The Final Stage of Silicic Island Arc Magmatism in the Northern Urals

Doklady Earth Sciences - The date of cessation of calc-alkaline volcanic complex accumulation within the Devonian (Frasnian) island arc in the Northern Urals was determined for the first time. It...     

Cainozoic igneous activity in eastern australia

During the Cainozoic there was widespread basaltic igneous activity in eastern AustraIia along and adjacent to the Eastern Highlands. The activity commenced about 70 m.y. ago, and has continued through the Cainozoic at a nearly constant rate. More than fifty igneous provinces are recognized. Each province consists of similar volumes of volcanic material and crustal intrusives, the volcanism generally lasting less than 5 m.y. and resulting in lavas that cover a region 50–200 km across. Three main types of igneous province are recognized:

1. (1) central volcano provinces, which are composed predominantly of slightly undersaturated to saturated basaltic lavas, but with some felsic flows and intrusions;

2. (2) lava field provinces, which consist of basaltic flows, commonly strongly undersaturated;

3. (3) a single, mafic, strongly undersaturated, high potassium province.

Within each type of province the average potassium content of the lavas has not changed with time. Most provinces are consistent with an apparent migration of volcanism westward at 5 mm/year across the Eastern Highlands. This migration is thought to be caused by crustal processes. The distribution and age of the slightly undersaturated provinces suggest a migration of the centre of volcanism southward at 66 ± 5 mm/year. Magmas for these provinces are thought to originate from a magma source or sources, with a limited latitudinal extent, within the asthenosphere. The migration is considered to be related to the movement of the Indian (Australian) lithospheric plate relative to the underlying asthenosphere.     

Cenozoic Island Arc Magmatism in Java Island (Sunda Arc, Indonesia): Clues on Relationships between Geodynamics of Volcanic Centers and Ore Mineralization

Abstract. Java island, regarded as a classic example of island arcs, is built through multi events of Cenozoic arc magmatism produced by the subduction of Indian‐Australian oceanic crusts along the southern margin of Eurasian plate. Regional crustal compositions, subducted slabs, and tectonics determined the spatial‐geochemical evolution of arc magmatism and regional metallogeny. Tertiary geodynamics of island arc was dominated by backarc‐ward migrations of volcanic centers. Only after the Miocene‐Pliocene roll‐back effects of retreating slab, slab detachment, and backarc magmatism took place in central Java. The source of arc magmas is mainly partial melting of mantle wedge, triggered by fluids released from dehydrated slabs. Increasing potassium contents of arc magmas towards the backarc‐side and younger magmas is typical for all magmas, while alkali and incompatible trace elements ratios are characteristics for different settings of volcanic centers. The oceanic nature of crust and the likely presence of hot slab subducted beneath the eastern Java determine the occurrences of adakitic magmas. Backarc magmatism has a deeper mantle source with or without contributions from subduction‐related materials. The domination of magnetite‐series magmatism determines the sulfide mineralization for the whole island. District geology, geodynamics, and magma compositions in turn control particular styles and scales of precious metals concentrations. Deep‐seated crustal faults have focused the locations of overlapping volcanic centers and metalliferous fluids into few major gold districts. Porphyry deposits are mostly concentrated within Lower Tertiary (early stage) volcanic centers in eastern Java which are not covered by younger volcanic centers, and whose sulfides are derived from partial melting of basaltic parental materials. On the other hand, high‐grade low‐sulfidation epithermal gold deposits formed in later stages of arc development and are spatially located within younger volcanic centers (Upper Miocene‐Pliocene) that overlap the older ones. Gold in low‐sulfidation epithermal system is likely to be derived from crustal materials. The overall interacting factors resulting in the petrochemical systematics that are applicable for exploration: 1) early‐stage volcanic centers with high Sr/Y and Na₂O/K₂O ratios are more prospective for porphyry mineralization, while 2) later‐stage volcanic centers with high K₂O, total alkali, and K₂O/Na₂O ratios are more prospective for low‐sulfidation epithermal mineralization.     

云南省格咱岛弧印支期岩浆演化及普朗斑岩型铜矿成矿作用

格咱岛弧位于西南三江构造火成岩带义敦岛弧南端,是晚三叠世甘孜-理塘洋壳向德格-中甸陆块俯冲的产物,是我国近年来新发现的重要铜多金属成矿带.该区强烈的构造-岩浆活动贯穿于岛弧造山作用的始终,丰富的成矿作用发育于岛弧造山的不同时代、不同环境,产出了普朗、雪鸡坪等大型铜多金属矿床.其中普朗斑岩型铜矿是印支期斑岩铜矿的典型代表,其含矿斑岩体SiO2含量在60.07％～65.78％,属于中酸性岩.Al2O3含量为14.80％～15.97％,(K2O+Na2O)为5.79％～10.99％,K2O/Na2O为0.7～1.14,里特曼指数(σ)为1.02～1.9,固结指数(SI)为7.65～24.77,分异指数(DI)为61.17～70.58,岩石属钙碱性系列,岩浆分异一般较好.A/CNK为0.74～0.981,平均为0.87,岩石属准铝质花岗岩.锆石U-Pb同位素测年结果表明,石英闪长玢岩的形成年龄大致在220.8±4.1Ma,普朗石英二长斑岩的形成年龄为214.8±3.5Ma,其成岩成矿时代主要集中于印支期.     

Phase relations of a high-Mg basalt from the Aleutian Island Arc: Implications for primary island arc basalts and high-Al basalts

Many volcanic centers in the Aleutian Islands have erupted lavas that range in composition from high-Mg basalt (MgO>9 wt%) to more fractionated and voluminous high-Al basalts and basaltic andesites. The petrogenetic relationships between these rock types and the composition of primary magmas has been vigorously debated. The phase relations of a typical high-Mg basalt from the Makushin volcanic field on Unalaska Island provide important constraints on petrogenetic models. Results of one-atmosphere and moderate-to high-pressure (5–20 kb) anhydrous experiments are similar to results obtained from primitive MORB. At low pressures olivine is the liquidus phase joined by plagioclase and clinopyroxene at progressively lower temperatures. Clinopyroxene is the second phase to crystallize at pressures greater than 5 kb and replaces olivine on the liquidus at approximately 10 kb. Above 10 kb the liquidus pyroxene is aluminous augite and orthopyroxene is the second phase to crystallize. Glasses in equilibrium with olivine and clinopyroxene at intermediate-pressure (5 to 10 kb) are similar in composition to high-Al basalt. Plagioclase is not involved and most likely does not become a liquidus phase until the liquid has evolved significantly. Although our studies do not confirm the primary nature of high-Mg basalts they do support a model in which high-Al basalts are generated by moderate amounts of crystal fractionation from more primitive (high Mg/Mg+Fe, lower Al₂O₃) basaltic magmas near the arc crust-mantle boundary.Abbreviations Ol olivine - Cpx Clinopyroxene - Pl plagioclase - L liquid - Sp spinel - Pig pigeonite - Opx Orthopyroxene     

海南岛中酸性岩浆活动与成矿作用关系探讨

海南岛位于我国环太平洋构造--岩浆成矿带的重要地段。根据岩石和同位素特征分析,认为海南岛海西—印支期的中酸性侵入岩可分为S型和I型两种成因的花岗岩,燕山期为I型花岗岩。由于复式岩体多次多阶段侵位,晚阶段的岩浆活动对早阶段岩石进一步改造并溶萃其中的成矿物质,形成大量的富含成矿物质的流体,构造应力体制转换及反复多次应力集中—释放过程式,在有利的空间位置成矿。根据中酸性侵入岩的控矿作用和对海南岛成矿规律的认识及DPIS成矿预测系统对岛内成矿远景区的预测,划分了与中酸性侵入岩有关矿床的成矿远景区,指明了多金属矿找矿方向。     

Ultramafic and Mafic Inclusions from Adak Island: Crystallization History, and Implications for the Nature of Primary Magmas and Crustal Evolution in the Aleutian Arc

Xenolithic inclusions in calc-alkaline andesite from Mt. Moffettvolcano, Adak Island, Aleutian arc, reveal a nearly continuousrecord of crystallization of basaltic magmas in the crust, andpossibly upper mantle, of the arc. The record is more detailedand continuous than that obtained from study of calc-alkalinevolcanic rocks in the arc. Cumulate xenoliths form a progressiveseries in modal mineralogy from ultramafic, hornblende-bearingolivine clinopyroxenite to both hornblende-bearing and hornblende-freegabbros. The cumulate hornblende gabbro xenoliths are typicalof those found in island arc andesites worldwide. Xenolithicinclusions without cumulate textures, here termed compositexenoliths, are characterized by forsteritic olivine, zoned Cr-diopsideand hornblende, and are interpreted to have resulted from reactionand chilling upon magma mixing at depth. The olivine and clinopyroxene in both cumulate and compositexenoliths show the largest and the most complete variation trendsfor Ni, Cr, and FeO/MgO ratio yet reported in igneous xenolithsfrom island arc volcanic rocks. Variation of Ni in olivine indicatesthat the parent magmas for the xenoliths had minimum MgO contentsof 9 wt. per cent. Variation of Cr in clinopyroxene indicatesthat the magmas were basaltic rather than picritic, probablyin equilibrium with spinel lherzolite at near Moho depths. Successiveinjections of batches of primary melt into a magma chamber fractionatingolivine and clinopyroxene can reproduce observed compatibleelement depletion trends. A steady-state process of cotecticcrystallization in a magma chamber continually replenished withbasaltic magma is a possible mechanism for producing large accumulationsof olivine and clinopyroxene, suggesting that Alaskan-type ultramaficcomplexes are related to hydrous basaltic magmas in island arcs.This steady-state open-system crystallization process can alsoyield the abundant high-alumina basalt type in the Aleutianarc. Continued crystallization of high-alumina basalt in lowercrustal magma chambers, recorded in a mineralogically coherentseries of pyroxenite to hornblende gabbro xenoliths, can yieldbasaltic to andesitic magmas of the calc-alkaline series. No xenoliths with a sedimentary protolith have been found atMt Moffett, evidence that the arc crust is igneous in origin,with the lower crust formed of gabbro crystallized from mantle-derivedmelts. Ultramafic cumulates may reside in both the lower crustor upper mantle beneath the arc. A model is proposed wherebythe cumulate crystallization products of hydrous, mantle beneaththe arc. A model is define the upper mantle and lower crustof the arc over time.The net composition added to the crustof the arc is that of high-alumina basalt.     

Variable Impact of the Subducted Slab on Aleutian Island Arc Magma Sources: Evidence from Sr, Nd, Pb, and Hf Isotopes and Trace Element Abundances

Major and trace element compositions and Sr, Nd, Pb, and Hfisotope ratios of Aleutian island arc lavas from Kanaga, Roundhead,Seguam, and Shishaldin volcanoes provide constraints on thecomposition and origin of the material transferred from thesubducted slab to the mantle wedge. ⁴⁰Ar/³⁹Ar dating indicatesthat the lavas erupted mainly during the last     

A Detailed Geochemical Study of Island Arc Crust: the Talkeetna Arc Section, South-Central Alaska

The Early to Middle Jurassic Talkeetna Arc section exposed inthe Chugach Mountains of south–central Alaska is 5–18km wide and extends for over 150 km. This accreted island arcincludes exposures of upper mantle to volcanic upper crust.The section comprises six lithological units, in order of decreasingdepth: (1) residual upper mantle harzburgite (with lesser proportionsof dunite); (2) pyroxenite; (3) basal gabbronorite; (4) lowercrustal gabbronorite; (5) mid-crustal plutonic rocks; (6) volcanicrocks. The pyroxenites overlie residual mantle peridotite, withsome interfingering of the two along the contact. The basalgabbronorite overlies pyroxenite, again with some interfingeringof the two units along their contact. Lower crustal gabbronorite(10 km thick) includes abundant rocks with well-developed modallayering. The mid-crustal plutonic rocks include a heterogeneousassemblage of gabbroic rocks, dioritic to tonalitic rocks (30–40%area), and concentrations of mafic dikes and chilled mafic inclusions.The volcanic rocks (7 km thick) range from basalt to rhyolite.Many of the evolved volcanic compositions are a result of fractionalcrystallization processes whose cumulate products are directlyobservable in the lower crustal gabbronorites. For example,Ti and Eu enrichments in lower crustal gabbronorites are mirroredby Ti and Eu depletions in evolved volcanic rocks. In addition,calculated parental liquids from ion microprobe analyses ofclinopyroxene in lower crustal gabbronorites indicate that theclinopyroxenes crystallized in equilibrium with liquids whosecompositions were the same as those of the volcanic rocks. Thecompositional variation of the main series of volcanic and chilledmafic rocks can be modeled through fractionation of observedphase compositions and phase proportions in lower crustal gabbronorite(i.e. cumulates). Primary, mantle-derived melts in the TalkeetnaArc underwent fractionation of pyroxenite at the base of thecrust. Our calculations suggest that more than 25 wt % of theprimary melts crystallized as pyroxenites at the base of thecrust. The discrepancy between the observed proportion of pyroxenites(less than 5% of the arc section) and the proportion requiredby crystal fractionation modeling (more than 25%) may be bestunderstood as the result of gravitational instability, withdense ultramafic cumulates, probably together with dense garnetgranulites, foundering into the underlying mantle during thetime when the Talkeetna Arc was magmatically active, or in theinitial phases of slow cooling (and sub-solidus garnet growth)immediately after the cessation of arc activity. KEY WORDS: island arc crust; layered gabbro; Alaska geology; island arc magmatism; lower crust     

A middle Triassic extensional event in the Hainan Island: Geochronologic and geochemical evidence from igneous rocks from Dazhou Island

Hainan Island lies at the southern tip of the South China block that was affected by the opening and closure of the paleo-Tethys from Paleozoic to Mesozoic and subsequently by the subduction of the paleo- Pacific plate beneath the Euroasian plate since the late Mesozoic era. This study presents zircon U-Pb ages and geochemistry of granites and basic diabase veins in Dazhou Island located 5 km southeast of Hainan Island. The age for granites is 237.2 Ma (MSWD = 0.14, n = 14), suggesting that they were emplaced in the late middle Triassic and are products of magmatic-tectonic episodes during the closure of paleo-Tethys. The granites belong to the shoshonitic magmatic series and are metaluminous I-type granites. They are systematically depleted in Ba, Nb, Sr, P, and Ti, and have variable ranges of initial ⁸⁷Sr/⁸⁶Sr ratios (0.71005 ~ 0.71110), but relatively constant εNd(t) values (–6.4~–6.8). The two-stage depleted mantle Nd model ages (T_DM2 = 1531–1562 Ma) for these granites suggest that they may have originated from partial melting of the lower crust with a crustal residence age of early Middle Proterozoic Era. The zircon U-Pb ages for diabase veins can be divided into three groups. The age for Group I is 236.8 Ma (n = 4), which represents the crystallization age of the veins and somewhat younger than the accompanying granites. The ages for Group II and III are 1767 Ma (n = 6) and 2432 Ma (n = 3), respectively. These suggest that the veins have inherited some old zircons that possibly represent old geological events in the Hainan Islands. The diabase also belongs to the shoshonitic magmatic series and has arc-like trace element characteristics. The veins have high 87Sr/86Sr and low 143Nd/144Nd ratios, which suggest that they may have originated from an enriched mantle II type source or a mantle source metasomatized by aqueous fluids released from previously subducted slab. The new ages obtained for the Dazhou igneous rocks in the present study are similar to those for the Qiongzhong batholith in the Hainan Island, which implies that the Hainan Island had also regionally experienced a middle Triassic extension event after the Indosinian Orogeny.     

The Petrology and Geochemistry of Volcanic Rocks on Jeju Island: Plume Magmatism along the Asian Continental Margin

The incompatible element signatures of volcanic rocks formingJeju Island, located at the eastern margin of the Asian continent,are identical to those of typical intraplate magmas. The sourceof these volcanic rocks may be a mantle plume, located immediatelybehind the SW Japan arc. Jeju plume magmas can be divided intothree series, based on major and trace element abundances: high-aluminaalkalic, low-alumina alkalic, and sub-alkalic. Mass-balancecalculations indicate that the compositional variations withineach magma series are largely governed by fractional crystallizationof three chemically distinct parental magmas. The compositionsof primary magmas for these series, using inferred residualmantle olivine compositions, suggest that the low-alumina alkalicand sub-alkalic magmas are generated at the deepest and shallowestdepths by lowest and highest degrees of melting, respectively.These estimates, together with systematic differences in traceelement and isotopic compositions, indicate that the upper mantlebeneath Jeju Island is characterized by an increased degreeof metasomatism and a change in major metasomatic hydrous mineralsfrom amphibole to phlogopite with decreasing depth. The originalplume material, having rather depleted geochemical characteristics,entrained shallower metasomatized uppermost mantle material,and segregated least-enriched low-alumina alkalic, moderatelyenriched high-alumina alkalic, and highly enriched sub-alkalicmagmas, with decreasing depth. KEY WORDS: Jeju Island; magma genesis; mantle plume; subcontinental mantle     

The Petrology and Geochemistry of the Aniakchak Caldera-forming Ignimbrite, Aleutian Arc, Alaska

Aniakchak caldera, Alaska, produced a compositionally heterogeneousignimbrite 3400 years ago, which changes from rhyodacitic atthe base to andesitic at the top of the eruptive sequence. Interpretationsof compositionally heterogeneous ignimbrites typically includeeither in situ fractional crystallization of mafic magma andgeneration of a stratified magma body or replenishment of asilicic magma chamber by mafic inputs. Another possibility,silicic replenishment of a more mafic chamber, exists. Geochemicalcharacteristics of the caldera-forming rhyodacite and severallate pre-caldera rhyodacites indicate independent origins foreach, within a maximum of 5000 years prior to caldera formation.Isotopic considerations preclude derivation of the caldera-formingrhyodacite from the caldera-forming andesite. However, the caldera-formingrhyodacite can be explained as the residual liquid of a mostlycrystallized basalt, with addition of crustal material. TheAniakchak andesite probably formed in a shallow chamber by successivemixing events involving small volumes of basalt and rhyodacite,together with contamination. The pre-caldera rhyodacites representerupted portions of intruding silicic magma, whereas anotherportion homogenized with the resident mafic magma. The caldera-formingevent reflects a large influx of rhyodacite, which erupted beforesignificant mixing occurred and also triggered draining of muchof the andesitic magma from the chamber. KEY WORDS: Aniakchak; caldera-forming eruption; geochemistry; ignimbrite; silicic replenishment     

Esmeralda Bank: Geochemistry of an active submarine volcano in the Mariana Island Arc

Esmeralda Bank is the southernmost active volcano in the Izu-Volcano-Mariana Arc. This submarine volcano is one of the most active vents in the western Pacific. It has a total volume of about 27 km³, rising to within 30 m of sea level. Two dredge hauls from Esmeralda recovered fresh, nearly aphyric, vesicular basalts and basaltic andesites and minor basaltic vitrophyre. These samples reflect uniform yet unusual major and trace element chemistries. Mean abundances of TiO₂ (1.3%) and FeO^* (12.6%) are higher and CaO (9.2%) and Al₂O₃ (15.1%) are lower than rocks of similar silica content from other active Mariana Arc volcanoes. Mean incompatible element ratios K/Rb (488) and K/Ba (29) of Esmeralda rocks are indistinguishable from those of other Mariana Arc volcanoes. On a Ti-Zr plot, Esmeralda samples plot in the field of oceanic basalts while other Mariana Arc volcanic rocks plot in the field for island arcs.Incompatible element ratios K/Rb and K/Ba and isotopic compositions of Sr (⁸⁷Sr/⁸⁶Sr=0.70342–0.70348), Nd (_ND=+7.6 to +8.1), and O(¹⁸O=+5.8 to +5.9) are incompatible with models calling for the Esmeralda source to include appreciable contributions from pelagic sediments or fresh or altered abyssal tholeiite from subduction zone melting. Instead, incompatible element and isotopic ratios of Esmeralda rocks are similar to those of intra-plate oceanic islands or hot-spot volcanoes in general and Kilauean tholeiites in particular. The conclusion that the source for Esmeralda lavas is an ocean-island type mantle reservoir is preferred.Esmeralda Bank rare earth element patterns are inconsistent with models calling for residual garnet in the source region, but are adequately modelled by 7–10% equilibrium partial melting of spinel lherzolite. This is supported by consideration of the results of melting experiments at 20 kbars, 1,150° C with CO₂ and H₂O as important volatile components. These experiments further indicate that low MgO (4.1%), MgO/FeO^*(0.25) and Ni(12 ppm) in Esmeralda Bank melts are characteristic of initial melts generated by moderate degrees of melting of hydrous and carbonated mantle. Consideration of experimental determinations and spinel-lherzolite to garnet-lherzolite stabilities indicates Esmeralda Bank melts were generated by partial melting within the upper 60–110 km of the mantle.     

Magmatism and metallogeny of the Ridanj-Krepoljin belt (eastern Serbia) and their correlation with northern and eastern analogues

In the southwestern Carpathians of SW Romania and E Serbia calc-alkaline Upper Cretaceous-Paleogene subduction related magmatic rocks occur in two main zones. The Ridanj-Krepoljin belt (E Serbia) represents the southern part of the western zone. In this belt Early Maastrichtian dacites and subordinate andesites occur, but there are indications of an unexposed granodioritic (?) magmatism about 60 Ma old. Pb-Zn-Ag and only subordinate Cu mineralization is associated with this igneous activity. The Timok magmatic complex (E Serbia) composed of Upper Cretaceous (-Paleocene?) andesites and analogue intrusives represents the eastern zone south of the Danube. Associated with this magmatism are Cu (+Au) and very subordinate Zn (±Pb) mineralization. To the north (SW Romania) in both zones mainly intrusive bodies (diorites, quartz diorites, granodiorites, monzonites) of Campanian-Paleocene age are exposed. Porphyry copper (+Mo) and in north Pb-Zn (±Cu) mineralization are related to those intrusives. The petrological and geochemical features of all these calc-alkaline rocks are very similar. It appears, however, that the associated polymetallic and the copper mineralization are both spatially and vertically separated.     

三江地区义敦岛弧安山岩成因

义敦岛弧是一个发育于张性薄陆壳基底上的三叠纪岛弧,以发育岛弧裂谷和成对的火山弧为特征。弧火山岩主要为钙碱性火山岩系,以安山岩为主,其岩石组合为玄武岩—安山岩—英安岩。岩相学、地球化学、相平衡及定量模型计算均证实,分离结晶作用是产生钙碱性火山岩系的主导作用,安山岩是钙碱性玄武岩浆在不同压力条件下发生多阶段分离结晶的产物。岩浆演化具有相对封闭系统特征。玄武岩浆在较大压力条件下发生Ol+Cpx+Pl±Mt±Ap结晶(F=0.46)后,派生岩浆发生Hb+Cpx+Pl+Mt结晶分离(F=0.25),产生安山岩浆。该岩浆可能在较低压力下再次发生结晶分异,从而派生出英安岩。角闪石在相对较高的压力条件下的大量结晶是安山岩形成的主要原因。     

On the origin of andesite in the northern Mariana Island Arc: Implications from Agrigan

Magmatic evolution on the active volcano of Agrigan in the northern Mariana Island Arc is interpreted as resulting in the production of calc-alkaline andesites by the fractional crystallization of high-alumina basalt. Basaltic products predominate, but the ratio of andesites to basalts increases with time up to an event of voluminous andesitic pyroclastic ejection accompanied by caldera-collapse; post-collapse lavas are entirely basaltic. Moderate iron-enrichment is demonstrated for the volcanic suite, with indications of a progressive, pre-caldera decrease in iron-enrichment; post-caldera lavas display a return to moderate Fe-enrichment. Overall, the lavas are enriched in the LIL elements (K, Rb, Ba, Sr) and depleted in Ti, Mg, Cr, and Ni. From the oldest to the youngest pre-caldera volcanic sequence, the LIL elements increase 3-6X while Ca and Mg decrease by 50% or more. Approximately constant K/ Rb (430±60) and ⁸⁷Sr/⁸⁶Sr (0.7032–0.7034) indicate consanguinity of the basalts and the andesites. Cumulate plutonic xenoliths, common in the lavas, are composed of mineral phases also encountered as phenocrysts. The following order of crystallization is indicated: olivine; anorthite-bytownite; clinopyroxene; orthopyroxene and titanomagnetite. Co-existing xenolithic olivines (Fo_74–83) and plagioclase (An_88–96) are typical of calc-alkaline island-arc assemblages and contrast with assemblages in the tholeiites from the Mariana Trough to the west. The relatively fayalitic composition and low abundances of Ni in olivines and Cr in clinopyroxenes indicate equilibrium with an already-fractionated liquid. These data, along with structural evidence, high Ca in the olivines, and comparison of the observed assemblages with experimental studies, suggests that these xenoliths formed as crystal cumulates at the floor of a shallow ( 7 km) crustal magma chamber.Major element modeling studies using the separation of observed xenocrystic and phenocrystic phases from assumed parental liquids reproduce the observed temporal and geochemical variations in the lavas. Trace element modeling parallels this evolution with the exception of Cr and Ni in the andesites. An extensive (16.3 km³) gabbroic body is required by this modeling to be present beneath Agrigan to produce the inferred volumes of the various lithologies preserved in the volcano's evolution. The sum of stratigraphic, geochemical, and isotopic evidence on Agrigan supports the derivation of calc-alkaline andesite by the removal of about 75% solids from a high-alumina basalt accompanied by a process of K and Rb enrichment, such as volatile-transfer. Considerations of ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and ³He/⁴He isotopic data indicate that the source region of these parental liquids lies in the mantle, not subducted crust. In the northern Marianas, the model of a shallow-crustal origin for andesite is preferred over one requiring andesite generation in the deeper mantle and/or subducted slab.     

豫东地区岩浆活动对煤层结构及煤质的影响

为指导豫东地区煤矿合理部署、高效生产,基于大量钻井和测试资料的综合分析和对比研究,探讨了豫东地区岩浆活动的基本特征及其对煤层结构和煤质的影响。研究结果表明：燕山中晚期的岩浆活动对豫东地区的煤层结构和煤层连续性造成影响,导致煤层变薄、分叉、尖灭和吞噬;同时,岩浆活动导致本区煤层变质程度普遍增高,煤质参数发生变化。煤层的原始赋存状态、岩浆侵入的初始温度、侵入岩体的厚度、岩浆侵入煤层的方式、岩体离煤层的距离、围岩和岩体上覆地层的岩性等是影响煤层结构和煤变质程度的主要控制因素。