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The Middle Jurassic Kirkpatrick flood basalts and comagmatic Ferrar intrusions in the Transantarctic Mountains represent
a major pulse of tholeiitic magmatism related to early stages in the breakup of Gondwana. A record of the volcano-tectonic
events leading to formation of this continental flood-basalt province is provided by strata underlying and only slightly predating
the Kirkpatrick lavas. In the central Transantarctic Mountains, the lavas rest on widespread (≥7500 km2) tholeiitic pyroclastic deposits of the Prebble Formation. The Prebble Formation is dominated by lahar deposits and is an
unusual example of a regionally developed basaltic lahar field. Related, partly fault-controlled pyroclastic intrusions cut
underlying strata, and vents are represented by the preserved flanks of two small tephra cones associated with a volcanic
neck. Lahar and air-fall deposits typically contain 50–60% accidental lithic fragments and sand grains derived from underlying
Triassic – Lower Jurassic strata in the upper part of the Beacon Supergroup. Juvenile basaltic ash and fine lapilli consist
of nonvesicular to scoriaceous tachylite, sideromelane, and palagonite, and have characteristics indicating derivation from
hydrovolcanic eruptions. The abundance of accidental debris from underlying Beacon strata points to explosive phreatomagmatic
interaction of basaltic magma with wet sediment and groundwater, which appears to have occurred in particular where rising
magma intersected upper Beacon sand aquifers. Composite clasts in the lahar deposits exhibit complex peperitic textures formed
during fine-scale intermixing of basaltic magma with wet sand and record steps in subsurface fuel-coolant interactions leading
to explosive eruption.
The widespread, sustained phreatomagmatic activity is inferred to have occurred in a groundwater-rich topographic basin linked
to an evolving Jurassic rift zone in the Transantarctic Mountains. Coeval basaltic phreatomagmatic deposits of the Mawson
and Exposure Hill Formations, which underlie exposures of the Kirkpatrick Basalt up to 1500 km to the north along strike in
Victoria Land, appear to represent other parts of a regional, extension-related Middle Jurassic phreatomagmatic province which
developed immediately prior to rapid outpouring of the flood basalts. This is consistent with models which assign an important
role to lithospheric stretching in the generation of flood-basalt provinces.
Received: 28 August 1995 / Accepted: 18 April 1996 相似文献
2.
Tereza Cristina Junqueira-Brod Jos Carlos Gaspar Jos Affonso Brod Camilla Vasconcelos Kafino 《Journal of South American Earth Sciences》2005,18(3-4):337-353
Kamafugitic rocks intruded the Precambrian basement and Phanerozoic sediments at the northeast border of the Paraná basin as part of the Late Cretaceous Goiás alkaline province (GAP). Plutonic complexes dominate the north of the province, whereas lavas and pyroclastic rocks prevail in the south. The central GAP is characterized by kamafugitic diatremes, which may crop out continuously for up to 850 m and consist of a central breccia body, surrounded and overlain by lava flows and crosscut by dykes. The breccias contain some special spheroidal juvenile fragments—namely, accretionary and armored lapilli, frozen droplets, spinning droplets, and wrapped fragments—whose textural and mineralogical aspects are described in detail. Irregularly shaped tuff pockets that occur within the breccias contain textures and structures similar to those of subaerial surge deposits and formed in confined, high gas to solid+liquid ratio domains in the conduit. Diatreme emplacement affected the country rock through thermal metamorphism, development of columnar jointing, and formation of peperite-like mixtures. There is no evidence of phreatomagmatic activity in the diatremes, and CO2, rather than H2O, seems to have been the major volatile component of the kamafugitic magmas. This finding implies that features such as accretionary lapilli and peperites are not exclusively associated with H2O-dominated processes. 相似文献
3.
Large phreatomagmatic vent complex at Coombs Hills, Antarctica: Wet, explosive initiation of flood basalt volcanism in the Ferrar-Karoo LIP 总被引：2，自引：2，他引：2
The Mawson Formation and correlatives in the Transantarctic Mountains and South Africa record an early eruption episode related
to the onset of Ferrar-Karoo flood basalt volcanism. Mawson Formation rocks at Coombs Hills comprise mainly (≥80% vol) structureless
tuff breccia and coarse lapilli tuff cut by irregular dikes and sills, within a large vent complex (>30 km2). Quenched juvenile fragments of generally low but variable vesicularity, accretionary lapilli and country rock clasts within
vent-fill, and pyroclastic density current deposits point to explosive interaction of basalt with groundwater in porous country
rock and wet vent filling debris. Metre-scale dikes and pods of coherent basalt in places merge imperceptibly into peperite
and then into surrounding breccia. Steeply dipping to sub-vertical depositional contacts juxtapose volcaniclastic rocks of
contrasting componentry and grainsize. These sub-vertical tuff breccia zones are inferred to have formed when jets of debris
+ steam + water passed through unconsolidated vent-filling deposits. These jets of debris may have sometimes breached the
surface to form subaerial tephra jets which fed subaerial pyroclastic density currents and fall deposits. Others, however,
probably died out within vent fill before reaching the surface, allowing mixing and recycling of clasts which never reached
the atmosphere. Most of the ejecta that did escape the debris-filled vents was rapidly recycled as vents broadened via lateral
quarrying of country rock and bedded pyroclastic vent-rim deposits, which collapsed along the margins into individual vents.
The unstratified, poorly sorted deposits comprising most of the complex are capped by tuff, lapilli tuff and tuff breccia
beds inferred to have been deposited on the floor of the vent complex by pyroclastic density currents. Development of the
extensive Coombs Hills vent-complex involved interaction of large volumes of magma and water. We infer that recycling of water,
as well as recycling of pyroclasts, was important in maintaining water supply for phreatomagmatic interactions even when aquifer
rock in the vent walls lay far from eruption sites as a consequence of vent-complex widening. The proportion of recycled water
increased with vent-complex size in the same way that the proportion of recycled tephra did. Though water recycling leaves
no direct rock record, the volcaniclastic deposits within the vent complex show through their lithofacies/structural architecture,
lithofacies characteristics, and particle properties clear evidence for extensive and varied recycling of material as the
complex evolved.
Editorial responsibility: J. Donnelly-Nolan 相似文献
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5.
Processes of magma/wet sediment interaction in a large-scale Jurassic andesitic peperite complex, northern Sierra Nevada, California 总被引：3，自引：0，他引：3
The Middle Jurassic Tuttle Lake Formation in the northern Sierra Nevada, California, comprises a thick volcaniclastic sequence
deposited in a submarine island-arc setting and penetrated by numerous related hypabyssal intrusions. A composite andesite-diorite
intrusive complex ≥4.5 km long and ≥1.5 km thick was emplaced while the host Tuttle Lake sediments were still wet and unconsolidated.
Large parts of the intrusive complex consist of peperite formed where andesitic magma intruded and intermixed with tuff, lapilli-tuff
and tuff-breccia. The southern half of the complex consists of augite-phyric andesite containing peperite in numerous small,
isolated pockets and in more extensive, laterally continuous zones. The peperites comprise three main types recognized previously
in other peperite studies. Fluidal peperite consists of small (≤30 cm), closely spaced, at least partly interconnected, globular to amoeboid andesite bodies enclosed
by tuff. This peperite type developed during intrusion of magma into fine-grained wet sediment along unstable interfaces,
and fluidization of the sediment facilitated development of complex intrusive geometries. Blocky peperite and mixed blocky and fluidal peperite formed where magma intruded coarser sediment and underwent variable degrees of brittle fragmentation by quenching and dynamic
stressing of rigid margins, possibly aided by small steam explosions. The northern half of the intrusive complex consists
predominantly of a different type of peperite, in which decimetre-scale plagioclase-phyric andesite clasts with ellipsoidal,
elongate, or angular, polyhedral shapes are closely packed to widely dispersed within disrupted host sediment. Textural features
suggest the andesite clasts were derived from conduits through which magma was flowing, and preserved remnants of the conduits
are represented by elongate, sinuous bodies up to 30 m or more in length. Disruption and dispersal of the andesite clasts
are inferred to have occurred at least partly by steam explosions that ripped apart a network of interconnected feeder conduits
penetrating the host sediments. Closely packed peperite is present adjacent to mappable intrusions of coherent andesite, and
along the margin of a large mass of coarse-grained diorite. These coherent intrusions are considered to be major feeders for
this part of the complex. Examples of magma/wet sediment interaction similar in scale to the extensive peperites described
here occur elsewhere in ancient island-arc strata in the northern Sierra Nevada. Based on these and other published examples,
large-scale peperites probably are more common than generally realized and are likely to be important in settings where thick
sediment sequences accumulate during active volcanism. Careful mapping in well-exposed terrains may be required to recognize
large-scale peperite complexes of this type.
Received: 8 June 1998 / Accepted: 4 December 1998 相似文献
6.
Luigina Vezzoli Massimo Matteini Natalia Hauser Ricardo Omarini Roberto Mazzuoli Valerio Acocella 《Bulletin of Volcanology》2009,71(5):509-532
The Middle-Upper Miocene Las Burras–Almagro-El Toro (BAT) igneous complex within the Eastern Cordillera of the central Andes
(∼24°S; NW Argentina) has revealed evidence of non-explosive interaction of andesitic magma with water or wet clastic sediments
in a continental setting, including peperite generation. We describe and interpret lithofacies and emplacement mechanisms
in three case studies. The Las Cuevas member (11.8 Ma) comprises facies related to: (i) andesite extruded in a subaqueous
setting and generating lobe-hyaloclastite lava; and (ii) marginal parts of subaerial andesite lava dome(s) in contact with
surface water, comprising fluidal lava lobes, hyaloclastite, and juvenile clasts with glassy rims. The Lampazar member (7.8 Ma)
is represented by a syn-volcanic andesite intrusion and related peperite that formed within unconsolidated, water-saturated,
coarse-grained volcaniclastic conglomerate and breccia. The andesite intrusion is finger-shaped and grades into intrusive
pillows. Pillows are up to 2 m wide, tightly packed near the intrusion fingers, and gradually become dispersed in the host
sediment ≥50 m from the parent intrusion. The Almagro A member (7.2 Ma) shows evidence of mingling between water-saturated,
coarse-grained, volcaniclastic alluvial breccia and intruding andesite magma. The resulting intrusive pillows are characterized
by ellipsoidal and tubular shape and concentric structure. The high-level penetration of magma in this coarse sediment was
unconfined and irregular. Magma was detached in apophyses and lobes with sharp contacts and fluidal shapes, and without quench
fragmentation and formation of a hyaloclastite envelope. The presence of peperite and magma–water contact facies in the BAT
volcanic sequence indicates the possible availability of water in the system between 11–7 Ma and suggests a depositional setting
in this part of the foreland basin of the central Andes characterized by an overall topographically low coastal floodplain
that included extensive wetlands. 相似文献
7.
滇西北衙金矿区碱性斑岩岩石学、年代学和地球化学特征及其成因机制 总被引：15，自引：5，他引：15
报道了在北衙金矿区新发现的含矿石英钠长斑岩的特征以及某些罕见的地质现象,如石英正长斑岩侵位于岩溶洞穴,黑云正长斑岩侵位于未固结河湖相沉积物中,并形成诸如微晶与玻璃质基质共生和斑岩与围岩混积等奇特结构构造,并给出了矿区斑岩系统的~(39)Ar/~(40)Ar年龄。年龄结果显示:石英钠长斑岩形成于第三纪早期、年龄为65．56Ma,石英正长斑岩形成于25～33Ma之间,黑云正长斑岩侵位于上新世、两个岩体黑云母的年龄分别为3．66Ma和3．78Ma。矿区石英钠长斑岩、石英正长斑岩和黑云正长斑岩均为碱性斑岩,具有一些相似的地球化学特征,包括岩石中(K_2O+Na_2O)＞10%,SiO_2＞67%,富轻稀土、与OIB相似的REE稀土分配模式,Sr＞400×10~(-6),~(87)Sr/~(86)Sr＞0．707。矿区碱性斑岩的REE组成和Nd-Sr-Pb同位素组成显示斑岩成因于含有大洋岛屿玄武岩(OIB)、大洋中脊玄武岩(MORB)、正常沉积岩和上地幔(EMII+PREMA)组分的混合源,我们进一步推断北衙矿区碱性斑岩和滇西其它第三纪碱性斑岩一样成因于被掩埋的古金沙江特提斯大洋岩石圈与其上覆上地幔之间因印度-亚洲大陆俯冲-碰撞而导致的强烈剪切变形所引起和形成的熔体的上侵。 相似文献
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