆中。两个体系并不容易混合,仍作为独立的单元存在,直至达到了热平衡和粘度相近的时候才能发生混合,并且,沿着二者的边界基性岩浆冷却。酸性岩浆被加热。在第二阶段,基性岩浆既经历了伸展、对流搅动和与酸性岩浆混合的物理作用,又经历了晶体分离和与酸性岩浆混染(混染分离结晶作用)的化学作用。这些物理和化学作用的再循环便导致了微粒包体和进一步演化了的熔体的形成。这种熔体与酸性岩浆已达到了热平衡。因此,微粒包体记载了基性岩浆演化的各个阶段。在第三阶段,基性岩浆进一步演化的产物(成分上为英云闪长质到花岗闪长质)和酸性岩浆参与了两端元的混合作用。该作用可用于解释含微粒包体花岗岩类深成岩的地球化学演化。该模式阐明了复式岩基形成过程中产于深成环境中的各种岩浆作用,并且也提出了有关英云闪长质深成岩成因的某些想法。最终,所观察到的微粒包体单独产出的这一特性表明微小几何学(一种相对新的数学概念)在了解粘性流体混乱流机制方面可能起到了决定性作用,即基性岩浆的混合和伸展作用动力学。 相似文献
8.
包体提供了花岗岩浆起源和演化的重要信息。花岗岩中富云母包体残留物的存在是大陆壳产生岩浆的极好证据。而暗色微粒包体的存在,表明有地幔物质的加入。在同一花岗岩中,两种包体的共存,表明其岩浆起源远不止一种。通过包体的研究就可以确定混合作用的条件。包体形状可提供混合作用位置与观察位置之间距离的信息。尽管发生过混合作用,但有时仍能识别山混合物基性组分的原始成分。对同一造山带中不同花岗岩的这种组分加以系统鉴定,有助于理解造山带中基性岩浆作用的时空变化。岩浆混合是引起花岗岩不均一性的原因之一,但当基性岩浆提供的热能产生新的混合组分,有时还导致新的侵入作用发生,岩浆混合又是使花岗岩均一化的重要因素。 相似文献
9.
混合岩(Hybrid rocks)之发生,据A.Harker等氏解释,系某种岩浆未完凝结以前,与另一成份不同之侵入岩浆,发生混合所致;又同一岩浆上升时期,吞噬与吸取围岩之包体(Xenoliths),结果亦可产生混合岩,此类现象,在闪长岩(diorite)中,通常显著,Appinite为混合岩之一种,最初在苏格兰发现、一般认为先期之基性岩浆或基性岩,与后来之酸性岩浆如花岗岩浆等发生作用而成,三十五年冬,作者等调查浙江北部地质,于诸暨一带之 相似文献
10.
位于吉林省安图县东南的石门岩体中，含有大量细粒、斑状闪长岩包体，通过对其形态、产状及特征，特别是包体的结构、矿物成分及矿物生长习性等岩相学特征的研究，初步确定石门岩体岩浆至少是由酸性、基性两种不同成分的岩浆在未完全结晶状态下相互混合形成，首次提出该岩体具岩浆混合花岗岩特征，为浆混成因花岗岩。 相似文献
11.
Basaltic lava flows and high-silica rhyolite domes form the Pleistocene part of the Coso volcanic field in southeastern California. The distribution of vents maps the areal zonation inferred for the upper parts of the Coso magmatic system. Subalkalic basalts (<50% SiO2) were erupted well away from the rhyolite field at any given time. Compositional variation among these basalts can be ascribed to crystal fractionation. Erupted volumes of these basalts decrease with increasing differentiation. Mafic lavas containing up to 58% SiO2, erupted adjacent to the rhyolite field, formed by mixing of basaltic and silicic magma. Basaltic magma interacted with crustal rocks to form other SiO2-rich mafic lavas erupted near the Sierra Nevada fault zone.Several rhyolite domes in the Coso volcanic field contain sparse andesitic inclusions (55–61% SiO2). Pillow-like forms, intricate commingling and local diffusive mixing of andesite and rhyolite at contacts, concentric vesicle distribution, and crystal morphologies indicative of undercooling show that inclusions were incorporated in their rhyolitic hosts as blobs of magma. Inclusions were probably dispersed throughout small volumes of rhyolitic magma by convective (mechanical) mixing. Inclusion magma was formed by mixing (hybridization) at the interface between basaltic and rhyolitic magmas that coexisted in vertically zoned igneous systems. Relict phenocrysts and the bulk compositions of inclusions suggest that silicic endmembers were less differentiated than erupted high-silica rhyolite. Changes in inferred endmembers of magma mixtures with time suggest that the steepness of chemical gradients near the silicic/mafic interface in the zoned reservoir may have decreased as the system matured, although a high-silica rhyolitic cap persisted.The Coso example is an extreme case of large thermal and compositional contrast between inclusion and host magmas; lesser differences between intermediate composition magmas and inclusions lead to undercooling phenomena that suggest smaller T. Vertical compositional zonation in magma chambers has been documented through study of products of voluminous pyroclastic eruptions. Magmatic inclusions in volcanic rocks provide evidence for compositional zonation and mixing processes in igneous systems when only lava is erupted. 相似文献
12.
A.
nal 《Geological Journal》2008,43(1):95-116
The Middle Miocene Orduzu volcanic suite, which is a part of the widespread Neogene Yamadağ volcanism of Eastern Anatolia, consists of a rhyolitic lava flow, rhyolitic dykes, a trachyandesitic lava flow and basaltic trachyandesitic dykes. Existence of mafic enclaves and globules in some of the volcanic rocks, and microtextures in phenocrysts indicate that magma mingling and mixing between andesitic and basaltic melts played an important role in the evolution of the volcanic suite. Major and trace element characteristics of the volcanic rocks are similar to those formed in convergent margin settings. In particular, incompatible trace element patterns exhibit large depletions in high field strength elements (Nb and Ta) and strong enrichments in both large ion lithofile elements (Ba, Th and U) and light rare earth elements, indicating a strong subduction signature in the source of the volcanic rocks. Furthermore, petrochemical data obtained suggest that parental magmas of rhyolite lava and dykes, and trachyandesite lava and basaltic trachyandesite dykes were derived from subduction‐related enriched lithospheric mantle and metasomatized mantle (± asthenosphere), respectively. A detailed mineralogical study of the volcanic suite shows that plagioclase is the principal phenocryst phase in all of the rock units from the Orduzu volcano. The plagioclase phenocrysts are accompanied by quartz in the rhyolitic lava flows and by two pyroxenes in the trachyandesitic lava flows and basaltic trachyandesitic dykes. Oxide phases in all rocks are magnetite and ilmenite. Calculated crystallization temperatures range from 650°C to 800°C for plagioclase, 745°C–1054°C for biotite, 888°C–915°C for pyroxene and 736°C–841°C for magnetite–ilmenite pairs. Calculated crystallization pressures of pyroxenes vary between 1.24–5.81 kb, and oxygen fugacity range from −14.47 to −12.39. The estimates of magmatic intensive parameters indicate that the initial magma forming the Orduzu volcanic unit began to crystallize in a high‐level magma chamber and then was stored in a shallow reservoir where it underwent intermediate‐mafic mixing. The rhyolitic lava flow and dykes evolved in relatively shallower crustal magma chambers. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
13.
浙江拔茅破火山岩浆作用：开放体系多机制复合演化 总被引：1，自引：3，他引：1
拔茅火山杂岩的成分变化范围广泛，包括玄武质，玄武粗安质，粗面安山质，粗面英安质，流纹英安质到流纹质和高硅流纹质岩石都有，它们不是由单一母岩浆演化而来，也不是由单一岩浆房喷发形成的，而是来自多种类型的岩浆房，并经历过复杂的演化过程，为开放体系多机制复合演化，其中早期双峰式基性端元是由上地幔部分熔融形成的，而酸性端无则是地壳物质边熔融边喷发（分离熔融）的结果，作为火山杂岩主体的粗面英安质岩石，其岩浆是在高位主岩浆房内由玄武质岩浆与流纹英安质岩浆混合形成的，发生混合的流纹英安质与玄武质岩浆的比例为57：43，而粗面安山质岩浆则是溶部岩浆房内由玄武质岩浆分离结晶形成的，晚期侵出－侵入的流纹英安质岩穹和高硅流纹斑岩株则分别代表高位次级岩浆房的成分及其硅质帽。 相似文献
14.
Evolution of Holocene Dacite and Compositionally Zoned Magma, Volcan San Pedro, Southern Volcanic Zone, Chile 总被引：1，自引：2，他引：1
Volcán San Pedro in the Andean Southern Volcanic Zone(SVZ) Chile, comprises Holocene basaltic to dacitic lavas withtrace element and strontium isotope ratios more variable thanthose of most Pleistocene lavas of the underlying Tatara–SanPedro complex. Older Holocene activity built a composite coneof basaltic andesitic and silicic andesitic lavas with traceelement ratios distinct from those of younger lavas. Collapseof the ancestral volcano triggered the Younger Holocene eruptivephase including a sequence of lava flows zoned from high-K calc-alkalinehornblende–biotite dacite to two-pyroxene andesite. Notably,hornblende–phlogopite gabbroic xenoliths in the daciticlava have relatively low 87Sr/86Sr ratios identical to theirhost, whereas abundant quenched basaltic inclusions are moreradiogenic than any silicic lava. The latest volcanism rebuiltthe modern 3621 m high summit cone from basaltic andesite thatis also more radiogenic than the dacitic lavas. We propose thefollowing model for the zoned magma: (1) generation of hornblende–biotitedacite by dehydration partial melting of phlogopite-bearingrock similar to the gabbroic xenoliths; (2) forceful intrusionof basaltic magma into the dacite, producing quenched basalticinclusions and dispersion of olivine and plagioclase xenocryststhroughout the dacite; (3) cooling and crystallization–differentiationof the basalt to basaltic andesite; (4) mixing of the basalticandesite with dacite to form a small volume of two-pyroxenehybrid andesite. The modern volcano comprises basaltic andesitethat developed independently from the zoned magma reservoir.Evolution of dacitic and andesitic magma during the Holoceneand over the past 350 kyr reflects the intrusion of multiplemafic magmas that on occasion partially melted or assimilatedhydrous gabbro within the shallow crust. The chemical and isotopiczoning of Holocene magma at Volcán San Pedro is paralleledby that of historically erupted magma at neighboring VolcánQuizapu. Consequently, the role of young, unradiogenic hydrousgabbro in generating dacite and contaminating basalt may beunderappreciated in the SVZ. KEY WORDS: Andes; dacite; gabbro; Holocene; strontium isotopes 相似文献
15.
Enrichment of basalt and mixing of dacite in the rootzone of a large rhyolite chamber: inclusions and pumices from the Rattlesnake Tuff, Oregon 总被引：3，自引：0，他引：3
A variety of cognate basalt to basaltic andesite inclusions and dacite pumices occur in the 7-Ma Rattlesnake Tuff of eastern
Oregon. The tuff represents ∼280 km3 of high-silica rhyolite magma zoned from highly differentiated rhyolite near the roof to less evolved rhyolite at deeper
levels. The mafic inclusions provide a window into the processes acting beneath a large silicic chamber. Quenched basaltic
andesite inclusions are substantially enriched in incompatible trace elements compared to regional primitive high-alumina
olivine tholeiite (HAOT) lavas, but continuous chemical and mineralogical trends indicate a genetic relationship between them.
Basaltic andesite evolved from primitive basalt mainly through protracted crystal fractionation and multiple cycles (≥10)
of mafic recharge, which enriched incompatible elements while maintaining a mafic bulk composition. The crystal fractionation
history is partially preserved in the mineralogy of crystal-rich inclusions (olivine, plagioclase ± clinopyroxene) and the
recharge history is supported by the presence of mafic inclusions containing olivines of Fo80. Small amounts of assimilation (∼2%) of high-silica rhyolite magma improves the calculated fit between observed and modeled
enrichments in basaltic andesite and reduces the number of fractionation and recharge cycles needed. The composition of dacite
pumices is consistent with mixing of equal proportions of basaltic andesite and least-evolved, high-silica rhyolite. In support
of the mixing model, most dacite pumices have a bimodal mineral assemblage with crystals of rhyolitic and basaltic parentage.
Equilibrium dacite phenocrysts are rare. Dacites are mainly the product of mingling of basaltic andesite and rhyolite before
or during eruption and to a lesser extent of equilibration between the two. The Rattlesnake magma column illustrates the feedback
between mafic and silicic magmas that drives differentiation in both. Low-density rhyolite traps basalts and induces extensive
fractionation and recharge that causes incompatible element enrichment relative to the primitive input. The basaltic root
zone, in turn, thermally maintains the rhyolitic magma chamber and promotes compositional zonation.
Received: 1 June 1998 / Accepted: 5 February 1999 相似文献
16.
Archean volcanic rocks in the Confederation Lake area, northwestern Ontario, Canada, are in three mafic to felsic cycles collectively 8,500 to 11,240 m thick. Each cycle begins with pillowed basalt and andesite flows and is capped with andesitic to rhyolitic pyroclastic rocks and minor flows. Seventy five samples from this succession were analyzed for major and trace elements including the rare earth elements. In two cycles, tholeiitic basalts are overlain by calcalkaline andesite to rhyolite. In the third, cycle, the tholeiitic basalts are overlain by tholeiitic rhyolites. Fe enrichment in basalts is accompanied by depletion of Ca, Al, Cr, Ni, and Sr, and enrichment in Ti, P, the rare earth elements, Nb, Zr, and Y. This is interpreted as open system fractionation of olivine, plagioclase, and clinopyroxene. Si enrichment in dacites and rhyolites is attributed to fractional crystallization of plagioclase, K-feldspar, and biotite. Tholeiitic basalt liquids are believed to be mantle-derived. Intercalated andesites with fractionated rare earth patterns appear to be products of mixing of tholeiitic basalt and rhyolite liquids and, andesites with flat rare earth patterns are probably produced by melting of previously depleted mantle. Felsic magmas are partial melts of tholeiitic basalt or products of liquid immiscibility in a tholeiitic system perhaps involving extreme fractionation in a high level magma chamber, and assimilation of sialic crust. It is concluded that Archean cyclical volcanism in this area involves the interplay of several magmatic liquids in processes of fractional crystallization, magma mixing, liquid immiscibility, and the probable existence of compositionally zoned magma chambers in the late stages of each cycle. The compositionally zoned chambers existed over the time period represented by the upper felsic portion of each cycle. 相似文献
17.
Sugarloaf Mountain is a 200-m high volcanic landform in central Arizona, USA, within the transition from the southern Basin and Range to the Colorado Plateau. It is composed of Miocene alkalic basalt (47.2–49.1?wt.% SiO2; 6.7–7.7?wt.% MgO) and overlying andesite and dacite lavas (61.4–63.9?wt.% SiO2; 3.5–4.7?wt.% MgO). Sugarloaf Mountain therefore offers an opportunity to evaluate the origin of andesite magmas with respect to coexisting basalt. Important for evaluating Sugarloaf basalt and andesite (plus dacite) is that the andesites contain basaltic minerals olivine (cores Fo76-86) and clinopyroxene (～Fs9-18Wo35-44) coexisting with Na-plagioclase (An48-28Or1.4–7), quartz, amphibole, and minor orthopyroxene, biotite, and sanidine. Noteworthy is that andesite mineral textures include reaction and spongy zones and embayments in and on Na-plagioclase and quartz phenocrysts, where some reacted Na-plagioclases have higher-An mantles, plus some similarly reacted and embayed olivine, clinopyroxene, and amphibole phenocrysts.Fractional crystallization of Sugarloaf basaltic magmas cannot alone yield the andesites because their ～61 to 64?wt.% SiO2 is attended by incompatible REE and HFSE abundances lower than in the basalts (e.g., Ce 77–105 in andesites vs 114–166?ppm in basalts; Zr 149–173 vs 183–237; Nb 21–25 vs 34–42). On the other hand, andesite mineral assemblages, textures, and compositions are consistent with basaltic magmas having mixed with rhyolitic magmas, provided the rhyolite(s) had relatively low REE and HFSE abundances. Linear binary mixing calculations yield good first approximation results for producing andesitic compositions from Sugarloaf basalt compositions and a central Arizona low-REE, low-HFSE rhyolite. For example, mixing proportions 52:48 of Sugarloaf basalt and low incompatible-element rhyolite yields a hybrid composition that matches Sugarloaf andesite well ? although we do not claim to have exact endmembers, but rather, viable proxies. Additionally, the observed mineral textures are all consistent with hot basalt magma mixing into rhyolite magma. Compositional differences among the phenocrysts of Na-plagioclase, clinopyroxene, and amphibole in the andesites suggest several mixing events, and amphibole thermobarometry calculates depths corresponding to 8–16?km and 850° to 980?°C. The amphibole P-T observed for a rather tight compositional range of andesite compositions is consistent with the gathering of several different basalt-rhyolite hybrids into a homogenizing ‘collection' zone prior to eruptions. We interpret Sugarloaf Mountain to represent basalt-rhyolite mixings on a relatively small scale as part of the large scale Miocene (～20 to 15 Ma) magmatism of central Arizona. A particular qualification for this example of hybridization, however, is that the rhyolite endmember have relatively low REE and HFSE abundances. 相似文献
18.
Composite dikes at Hell Hole Meadow, in the central Sierra Nevada, contain hybrids created by small scale mixing of andesitic and rhyolitic magmas. Early rhyolitic injections had partially solidified when subsequent andesitic magmas arrived and mixed with small increments of remnant rhyolitic magma. In major element chemistry, the hybrid rocks define linear MgO-variation diagrams that closely resemble those for the Half Dome, Mt. Givens, and Eagle Peak granodiorites. The patterns suggest that mixing of mafic and felsic magmas has been important in the evolution of these plutons.Hornblendes in three Hell Hole Meadow hybrid rocks ranging from dacite to andesite display indistinguishable crystal-chemical variation patterns. The crystals apparently developed as phenocrysts of endmember andesitic magma prior to the mixing event and retained their compositional character in the hybridization event. Plutonic hornblendes (Dodge et al. 1968; Noyes et al. 1983) display crystal-chemical patterns nearly identical to those in the dike cores making it unlikely that the plutonic hornblendes represent restite of a complex lower crust.Hornblende chemical data from a suite of rocks collected across the width of the compositionally zoned Half Dome granodiorite (Yosemite National Park) define clusters, like their Hell Hole Meadow counterparts, that are very similar to one another despite the large range in host rock bulk composition. Sr isotopic data (Kistler et al. 1986) and linear MgO-variation patterns for the major elements (Reid et al. 1983) suggest that the Half Dome is a mixture of high-alumina basalt and rhyolitic magmas. We propose that while the bulk chemistry of hybrid granodiorites is determined by the proportions of the constituent mafic and felsic magmas, the hornblendes in the mixed rocks largely retain compositions created in the mafic component prior to mixing. Mixing may occur either by incremental addition of felsic magma into a relatively large volume of mafic magma (as at Hell Hole Meadow), or by the chilling and subsequent disaggregation of mafic pillows in a largely felsic host. 相似文献
19.
The volcanics exposed in the northeast Niğde area are characterized by pumiceous pyroclastic rocks present as ash flows and fall deposits and by compositions ranging from dacite to rhyolite. Xenoliths found in the volcanics are basaltic andesite, andesite and dacite in composition. These rocks exhibit linear chemical variations between end‐member compositions and a continuity of trace element behaviour exists through the basaltic andesite–andesite–dacite–rhyolite compositional range. This is consistent with the fractionation of ferromagnesian minerals and plagioclase from a basaltic andesite or andesite parent. These rocks are peraluminous and show typical high‐K calc‐alkaline differentiation trends with total iron content decreasing progressively with increasing silica content. Bulk rock and mineral compositional trends and petrographic data suggest that crustal material was added to the magmas by subducted oceanic crust and is a likely contaminant of the source zone of the Niğde magmas. The chemical variations in these volcanics indicate that crystal liquid fractionation has been a dominant process in controlling the chemistry of the northeast Niğde volcanics. It is also clear, from the petrographic and chemical features, that magma mixing with disequilibrium played a significant role in the evolution of the Niğde volcanic rocks. This is shown by normal and reverse zoning in plagioclase and resorption of most of the observed minerals. The xenoliths found in the Niğde volcanics represent the deeper part of the magma reservoir which equilibrated at the higher pressures. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献