bluuebunny/crossref_metadata_2025_split huggingface dataset:
Partial melting of a lithologically heterogeneous mantle: inferences from crystallization histories of magnesian abyssal tholeiites from the Siqueiros Fracture Zone
Authors: Natland James H. 
Published: April 2008

Summary

Complex processes of magma mixing are revealed by the crystallization histories of several magnesian, porphyritic abyssal tholeiites dredged from the Siqueiros Fracture Zone at about 8°30′ N near its intersection with the East Pacific Rise. The compositions of chromian spinels, olivines, plagioclases and glass inclusions in these rocks indicate that polybaric crystallization and mixing occurred at high temperatures involving a range of refractory (low-TiO
2
, low-Na
2
O) melts. These later coalesced with cooler much less refractory components to produce the bulk rocks.

The crystallization histories and comparisons with experimental data indicate that mantle sources of these basalts were lithologically heterogeneous, ranging from fertile (lherzolitic) to refractory (in the extreme, harzburgitic) with respect to a basaltic melt fraction. The separate magma strains going into a typical magnesian abyssal tholeiite were produced by simultaneous partial melting of the different lithofacies, after which they coalesced and mixed. Such mixing is more likely than batch melting to explain the ranges in CaO/Al
2
O
3
, and abundances of Na
2
O and TiO
2
, within the fairly small range and low (non-picritic) magnesium numbers (0.75–0.64) exhibited by all liquids including those inferred from mineral compositions.

The early crystallization of minerals from refractory magma strains suggests that melting domains beneath spreading ridges consist of cores of refractory peridotite surrounded by cooler, more fertile peridotite. Sequential mixing of refractory and fertile magma strains is an unavoidable consequence of the ascent of magma batches from the interiors of complex melting regimes (diapirs) in the mantle. The diapirs may have been triggered into ascent by the buoyancy of low-density (low-iron) refractory peridotite.
Several of the Siqueiros magnesian abyssal tholeiites are among the most depleted basalts ever obtained from the eastern Pacific, and yet alkalic basalt was also dredged from the fracture zone a few kilometres away. The tholeiites are even more depleted than typical, more fractionated, summit eruptives of the East Pacific Rise. The contrast may arise because of the homogenizing effects of axial magma chambers, within which truly depleted tholeiites are mixed with enriched alkalic magmas instead of erupting separately, as they do in the fracture zone. Mantle in this region thus appears to be segregated into strongly contrasting depleted and enriched peridotites, with depleted peridotite also ranging from refractory to fertile lithofacies. Primary lithological variability in the mantle thus was probably imposed by one or more ancient melting events that left distinctively enriched and depleted reservoirs in their wake.
Mineral relationships in gabbroic rocks from fracture zones of Indian Ocean ridges: evidence for extensive fractionation, parental diversity and boundary-layer recrystallization
Authors: Bloomer Sherman H. , Natland James H. , Fisher Robert L. 
Published: April 2008

Summary

Evolved two-pyroxene gabbros and ferrogabbros predominate in gabbroic suites dredged from five fracture zones on the SW and Central Indian Ridges. Compositions of olivines (Fo
56
-Fo
85
) and plagioclases (An
10
-An
80
) and the generally low magnesium numbers of orthopyroxene (0.58–0.84) and clinopyroxene (0.50–0.89) indicate that most gabbros crystallized from liquids more fractionated than those represented by basalts from adjacent ridge segments and the fracture zones themselves. This disparity, the paucity of diabases and gabbros, the absence of more magnesian gabbros and olivine-spinel cumulates, and the abundance of serpentinite in the dredge collections suggest that the fracture-zone gabbros crystallized in small magma bodies such as dykes or sills. These were emplaced laterally from large central magma chambers along ridge axes towards the fracture zones beneath a carapace of less fractionated basalt.

Rare olivine gabbros, however, contain magnesian clinopyroxene (magnesium number >0.84) and orthopyroxene (magnesium number >0.70), compositions which are more magnesian than those of pyroxenes that have been crystallized experimentally from certain ocean-ridge basalts at 1 atm. It has been suggested that such high-magnesium pyroxenes result from moderate- to high-pressure fractionation. However, the magnesian pyroxenes in the Indian Ocean samples follow olivine and plagioclase in the crystallization sequence and have compositions appropriate for crystallization at low pressure. For example, they match compositions of phenocrysts in refractory (low-Na
2
O, low-TiO
2
) siliceous (51–53% SiO
2
) basalts from Deep Sea Drilling Project sites in Eocene-Cretaceous portions of the Indian Ocean. Such refractory basalts have not been studied experimentally, but the early crystallization of bronzite at low pressure in one of them is consistent with the siliceous and magnesian composition of the host glass.

Alternatively, the more magnesian pyroxenes in the gabbros may have resulted from a process of
in situ
boundary-layer recrystallization (resorption and recrystallization) of minerals on the walls of small magma bodies as high-temperature magmas were repeatedly injected along them. This mechanism is suggested by phase relationships in simplified basaltic systems at low pressure and by the compositions of rounded (resorbed) mega-crystal minerals which occur in many tholeiites from the Indian Ocean and elsewhere.
The Fjallgardar volcanic ridge in NE Iceland: an aborted early stage plate boundary or a volcanically dormant zone?
Authors: Helgason Johann 
Published: April 2008

Summary
The extinct Fjallgardar volcanic ridge (FVR) in NE Iceland is approximately 15–20 km wide and extends for about 190 km from the Vatnajökull glacier toward the Slétta peninsula in the far N. The FVR was mainly formed subglacially and strikingly parallels the curvature of the present median line (Askja to Theistareykir centres) of the neovolcanic zone (NVZ) which lies about 40 km to the W.

The basalts from the Fjallgardar and Slétta regions are dominantly evolved tholeiites as shown by major- and trace-element compositions of 42 samples. The average magnesium number is 47 (range 37–58). 90 per cent of the samples can be grouped into either ‘low-’ potassium (0.1%–0.4% K
2
O) or ‘high-’ potassium (0.5%–0.7% K
2
O) units. The high-potassium lavas in the FVR are chemically similar to Tertiary tholeiites of eastern Iceland.

A test for temporal geochemical variations for three stratigraphic formations was carried out for the Mödrudalur area of the FVR. The oldest of the three formations (M3) consists of low-potassium basalts only, whereas the two younger formations (M4 and M5) consist predominantly of more evolved high-potassium basalts. Formation M5 is most probably fed into Mödrudalur from the Kverkfjöll volcanic system some 75 km further S.
Volcanism in the FVR S of Slétta was short-lived and lasted for perhaps 200 000–300 000 years. Subsurface lateral flow of magma from the NVZ is probably responsible for the Recent subaerial volcanism in Slétta. In the Slétta area the older subglacially formed ridges have only a low-potassium basalt composition.
The FVR may be viewed as ‘abortive’ and as the precursor to the NVZ in northeastern Iceland. During the Brunhes magnetic polarity chron (less than 0.7 Ma) a rift jump is postulated to have occurred from the FVR to the present NVZ, approximately 40 km to the W. E.-W lateral rift jumps (20–40 km) may have been common in the eastern half of Iceland in the vicinity of the Iceland hot-spot located 150–200 km E of the Reykjanes and Kolbeinsey mid-ocean ridges.
Inversion of the Kechika Trough, Northeastern British Columbia, Canada
Authors: McClay K. R. , Insley M. W. , Anderton R. 
Published: April 2008

Summary
The Kechika Trough, in the western Rocky Mountains of northern Canada, is a NW-trending fold and thrust belt of Hadrynian through Mississippian strata within the telescoped passive margin of ancestral North America. The Kechika Trough underwent major crustal extension in the Mid-Late Devonian with the deposition of coarse to fine ‘Antler’ type black clastics in an asymmetrical basin consisting of a terraced half-graben system to the west and a roll-over structure to the east. Inversion and contraction of the Kechika Trough occurred during the Jurassic-Cretaceous Columbian-Laramide orogeny. The geometries of the resultant fold and thrust systems were strongly controlled by the half-graben extensional architectures and by the geometries of the synrift sediment packages. Inversion structures include contractionally reactivated extensional faults, rotated and re-utilized extensional faults, and thrust faults pinned by extensional faults as a result of buttressing against rigid sediment wedges. Inversion of the synrift sediments is associated with strongly developed chevron folding, pervasive cleavage development and imbrication by out-of-half-graben thrust faults. These thrust faults may locally climb down stratigraphic section in the direction of transport as a result of cutting through strata tilted by Mid-Late Devonian extensional faults. Major stratigraphic contrasts between adjacent thrust packages have been resolved in terms of lateral facies differences produced by extension, rather than by over-complex thrust geometries. Although syndepositional extensional faults are not preserved in the thrust belt, their position and geometry may be inferred from an analysis of structural, stratigraphic and sedimentological relationships.
Fold and fault geometry in the western Helvetic nappes of Switzerland and France and its implication for the evolution of the arc of the western Alps
Authors: Ramsay J. G. 
Published: April 2008

Summary
The main features of the fold and fault geometry of the Helvetic nappes and Chaînes Subalpines of Switzerland and France have been determined by the construction of tie lines between outcropping hinge points of folds and cut-off points of strata against thrusts. Longitudinal correlation of these structures is now possible. Folds are very persistent along their hinge lines and can often be followed for tens of kilometres. Fold hinges and thrust plane cut-off lines are extremely regularly oriented. The Alpine structures developed in the external massifs are obliquely superposed on previously emplaced nappes. The overall structural pattern is attributed to a progressive change of the relative displacement directions between the northern (European) plate and southern (Apulian) plate from N-S, through NW-SE, concluding with movements in an E-W sense. The well known structural culminations and depressions are interpreted as the result of the latest shortening in an E-W direction and not to complex ramp geometry or to duplex stacking.
South-Alpine thrusting and trans-Alpine convergence
Authors: Roeder D. 
Published: April 2008

Summary
The south front of the Alps in northern Italy contains a Neogene S-vergent fold-thrust belt. It affects the crust of the Adria plate, a series of shelf sediments, and S Alpine foredeep fill. The belt shows that after collision, the Alps assumed the bivergent wedge shape of an intracratonic compressional belt, notwithstanding its long prior history of N-vergent subduction and thrust imbrication.
Parts of the S Alpine fold-thrust belt are buried beneath Apennine foredeep fill. Thrust transport is about 60 km to the SE; it has produced detached foothills folds, a triangle zone of stacked blind and emergent thrusts, and a basement front with out-of-sequence thrusting above a line of duplex anticlines. The structure of the foreland can be described by the curvature of an elastic line-load deflection. Details not predicted by the load model include Mesozoic rifting, foreland upthrusts, and perhaps complex map patterns of thrust fronts.
A match between trans-Tethys convergence and Alpine kinematics suggests either that a 150 km wide oceanic gap north of the Tauern was closed in late Miocene. Or it suggests an origin of the Tauern terranes on the S margin of a sub-Apennine crustal slab. The first choice is consistent with traditional Alpine crustal data but inconsistent with Mesozoic palaeogeography. The second choice is consistent with newer, EGT-generated, crustal data from the Ligurian—Apennine area. It admits 26% compression or 80 km bulk strain within the upper crustal slab while plate convergence generates 380 km of thrust overlap.
A conceptual model of Neogene Alpine convergence, consistent with the second choice, assumes increased traction where the basal detachment of the upper crustal slab passes upward into the brittle shell. Part of the viscous overlap at plate convergence rates is translated upward into compression within a back-to-back pair of thickening and over-critical Coulomb wedges.
Deep-reaching geodynamic processes in the Alps
Authors: Mueller St. 
Published: April 2008

Summary
The Alps are a deep-reaching crust-mantle structure in Europe situated at the northernmost tip of the Adriatic promontory of the African plate. In that region the contact against the Eurasian plate is presently characterized by a NW-SE oriented compressional stress regime. A detailed regional analysis of seismic surface-wave dispersion and of P-wave travel time residuals (‘seismic tomography’) as well as long-range seismic refraction measurements and the interpretation of ‘stripped’ residual gravity anomalies have revealed a rather anomalous structure within the uppermost mantle under the Alpine arc. As a result of the massive continent-continent collision, parts of the lithosphere have been delaminated — a process which has led to ‘flaking’ in the upper crust, combined with a thickening of the entire crust and the formation of a pronounced relatively cold, dense and slowly subsiding ‘lithospheric root’ beneath the mountain chain. In this still continuing plate collision the lower parts of the lithosphere apparently have penetrated into the asthenosphere to a depth of 130 to 200 km in a steep zone of ‘subfluence’ (‘Verschluckung’). On this scale the presently measurable uplift of the Alpine chain (about 0.1 cm per year) is a secondary effect due to isostatic rebound of less dense crustal masses which previously had been forced to greater depths. A configuration of this type generates regionally a compressive dynamics of its own on which — within a wider framework — rotational processes may be superimposed.
Indentation and rotation in the western Alpine arc
Authors: Vialon P. , Rochette P. , Ménard G. 
Published: April 2008

Summary
The overall structural aspects of the external western Alpine arc and foreland may be consequences of NW-SE compression ahead of crustal thrusting on the northern front and of mainly sinistral wrenching along its southwestern lateral border. However such a model assumes a simple straight-line displacement which, while possibly valid on the continental scale, cannot be considered for the external arc since a model of linear translation and rotation around a distant pole is not compatible with the structural organization of the western arc of the Alps. The constant tangential stretching parallel to the curved front, the fan-ring shortening and shear directions on décollement levels, the complex nappe transport trajectories and the widespread dextral wrench-faulting around the arc, can be better explained by local rotation of a block belonging to the European margin, dragged along the western sidewall of a large indenter moving northwestwards. This laterally displaced block produces an anticlockwise ring shear about a pole of rotation within it. Consequently, the Alpine arc cannot be understood in terms of plane-strain models along local ‘transport’ directions. The deformation process corresponds instead to lateral rotation-expulsion of a deformed unit belonging to the European Plate margin, jammed against its stable foreland.
Phyllosilicate formation in berthierine peloids and iron oolites
Authors: Velde B. 
Published: April 2008

Summary
Electron microprobe compositions of phyllosilicates formed in berthierine pellets and iron oolites suggest two different reaction paths which result in slightly different bulk chemical compositions.
In the case of berthierine peloids, for mineral aggregates formed and transformed at or very near the sediment—sea water interface, the starting materials seem to be most often dominated by kaolinite. The gradual incorporation of iron and magnesium produces the chlorite composition within the pellet. Systematic compositional zoning is apparent when pellet edges are compared to pellet centres. The compositions of the chlorite end-product have a rather large range of Fe/Fe + Mg (0.65–0.88). The general compositional range of the transformed berthierine peloids is less aluminous than kaolinite suggesting a high silica activity in the solutions, which is independent of the dissolution of kaolinite.

In the case of berthierine/chamosite-bearing iron oxide oolites, the system is dominated by the presence of iron, as would be expected. The range of compositions observed using the electron microprobe on oolites which still contain iron oxides gives an Al/Si ratio of one, that of kaolinite. The formation of these chlorites is through a combination of divalent iron and kaolinite. These minerals have Fe/ Fe + Mg = 0.88–1.0. The bulk composition is Si
26
Al
26
(Fe,Mg)
48
. Multiple point analyses in the same oolite or pellet indicate that the formation of chlorite occurs at various points in the mass with no specific zoning pattern.

When the iron oxide in a rock is completely reacted to form silicate and/or phosphate phases (the oolites are completely chloritized), the berthierine/chamosite mineral composition can become less aluminous due to a general chemical equilibrium established throughout the sample. In more metamorphic iron ore rocks, where the oolitic structure is absent and where new iron silicates are present (such as stilpnomelane) and iron oxides are present, the chlorite composition can be found to vary from that formed in the initial reaction, i.e. Si/Al = 1. They can be more silica-rich or more aluminous.
These analytical results indicate that the composition of the chlorites formed in high iron environments reflect the chemical system in which they form and the scale of that system, a pellet or a rock.
A depositional model for the Liassic Minette ironstones (Luxemburg and France), in comparison with other Phanerozoic oolitic ironstones
Authors: Teyssen T. 
Published: April 2008

Summary
Oolitic ironstones generally mark the top of a regressive, coarsening- and shallowing-upward depositional megasequence. The iron ooids themselves are preferentially formed during sea-level lowstands or in condensed sections. This general model is valid for the Minette iron formation which was deposited in Toarcian and Aalenian times at the north-eastern margin of the Liassic Paris Basin in a near-shore, shallow marine, mostly high-energy environment. The top of the regressive sequence shows intraformational reworking. The Minette, however, is a remarkably thick unit (up to 60 metres), also internally composed of coarsening-up sequences. In many places these sequences represent large-scale subtidal sand waves, elsewhere large subtidal shoals advancing over their distal finer grained facies. Strong tidal currents building up large sand waves are preferentially active during transgressions. For the Minette base level rise events (transgressions) are therefore required at the top of a base level fall sequence (regression). This apparent discrepancy can be resolved when considering the tectonic setting. Reactivation of structures of the underlying Variscian basement led to subsidence events in syndepositionally formed troughs. This model can explain a high aggradational potential and deposition of thick ironstone deposits at the top of a regressive sequence while erosion took place laterally along the same palaeocoastline.
Late Cambrian molluscan faunas and the origin of the Cephalopoda
Authors: Webers Gerald F. , Yochelson Ellis L. 
Published: April 2008

Abstract
Late Cambrian gastropod and monoplacophoran faunas from western Antarctica, eastern and mid-western North America, and northern China, are diverse and provide some insight into palaeogeography and faunal distribution. The oldest of these, a well-preserved trilobite-mollusc fauna from the Ellsworth Mountains, Antarctica is dated as latest Dresbachian (Idamean); the Antarctic fauna is not unlike a slightly younger one (Franconian) from east-central Minnesota, USA. Slightly younger Franconian-age rocks from north China contain the first authentic cephalopods. A Trempealeauan fauna from eastern New York, USA, is closely comparable to another Trempealeauan fauna from north China.
Relatively high taxonomic diversity suggests tropical to sub-tropical marine environments for all these faunas. Plate tectonic position as interpreted from palaeomagnetism supports this interpretation. The occurrence of these molluscs on separate continents, and the presumed widely separated position of these continents in the Late Cambrian, makes it likely that the distribution mechanism for the molluscs was long-lived larval forms.

A key member of the Antarctic faunule is the monoplacophoran
Knightoconus
which has been considered representative of a group directly ancestral to the oldest cephalopod
Plectronoceras
. This view is reconsidered in the light of criticisms and alternatives proposed during the last 15 years; these occupy the bulk of the paper. Reaffirmation of evolution from the Antarctic
Knightoconus
is supported both by refutation of the criticisms and by ontogenetic studies of new topotypical material of the genus.
Antarctica: Cretaceous cradle of austral temperate rainforests?
Authors: Dettmann Mary E. 
Published: April 2008

Abstract

Although it is well known that certain Gondwanic elements of present-day austral temperate rainforests occurred on Antarctica during latest Cretaceous to early Tertiary times, there has been insufficient factual evidence for pinpointing the cradle of these forests. Fossil evidence from Antarctica and closely associated regions in the Creataceous southern Gondwanan assembly confirms that Antarctica was a Cretaceous origination and dispersal region of certain elements of today’s southern hemispheric humid and perhumid forests. Antarctic origins are indicated for the fern
Lophosoria
, the podocarp gymnosperms
Lagarostrobus
and
Dacrydium, Nothofagus, Ilex
, and several lineages of the Proteaceae; migration to their present regions of distribution was probably step-wise. Antarctica also served as a Cretaceous dispersal corridor for other angiosperms represented today in mid to low latitude austral regions. These include
Ascarina
(or its stock), Myrtaceae, Gunneraceae, and Winteraceae, all of which had earlier histories in northern Gondwana or southern Laurasia. Origination and dispersal appears to be related to changing environmental circumstances associated with fragmentation of Gondwana and opening and enlargement of the southern oceans.