长白山天池火山造锥阶段玄武质火山活动期次划分及成因探讨

在天池火山造锥阶段,长白山火山区玄武质火山活动频繁。文章在野外调查的基础上,通过年代学及地球化学研究,对其活动期次进行划分,并探讨其岩浆来源与演化。天池火山造锥阶段的玄武质火山岩主要呈火山渣锥或小型河谷玄武岩形式分布,其形成可划分为两期:一期为老房子小山期,形成时限为0.87~0.54 Ma,属碱性岩石系列;另一期为老虎洞期,形成时限为0.34~0.1 Ma,属碱性岩石系列和拉斑岩石系列。地球化学特征显示,碱性系列玄武岩具高Al、Ti、K、P和低Mg特征,拉斑系列玄武岩具高Mg、富Fe、Ca和低Na特征;二者稀土和微量特征较为一致,稀土元素配分曲线呈右倾型,略显正铕异常,并富集Ba、K、Pb、P、Ti,亏损Th、U、Sr,但拉斑系列玄武岩的稀土元素和微量元素含量及轻重稀土分馏强度均低于碱性系列。天池火山造锥阶段形成的玄武质火山岩均来源于进化岩浆,具有同源特征,经历了一致或相似的演化过程,岩浆房赋存位置相当于上地幔—下地壳的过渡部位,结晶分异岩浆作用显著、地壳混染作用微弱,其成分变化受控于多期次结晶分异作用和早期结晶再循环的岩浆作用过程。     

Sedimentary basin formation associated with a silicic large igneous province: stratigraphy and provenance of the Mesoproterozoic Roopena Basin,Gawler Range Volcanics

The Mesoproterozoic Gawler Silicic Large Igneous Province (SLIP) in the Gawler Craton and Curnamona Province, southern Australia, comprises extensive felsic and lesser mafic volcanic sequences, with only limited sedimentary successions. The Roopena Basin is a rare example of a synvolcanic sedimentary basin that formed within the Gawler SLIP in the eastern Gawler Craton. It is a north–south-trending basin with a preserved area of 75 km², bound by the Roopena and Wizzo Well faults, and contains three units of the lower Gawler Range Volcanics; the Angle Dam Dacite, Fresh Well Formation and Roopena Basalt. The Angle Dam Dacite is a porphyritic lava and the oldest part of the volcanic succession, directly overlying basement. The Fresh Well Formation overlies the Angle Dam Dacite or basement, comprises three coarsening-upwards volcaniclastic packages of claystone, siltstone, fine-grained to coarse-grained lithic sandstone and conglomerate deposited in a fluvio-lacustrine setting, and contains three tuff members. The Roopena Basalt is interlayered with the Fresh Well Formation, and comprises auto-brecciated lavas that exhibit only local interaction with water or wet sediment. Sharp basal contacts of the prograding packages within the Fresh Well Formation provide evidence of rapid flooding events within the basin. New detrital zircon geochronology of a sandstone within the Fresh Well Formation yielded a maximum depositional age of ca 1580 Ma, with provenance dominated by felsic volcanic units of the 1635–1605 Ma St Peter Suite. Sedimentation in the Gawler SLIP appears to have occurred in isolated basins with limited areal extent. It was largely restricted to the eastern Gawler Craton, and as well as the Roopena Basin, and includes similar basins at the Olympic Dam and Prominent Hill iron oxide–copper–gold ± uranium (IOCG ± U) deposits. The coincidence of sedimentation and mafic volcanism in the eastern Gawler Craton suggests that this region underwent extension at this time, although high-temperature metamorphism and compressional deformation occurred in some parts of the Gawler Craton and Curnamona Province synchronous with the Gawler SLIP. The Roopena Basin sedimentary rocks and underlying basement contain hematite–chlorite–sericite–white mica assemblages, permissive of hematite-style IOCG mineral deposits; however, no significant ore deposit has yet been discovered in the Roopena Basin.     

Basaltic pillow mounds in the Vinalhaven intrusion, Maine

The Vinalhaven intrusion is a dominantly granitic pluton of probable Devonian age, located on Vinalhaven Island and adjacent islands, Maine. It consists of four main units: coarse-grained granite, fine-grained granite, a gabbro-diorite unit consisting of interlayered mafic, hybrid and granitic rocks, and a heterogeneous granitic unit. The gabbro-diorite unit occurs along the south and east coast of the island as a sheet-like body, hundreds of meters to more than 1 km thick, that dips beneath the central granitic units and rests on heterogeneous granitic rocks that form the base of the intrusion and are exposed on islands to the southeast. Load-cast and pipe structures at the bases of mafic sheets indicate that the gabbro-diorite unit represents a sequence of basaltic injections that ponded on crystal-rich mush at the base of a silicic magma chamber and variably interacted with overlying crystal-poor granitic magma. The pluton, therefore, represents a fossilized silicic magma chamber that was periodically replenished by basaltic magma. Near the base of the gabbro-diorite unit, some basaltic injections produced large mounds up to more than 10 m high and 100 m wide of tightly packed, meter-scale chilled basaltic pillows, tubes and sheets in a granitic matrix. The mounds appear to represent flow fronts of basaltic injections that entered and ponded on the floor of a silicic magma chamber. Although physical conditions differ significantly, these plutonic pillow mounds appear to share many characteristics with submarine pillow basalts and lava flows.     

Intracrustal origin of Arenal basaltic andesite in the light of solid–melt interactions and related compositional buffering

The origin of Arenal basaltic andesite can be explained in terms of fractional crystallization of a parental high-alumina basalt (HAB), which assimilates crustal rocks during its storage, ascent and evolution. Contamination of this melt by Tertiary calc-alkalic intrusives (quartz–diorite and granite, with ⁸⁷Sr/⁸⁶Sr ratios ranging 0.70381–0.70397, nearly identical with those of the Arenal lavas) occurs at upper crustal levels, following the interaction of ascending basaltic magma masses with gabbroic–anorthositic layers. Fragments of these layers are found as inclusions within Arenal lavas and tephra and may show reaction rims (1–5 mm thick, consisting of augite, hypersthene, bytownitic–anorthitic plagioclase, and granular titanomagnetite) at the gabbro–lava interface. These reaction rims indicate that complete `assimilation' was prevented since the temperature of the host basaltic magma was not high enough to melt the gabbroic materials (whose mineral phases are nearly identical to the early formed liquidus phases in the differentiating HAB). Olivine gabbros crystallized at pressure of about 5–6 kbar and equilibrated with the parental HAB at pressures of 3–6 kbar (both under anhydrous and hydrous conditions), and temperatures ranging 1000–1100°C. In particular, `deeper' interactions between the mafic inclusions and the hydrous basaltic melt (i.e., with about 3.5 wt.% H₂O) are likely to occur at 5.4 (±0.4) kbar and temperatures approaching 1100°C. The olivine gabbros are thus interpreted as cumulates which represent crystallized portions of earlier Arenal-type basalts. Some of the gabbros have been `mildly' tectonized and recrystallized to give mafic granulites that may exhibit a distinct foliation. Below Arenal volcano a zoned magma chamber evolved prior the last eruptive cycle: three distinct andesitic magma layers were produced by simple AFC of a high-alumina basalt (HAB) with assimilation of Tertiary quartz–dioritic and granitic rocks. Early erupted 1968 tephra and 1969 lavas (which represent the first two layers of the upper part of a zoned magma chamber) were produced by simple AFC, with fractionation of plagioclase, pyroxene and magnetite and concomitant assimilation of quartz–dioritic rocks. Assimilation rates were constant (r<sub>1</sub>=0.33) for a relative mass of magma remaining of 0.77–0.80, respectively. Lavas erupted around 1974 are less differentiated and represent the `primitive andesitic magma type' residing within the middle–lower part of the chamber. These lavas were also produced by simple AFC: assimilation rates and the relative mass of magma remaining increased of about 10%, respectively (r₁=0.36, and F=0.89). Ba enrichment of the above lavas is related to selective assimilation of Ba from Tertiary granitic rocks. Lava eruption occurred as a dynamic response to the intrusion of a new magma into the old reservoir. This process caused the instability of the zoned magma column inducing syneruptive mixing between portions of two contiguous magma layers (both within the column itself and at lower levels where the new basalt was intruded into the reservoir). Syneruptive mixing (mingling) within the middle–upper part of the chamber involved fractions of earlier gabbroic cumulitic materials (lavas erupted around 1970). On the contrary, within the lower part of the chamber, mixing between the intruded HAB and the residing andesitic melt was followed by simple fractional crystallization (FC) of the hybrid magma layer (lavas erupted in 1978–1980). By that time the original magma chamber was completely evacuated. Lavas erupted in 1982/1984 were thus modelled by means of `open system' AFCRE (i.e., AFC with continuous recharge of a fractionating magma batch during eruption): in this case assimilation rates were r₁=0.33 and F=0.86. Recharge rates are slightly higher than extrusion rates and may reflect differences in density (between extruded and injected magmas), together with dynamic fluctuations of these parameters during eruption. Ba and LREE (La, Ce) enrichments of these lavas can be related to selective assimilation of Tertiary granitic and quartz–dioritic rocks. Calculated contents for Zr, Y and other REE are in acceptable agreement with the observed values. It is concluded that simple AFC occurs between two distinct eruption cycles and is typical of a period of repose or mild and decreasing volcanic activity. On the contrary, magma mixing, eventually followed by fractional crystallization (FC) of the hybrid magma layer, occurs during an ongoing eruption. Open-system AFCRE is only operative when the original magma chamber has been totally replenished by the new basaltic magma, and seems a prelude to the progressive ceasing of a major eruptive cycle.     

扬子陆块西缘晚古生代玄武岩浆的性质和演化——玄武岩-辉绿玢岩稀土元素-微量元素地球化学研究

以四川和云南晚二叠世-早三叠世形成的峨嵋山玄武岩和同期形成的龙门山、墨江辉绿玢岩为主要研究对象,结合与勉略、三江蛇绿岩套内的玄武岩的对比,从岩石的化学成分、稀土元素和微量元素的含量和特征比值分析,提出晚古生代扬子陆块西缘的基性岩浆活动经历了由特提斯洋壳俯冲诱发陆块内玄武岩浆的发生、洋壳消减和陆块内大规模玄武岩浆喷发及局部小规模岩浆活动的萎缩消亡3个发展演化阶段.扬子陆块内玄武岩浆主要来源于正常或富集型地幔,在发生和萎缩阶段部分岩浆源自亏损型地幔.扬子陆块西缘晚古生代时期的基性岩浆活动是全球岩石圈构造活动的局域表现,与特提斯洋的俯冲-封闭有直接联系.     

莫来铁尖晶岩的发现和玄武岩浆的深部分异作用

莫来铁尖晶岩是首次发现的一种新型火山岩,由铁尖晶石、莫来石、铁板钛矿和火山玻璃组成,其平均化学成分相当于一个简化的SiO_2-FeO~*—Al_2O_3三元系。莫来铁尖晶岩浆是原始岩浆深部分异的结果。原始玄武岩浆在地幔深处分离结晶出富含硬玉分子的斜方辉石、单斜辉石,进化岩浆向富Al、Fe而贫Si的方向演化,最终产生莫来铁尖晶岩浆。后者喷出地表,冷凝结晶形成莫来铁尖晶岩。     

Variable involvements of mantle plumes in the genesis of mid-Neoproterozoic basaltic rocks in South China: A review

Ca. 825–720 Ma global continental intraplate magmatism is generally linked to mantle plumes or a mantle superplume that caused rifting and fragmentation of the supercontinent Rodinia. Widespread Neoproterozoic igneous rocks in South China are dated at ca. 825–760 Ma. There is a hot debate on their petrogenesis and tectonic affiliations, i.e., mantle plume/rift settings or collision/arc settings. Such competing interpretations have contrasting implications to the position of South China in the supercontinent Rodinia and in Rodinia reconstruction models.Variations in the bulk-rock compositions of primary basaltic melts can provide first order constraints on the mantle thermal–chemical structure, and thus distinguish between the plume/rift and arc/collision models. Whole-rock geochemical data of 14 mid-Neoproterozoic (825–760 Ma) basaltic successions are reviewed here in order to (1) estimate the primary melts compositions; (2) calculate the melting conditions and mantle potential temperature; and (3) identify the contributions of subcontinental lithosphere mantle (SCLM) and asenthospheric mantles to the generation of these basaltic rocks.In order to quantify the mantle potential temperatures and percentages of decompression melting, the primary MgO, FeO, and SiO₂ contents of basalts are calculated through carefully selecting less-evolved samples using a melting model based on the partitioning of FeO and MgO in olivine. The mid-Neoproterozoic (825–760 Ma) potential temperatures predicted from the primary melts range from 1390 °C to 1630 °C (mostly > 1480 °C), suggesting that most 825–760 Ma basaltic rocks in South China were generated by melting of anomalously hot mantle sources with potential temperatures 80–200 °C higher than the ambient Middle Ocean Ridge Basalt (MORB)-source mantle.The mantle source regions of these Neoproterozoic basaltic rocks have complex histories and heterogeneous compositions. Enriched mantle sources (e.g., pyroxenite and eclogite) are recognized as an important source for the Bikou and Suxiong basalts, suggesting that their generations may have involved recycled components. Trace elements variations show that interactions between asthenospheric mantle (OIB-type mantle) and SCLM played a very important role in generation of the 825–760 Ma basalts. Our results indicate that the SCLM metasomatized by subduction-induced melts/fluids during the 1.0–0.9 Ga orogenesis as a distinct geochemical reservoir that contributed significantly to the trace-elements and isotope inventory of these basalts.The continental intraplate geochemical signatures (e.g., OIB-type), high mantle potential temperatures and recycled components suggest the presence of a mantle plume beneath the Neoproterozoic South China block. We use the available data to develop an integrated plume-lithosphere interaction model for the ca. 825–760 Ma basalts. The early phases of basaltic rocks (825–810 Ma) were most likely formed by melting within the metasomatized SCLM heated by the rising mantle plume. The subsequent continental rift allowed adiabatic decompression partial melting of an upwelling mantle plumes at relatively shallow depth to form the widespread syn-rifting basaltic rocks at ca. 810–800 Ma and 790–760 Ma.     

江西中西部地区白垩纪玄武质岩石的40Ar/39Ar年代学和地球化学研究

对江西中西部两处中生代玄武质岩石进行了^40Ar／^39Ar年代学和元素一同位素地球化学研究,赣西北上高和赣中螺丝山地区玄武质岩石分别给出了137Ma和90Ma的全岩^40Ar／^39Ar年龄。赣西北上高为亚碱性的玄武安山岩,而赣中螺丝山为碱性玄武岩／粗玄岩,两者均以富集LREE和LILE、无明显HFSE亏损为特征。前者初始^86Sr=0．70492～0．70575,εxd(t)=0．61～3．36,落在Samoa-OIB同位素组成范围;后者初始ε=0．70323～0．70350,εxd(t)=6．19～6．93,与Hawaii-OIB的Sr-Nd同位素组成相似。与赣南早中生代(约173Ma)EMⅡ型玄武岩一起暗示该地区中生代岩浆源区有着自岩石圈向软流圈属性演化的时序迁移规律,表明该区自早中生代以来处于软流圈上隆的岩石圈伸展构造背景。     

Shifting styles of basaltic explosive activity during the 2002–03 eruption of Mt. Etna, Italy

The 2002–03 flank eruption of Etna was characterized by two months of explosive activity that produced copious ash fallout, constituting a major source of hazard and damage over all eastern Sicily. Most of the tephra were erupted from vents at 2750 and 2800 m elevation on the S flank of the volcano, where different eruptive styles alternated. The dominant style of explosive activity consisted of discrete to pulsing magma jets mounted by wide ash plumes, which we refer to as ash-rich jets and plumes. Similarly, ash-rich explosive activity was also briefly observed during the 2001 flank eruption of Etna, but is otherwise fairly uncommon in the recent history of Etna. Here, we describe the features of the 2002–03 explosive activity and compare it with the 2001 eruption in order to characterize ash-rich jets and plumes and their transition with other eruptive styles, including Strombolian and ash explosions, mainly through chemical, componentry and morphology investigations of erupted ash. Past models explain the transition between different styles of basaltic explosive activity only in terms of flow conditions of gas and liquid. Our findings suggest that the abundant presence of a solid phase (microlites) may also control vent degassing and consequent magma fragmentation and eruptive style. In fact, in contrast with the Strombolian or Hawaiian microlite-poor, fluidal, sideromelane clasts, ash-rich jets and plumes produce crystal-rich tachylite clasts with evidence of brittle fragmentation, suggesting that high groundmass crystallinity of the very top part of the magma column may reduce bubble movement while increasing fragmentation efficiency.     

Shallow Miocene basaltic magma reservoirs in the Bahia de Los Angeles basin, Baja California, Mexico

The basement in the Bahía de Los Angeles basin consists of Paleozoic metamorphic rocks and Cretaceous granitoids. The Neogene stratigraphy overlying the basement is formed, from the base to the top, by andesitic lava flows and plugs, sandstone and conglomeratic horizons, and Miocene pyroclastic flow units and basaltic flows. Basaltic dikes also intrude the whole section. To further define its structure, a detailed gravimetric survey was conducted across the basin about 1 km north of the Sierra Las Flores. In spite of the rough and lineal topography along the foothills of the Sierra La Libertad, we found no evidence for large-scale faulting. Gravity data indicates that the basin has a maximum depth of 120 m in the Valle Las Tinajas and averages 75 m along the gravimetric profile. High density bodies below the northern part of the Sierra Las Flores and Valle Las Tinajas are interpreted to be part of basaltic dikes. The intrusive body located north of the Sierra Las Flores is 2.5 km wide and its top is about 500 m deep. The lava flows of the top of the Sierra Las Flores, together with the distribution of basaltic activity north of this sierra, suggests that this intrusive body continues for 20 km along a NNW-trending strike. Between the sierras Las Flores and Las Animas, a 0.5-km-wide, 300-m-thick intrusive body is interpreted at a depth of about 100 m. This dike could be part of the basaltic activity of the Cerro Las Tinajas and the small mounds along the foothills of western Sierra Las Animas. The observed local normal faulting in the basin is inferred to be mostly associated with the emplacement of the shallow magma reservoirs below Las Flores and Las Tinajas.