Occurrence Of Breccias Research Articles

Long runout distance of a block breccia density flow along a half-graben axis: latest Jurassic rift climax in East Greenland

A basinally isolated breccia–pebbly sandstone couplet occurs in the axial part of the latest Jurassic rift-climax half-graben in Wollaston Forland, East Greenland. Synrift breccias are otherwise restricted to a narrow zone along the scarp of the basin-margin Dombjerg Fault. Towards the north the Dombjerg Fault sidesteps en echelon towards the east where it continues in the Thomsen Land Fault. The breccia part of the couplet was transported by a noncohesive debris flow and the pebbly sandstone by a high-density turbidity flow. The couplet is located about 15 km east of the Dombjerg scarp and flowed southwards along the half-graben axis. It is interpreted to have been derived from the southern end of the Thomsen Land Fault. This gives a runout distance of at least 25 km. Other isolated breccias have been reported towards the south along the half-graben axis, giving a runout distance of up to 40 km along the roughly horizontal half-graben axis. The angularity of the clasts is remarkable and reflects derivation directly from the fault scarp during a single catastrophic event. The occurrence of breccias thus cannot be taken as evidence of short transport and deposition at the foot of a scarp or steep slope.

The Serra da Borda Breccia Field, Aguapeí Mobile Belt, SW Amazonian Craton, western Brazil: geologic characterization based on field and petrographic data

Siliceous breccias with accompanying massive to banded metachert have been found hosted in metasandstones of the Fortuna Formation, the lower unit of the siliciclastic, mostly psammitic, Aguapei Group. This group developed mainly in a late Mesoproterozoic aborted continental rift or aulacogen of the Sunsas-Aguapei Province (1.2-0.95 Ga), whose evolution led to the stabilization of the southwestern fringe of the Amazonian Craton, promoting its final consolidation. Regionally, the breccias and related metachert form four main linear, NW-SE strike-parallel trends that lie symmetrically about the hinge zone of synclinal folds. These trends are known to occur for a length of 25 km and a width of up to 4 km, so that the main breccia occurrences are distributed over an area of ca. 100 km2 herein termed the Serra da Borda Breccia Field after the hills where they have been found in SW Mato Grosso state, western Brazil. The breccias display a multitude of textures and structures, e.g., massive, roughly tabular, bank-like outcrop morphology, dense to disperse packing and jig-saw to mosaic to chaotic clast distribution. They usually contain metachert fragments set in a fine-grained siliceous-clastic material, but fragments of crystalline quartz aggregates and of metasandstone are present as well. Stratified breccias with a clearly clastic sandy matrix also occur. Associated with the breccias are massive to banded metachert as well as sinter-like vuggy siliceous rocks with vugs following banding. This picture suggests strongly that the breccias constitute strata-bound syn-sedimentary bodies. On grounds of their outcrop and textural features, the breccias and associated metachert are provisionally interpreted as hydrothermal phreatic products of syn-sedimentary eruptive hot-spring activity completed by debris flow deposition, with the involvement of rift-related growth faults.

Impact-generated pseudotachylitic breccia in drill core BH-5 Hättberg, Siljan impact structure, Sweden

Pseudotachylitic breccia (PTB) in the form of cm-wide melt breccia veinlets locally occurs on the exposed central uplift of the 380 Ma Siljan impact structure. The host rock to the PTBs is the so-called Järna granite of quartz monzonitic to syenodioritic composition. The nearly 603 m long BH-5 drill core from Hättberg, near the centre of the Siljan central uplift, contains numerous veins and pods of PTB. In particular, two major zones of 60 m combined width contain extensive PTB network breccias (30% actual melt breccia component), with individual melt breccia occurrences up to >1 m in length. Core logging and petrographic and geochemical analysis of the core have been performed, and the data are interpreted to suggest the following. (1) The impact event caused low to moderate (at essentially < 20 GPa) shock deformation in the host rock and in clasts of this lithology within the PTB. (2) Macroscopic deformation of the basement mainly comprises fracturing, with only localised cataclasis. (3) No evidence for shock melting (i.e. compression/decompression melting early in the cratering process) could be observed. (4) Optical and scanning electron microscopy showed that dark PTB contains a definite melt component. (5) Shearing has significantly affected this part of the central uplift, but its effects are limited to very short displacements and likely did not result in extensive melting. (6) A frictional heating component upon melt generation can, however, not be excluded, as many PTB samples contain clasts of a mafic (gabbroic) component, although only in one place along the entire core, a 1.2 cm-wide section through such material in direct contact to host rock was observed. Consequently, we suggest that, upon uplift in the central part of the impact structure, considerable melt volumes were generated locally, especially in areas that had been affected by extensive cataclasis and where grain size comminution favoured melt formation. Rapid decompression related to central uplift formation is the preferred process for the generation of the PTB melt breccias.

Detailed volcanostratigraphy of an accreted seamount: Implications for intraplate seamount formation

Seamounts are a ubiquitous feature of the seafloor but relatively little is known about their internal structure. A seamount preserved in the Franciscan mélange of California suggests a sequence of formation common to all seamounts. Field mapping, geophysical measurements, and geochemical analyses are combined to interpret three stages of seamount growth consistent with the formation of other intraplate seamounts such as the Hawaiian volcanoes and the island of La Palma. A seamount begins to form as a pile of closely packed pillows with a high density and low porosity. Small pillow mound volcanoes common at mid‐ocean ridges are seamounts that do not grow beyond this initial stage of formation. The second stage of seamount formation is marked by the first occurrence of breccia. As the seamount grows and becomes topographically more complex, slope varies and fractured material may begin to accumulate. Magma supply may also become spatially diffuse as the seamount grows and new supply pathways develop through the edifice. The second stage thus exhibits variability in both flow morphologies and geophysical properties. The final cap stage is composed of thin flows of various morphologies. These sequences reflect the shoaling of the seamount and a greater variability in extrusion rate resulting from waning magma supply and increased mass wasting. Understanding the growth and structure of seamounts has important implications for intraplate volcanism and for models of hydrothermal circulation in the oceanic crust.

An aeromagnetic approach to revealing buried basement structures and their role in the Proterozoic evolution of the Wernecke Inlier, Yukon Territory, Canada

Gridded aeromagnetic data, 2D forward models and 3D potential field inversions are used in combination with surface geological and petrophysical data to reveal basement and supracrustal architecture of the Wernecke Inlier, northern Canada. Paleoproterozoic crystalline basement is modelled at ∼ 5–10 km depth, and is significantly offset by a major NW-trending normal fault. Basement fault development is interpreted to have occurred during lithospheric extension following accretion during the Wopmay Orogeny (2100–1840 Ma) utilising the existing structural architecture of the Richardson Fault Array. Forward models provided evidence for structural connectivity between basement faults and mapped normal faults within the Wernecke Supergroup. Basement faults appear to project to Wernecke Breccia occurrences, and may have been important fluid pathways during ca. 1590 Ma hydrothermal activity. Syn-breccia faulting also appears to have been important during Wernecke Breccia emplacement, accommodating and focusing brecciation and alteration.

A fluid inclusion study of the Suicide Ridge Breccia Pipe, Cloncurry district, Australia: Implication for Breccia Genesis and IOCG mineralization

The Suicide Ridge Barren Breccia Pipe (∼1527 ± 4 Ma) is one of numerous breccia occurrences that characterize the Cloncurry district, Australia. Interest in breccia systems in the Cloncurry district is related to their possible link with iron oxide copper–gold deposits. The Suicide Ridge Barren Breccia Pipe is temporally and spatially associated with a granite that outcrops at its southwestern boundary and which is ascribed to the Williams and Naraku magmatic event (ca. 1550–1500 Ma). The breccia is characterized by clasts of variable dimensions, degree of rounding and composition that includes dominant calc-silicate clasts of the Corella Formation that underlie rocks of the Soldiers Cap Group that host the pipe, indicating a dominant upward transport of the fragments. Other breccia fragments include granite bodies characterized by albite–quartz mineralogy and textures typical of the magmatic-hydrothermal transition. These rocks are interpreted to have formed in a fractionated granite carapace during the exsolution and accumulation of volatiles associated with the cooling magma. Two dominant types of inclusions have been identified at the Suicide Ridge Barren Breccia Pipe. These include early primary and pseudosecondary carbonic fluid inclusions, observed only in magmatic-hydrothermal transitional textures of granite breccia clasts, and secondary sodic (-calcic) brine inclusions that occur in all the collected samples. Variable homogenization temperatures in the primary and pseudosecondary pure CO 2 fluid inclusions indicate that they were trapped under different pressure conditions. Pressures estimated from the primary CO 2 fluid inclusions suggest that they were trapped at 4.2 kbar, at least 1 kbar above the inferred lithostatic pressure. This likely represents the overpressuring produced by release of CO 2-rich volatiles during the crystallization of magma and which ultimately caused the subsequent brecciation. Estimated pressures for pseudosecondary CO 2 fluid inclusions indicate pressure entrapment conditions around 2.7 kbar, similar to the lithostatic pressure and 1.5 kbar lower than pressures recorded by type Ia fluid inclusions. This significant drop of pressure likely represents the sudden pressure depression occurring in response to brecciation. Evidence from field relationships, breccia characteristics, mineralogy and textures of granite clasts within the pipe and fluid inclusion studies support an origin of the Suicide Ridge Barren Breccia Pipe in response to overpressuring generated by the release, expansion and possibly phase separation of fluids of dominant carbonic composition from a crystallizing magma beneath and adjacent to the pipe. Sodic (-calcic) fluid inclusions represent a later influx of fluids in the breccia system that produced widespread albitization. Significantly, high temperature, ultrasaline multisolid fluid inclusions abundant in IOCG deposits and considered to be samples of the mineralizing fluid were not observed at the Suicide Ridge Breccia Pipe. Their absence in the barren systems suggests that they were critical for the formation of IOCG deposits.

Ice‐marginal environment and ecosystem prior to initial advance of the late Palaeozoic ice sheet in the Mount Butters area of the central Transantarctic Mountains, Antarctica

Basal rocks of the Upper Carboniferous to Lower Permian Pagoda Formation at Mount Butters provide an unusual view of periglacial conditions in the central Transantarctic Mountains region prior to the initial advance of the Gondwanide ice sheet. These rocks were deposited on a high relief unconformity that developed on granite. Deposition within relief on the unconformity, possibly in the lee of a granite buttress, protected the rocks from erosion during subsequent overriding by the ice sheet. The succession reflects deposition in a glacial‐fed to ice‐contact lake that contained a freshwater crustacean fauna. Centimetre‐ to decimetre‐scale basal layers include breccia and coarse‐grained sandstone. The occurrence of breccia resting on weathered granite suggests sedimentation as scree and as mass flow deposits. Overlying decimetre‐to metre‐scale stratified diamictites interbedded with metre‐scale, coarsening‐upward successions of siltstone to cross‐laminated sandstone suggest lacustrine deposition by suspension settling, rain out of ice‐rafted debris, and deltaic progradation. Thin zones with abundant conchostracans and/or with prolific trace fossils, in addition to less common remains of other crustaceans, attest to the presence of a low diversity benthic fauna. Conchostracans are concentrated in a series of thin beds that reflect moderately lengthy, perhaps seasonal, periods of free‐flowing water. Patchy vertical and lateral distribution of intense bioturbation and profuse trace fossils probably reflect repeated colonization events during times of favourable environmental conditions. Massive diamictite overlies the basal rocks and indicates that the ice‐marginal lake was subsequently overridden by the late Palaeozoic ice sheet. Occurrences of lodgement till, glacitectonite and deformation till suggest deposition from temperate or warm‐based ice, whereas underlying lacustrine and deltaic deposits, along with a crustacean and trace fossil fauna, suggest temperate periglacial conditions. Previous studies have stressed that upper Palaeozoic glacigenic deposits in Antarctica, and in Gondwanaland, record deglaciation events. In contrast, rocks at Mt. Butters provide an unusual glimpse into an ice‐margin lake and its fauna just prior to ice sheet advance.

Multi-scalar structure at DSDP/ODP Site 504, Costa Rica Rift, II: fracturing and alteration. An integrated study from core, downhole measurements and borehole wall images

Abstract We used a database derived from the integration of core material and geophysical downhole measurements in order to investigate the relationships between fracturing and alteration in the volcanic section of DSDP/ODP Hole 504B. The studied crustal section (from top of the basement to 1000 mbsf (metres below sea floor)) consists of low resistivity/high porosity pillow lavas associated with breccias and rubble material, alternating with high resistivity/low porosity massive basalt flows. A positive correlation between DLL (Dual Laterolog)-derived porosity and occurrence of breccias in the core suggests that breccias more than fractures contribute to the electrical resistivity signal. A structural analysis performed from core suggests that most fractures and veins are steeply dipping, and may represent tectonic features or cracks due to contractional cooling of the crust, the latter being more abundant in pillows. Fractures and veins recorded on core tend to be clustered in massive units or thin flows. This result may derive from criteria adopted during structural measurements and must be taken with care. The natural radioactivity (GR) profile delineates two main alteration zones in the volcanic section: an oxidizing zone with increased potassium above, and a reducing one without K gain below. Most of the GR maxima are found to be correlated with celadonite-bearing alteration halos. GR minima are frequently located at the boundaries between domains of contrasting fracture orientation, where metasomatic reactions may have occurred due to contrasting permeability.

Geochemistry and age of metamorphosed felsic igneous rocks with A-type affinities in the Willyama Supergroup, Olary Block, South Australia, and implications for mineral exploration

Leucocratic quartzofeldspathic gneisses form a significant proportion of the lower part of the Palaeoproterozoic Willyama Supergroup sequence in the Olary Block, South Australia and have correlatives in the adjacent Broken Hill Block. Field and geochemical data demonstrate that these rocks were originally rhyolitic volcanics and granite, with A-type affinities consistent with magma production during intracratonic rifting, supporting tectonic models proposed for the Willyama Supergroup in the Broken Hill Block. Although the rocks have characteristic high-field-strength element enrichment, many have undergone extensive pervasive pre- or syn-metamorphic sodic alteration and are typically rich in albite. Sensitive high resolution ion microprobe (SHRIMP) U-Pb zircon data tightly constrain the depositional and early intrusive history. Zircons from an interpreted metavolcanic rock containing relict quartz phenocrysts yield an age of 1699 ± 10 Ma, whereas a metagranitoid sample has an age of 1703 ± 6 Ma. These results are compatible with recent geochronological data on felsic metavolcanic rocks from the Broken Hill Block (Page and Laing, 1992) and are indicative of widespread magmatism during deposition of the Willyama Supergroup. Nd signatures for the two Olary Block samples imply the presence of a significant component from a depleted mantle source. The A-type metavolcanic rocks are locally associated with small iron formations, some of which grade into stratiform barite-rich horizons. Although potentially favourable for sediment-hosted exhalative PbZn mineralisation, the Fe- and Ba-rich units, along with transgressive vein and breccia occurrences of Fe oxides ± quartz ± pyrite cutting both the metavolcanic and metagranitoid rocks, may be more prospective for epigenetic Cu-Au mineralisation related to later metamorphic and/or magmatic events. Partial melting of the A-type suite during high grade regional metamorphism at ~ 1600 ± 20 Ma led to the formation of local volumes of sodic granitoids and pegmatites containing U-Th-Ti-REE-F mineralisation.

IV.—On Some Brecciated Rock in the Archæan of Malvern

The question of the origin of the Archæan rocks is one of such great interest that it will not be thought unimportant on the one hand to record any facts that may bear upon it, and on the other to examine somewhat severely all the evidence that may be from time to time brought forward on the subject. Among the facts which may be adduced in support of the view that the Archæan rocks are altered sedimentary strata, the occurrence of breccias and conglomerates may well claim a foremost place; and a vast amount of superincumbent theory must rest on a very shaky foundation if there be any doubt whether some of those ancient fragmental beds be true conglomerates or not. The manner of occurrence, the mode of formation, and the character of pseudo-breccias and pseudoconglomerates become therefore questions of first importance.