Hannes K. Brueckner

Petrogenesis of Variscan high-temperature Group A eclogites from the Moldanubian Zone of the Bohemian Massif, Czechoslovakia

High-temperature (HT), Group A eclogites from three localities in the Moldanubian Zone of the Bohemian Massif are interpreted to have formed in the mantle and to have been transported into the crust by their enclosing garnet peridotites during Variscan orogenesis. Garnet and omphacite are compositionally zoned and contain homogeneous cores and retrograde rims. Cores of minerals yield minimum temperatures and pressures of 850 to 985°C and 16.0 to 22.5 kb, based on Fe−Mg exchange between garnet and clinopyroxene and the jadeite content of clinopyroxene. Sugh high temperatures indicate equilibration in, and derivation from, the upper mantle. Trace element compositions, including the REEs, high MgO contents, and high Mg numbers suggest that the rocks formed by high pressure accumulation of garnet and clinopyroxene and variable amounts of trapped melt. Sm-Nd ages determined on four garnet-clinopyroxene pairs from the three localities are 377±20, 342±9, 336±16, and 323±7 Ma. ɛNd and initial 87Sr/86Sr are negatively correlated, varying from +6.7 to -0.1 and 0.7027 to 0.7057, respectively. Field, compositional, and isotopic data indicate that the eclogites were derived from heterogeneous mantle that included depleted and enriched compositions; this heterogeneity may have resulted from subduction processes that occurred prior to the late Variscan collision of Gondwana and Baltica.

Zabargad and the isotopic evolution of the sub-Red Sea mantle and crust

Abstract Zabargad Island in the Red Sea exposes ultramafic bodies in contact with partially retrograded granulite-facies lower crustal rocks; the contact could represent the sub-Red Sea Moho prior to the opening of the Red Sea. The ultramafic rocks are largely protogranular spinel lherzolites that grade locally to porphyroclastic to mylonitic amphibole-peridotites. Some of the peridotites are infiltrated by high Mg-gabbros (troctolites). The peridotites and the lower crustal rocks are cut by diabase dikes. The Zabargad peridotites are isotopically heterogeneous on a scale of meters or less. Sr and Nd isotopic ratios of clinopyroxenes (cpx) from spinel lherzolites lie in the depleted portion of the 143 Nd 144 Nd − 87 Sr 86 Sr “mantle array” whereas cpx and amphiboles from amphibole-peridotites fall off the array towards the low 143 Nd 144 Nd and sol 87 Sr 86 Sr field that may characterize portions of the sub-continental mantle. Cpx from troctolites plot in the middle of the 143 Nd 144 Nd - 87 Sr 86 Sr mantle array and have higher 87 Sr 86 Sr ratios and more depleted LREE patterns than the intruded spinel-lherzolites and amphibole peridotites. All whole-rock lherzolites and peridotites and most diabase dikes from Zabargad, as well as a variety of rocks from other parts of the Red Sea area (MORB glasses, a diabase dike from The Brothers Island, basanites and clinopyroxenes from associated nodules from the Ataq diatreme in Yemen) define a linear array on a Sm-Nd isochron diagram with a best-fit “age” of ca. 675 Ma and an initial ϵNd of +6.7. This array is probably a mixing line between depleted MORB-type mantle and a low 143 Nd 144 Nd − 87 Sr 86 Sr metasomatic component. The “age” defined by the linear array may date either an episode of Pan-African mantle homogenization or, in our preferred model, the Pan-African segregation of a previously homogeneous mantle into two or more reservoirs of identical 143 Nd 144 Nd , but different Sm Nd ratios. The subsequent partial remixing of these isotopic reservoirs during metasomatism associated with the opening of the Red Sea did not perturbed the slope of the array. The LREE-enriched reservoir may have been the Pan-African lower crust since there is field and textural evidence that the metasomatic fluids were derived from, or equilibrated with, the adjacent crustal complex. Alternatively, the LREE-enriched component may have been a mantle reservoir similar to that which generated basalts and nodules from the nearby Ataq diatreme. The troctolites are isotopically distinct from the host peridotites and retain igneous textures suggesting they intruded the peridotites during the opening of the Red Sea. Their isotopic patterns are similar to those of recent volcanics from Afar, portions of the Ataq diatreme and some Zabargad dikes indicating derivation from a similar mantle source. Their very LREE-depleted patterns suggest they are either re-melted fractions of previously very depleted spinel lherzolites or early-formed mineral aggregates that became enmeshed in the peridotites as fractionating melt ascended to higher levels. Our results indicate that at least two major processes (metasomatism and intrusion) have modified the isotopic-pat-terns of the spinel lherzolites of Zabargad. Had the resultant chemically and isotopically heterogeneous bodies stayed in the mantle, instead of being uplifted to their present positions, they would have proven to be enigmatic sources for future basalts and xenoliths.

Reconnaissance geochronology, tectonothermal evolution, and regional significance of the middle proterozoic choma-kalomo block, Southern Zambia

Abstract New U-Pb zircon, 40 Ar- 39 Ar, Rb-Sr and Sm-Nd data provide the first numerical chronology for the tectonothermal evolution of the Choma-Kalomo block, an extensive Middle Proterozoic terrane in southern Zambia to the south of the Pan-African Zambezi belt. The Choma-Kalomo batholith, a large plutonic body in the central part of the block, is shown to be a composite intrusion emplaced from c. 1345 to 1200 Ma, based on U-Pb zircon ages and detailed mapping of intrusive relations. Early phases of the batholith were intruded as syntectonic to late-syntectonic plutons during development of a penetrative S 1 fabric in the metasedimentary country rocks of the Choma-Kalomo block. D 2 folding produced the regional NE-SW structural trends in the block and possibly occurred at c. 1230 Ma, based on an Rb-Sr isochron from an early phase of the batholith containing a penetrative S 2 foliation. An U-Pb zircon age of c. 1200 Ma from post-tectonic granite provides a younger limit to the timing of D 2 . 40 Ar- 39 Ar mineral ages from the posttectonic granite, and Sm-Nd and 40 Ar- 39 Ar mineral ages from amphibolites in country-rock paragneisses, range from c. 1200 to 500 Ma, recording post-D 2 cooling followed by subsequent low-grade Pan-African overprinting along the southern margin of the Zambezi belt. D 1 in the Choma-Kalomo block at c. 1345 Ma is substantially younger than Early Proterozoic orogenesis in adjacent terranes to the east in Zimbabwe, indicating that a major Precambrian discontinuity beneath the mid-Zambezi Valley separates these regions. D 1 in the block is similar in age to plutonic and deformational events dated at c. 1350–1400 Ma in the Irumide belt of central and northern Zambia, suggesting that these two terranes on each side of the younger, cross-cutting Zambezi belt are parts of a single, originally continuous Middle Proterozoic mobile belt. Continuity between the Choma-Kalomo block and broadly coeval terranes to the southwest in Namibia and South Africa also appears likely, and all of these areas are considered to form parts of a Middle Proterozoic orogenic province lying west of the Kaapvaal and Zimbabwe cratons.

REE, RbSr and SmNd studies of Norwegian eclogites☆

REE analyses and isotopic studies have been carried out on Norwegian eclogites and garnet peridotites to determine the age of metamorphism and the origin and history of the protoliths. n nWhole-rock REE patterns of eclogites in gneiss vary greatly, even within single outcrops. Many show positive Eu anomalies, indicating plagioclase fractionation and thus gabbroic protoliths. The irregular REE patterns and the common depletion of La and Ce relative to the middle REE, may reflect metasomatic effects during high-P metamorphism. The distribution of REE between clinopyroxene (cpx) and garnet (gnt) in the garnet peridotites is consistent with equilibration at lower T than the corresponding phases of gnt-lherzolite nodules in kimberlites. If the Norwegian gnt-peridotites are fragments of subcontinental mantle, they have undergone post-emplacement equilibration. n nMost eclogites that have high TMET (> 700°C) also show equilibrium distribution of REE between cpx and gnt, with large spreads in Sm/Nd. Gnt-cpx pairs from five such eclogites give Smue5f8Nd ages from 407–447 Ma. In lower-T eclogites (∼ 600°C) the REE have not equilibrated between gnt and cpx; no spread in Sm-Nd is observed, and no dates have been obtained. n nISr varies from 0.7025 to 0.7245 in 21 samples of eclogites in gneisses. Most orthopyroxene (opx)-free eclogites fall in the range 0.7025–0.7060, while 7 or 8 opx-bearing ones have ISr > 0.715. While most opx-free eclogites may be derived from gabbroic protoliths, the opx-eclogites must either be derived from old Rb-rich protoliths, or have interacted extensively with the surrounding gneisses. The Sr and Nd isotopic data suggest that the rocks studied here all had complex crustal histories prior to the eclogite-forming metamorphism. n nMica-cpx or mica-whole rock (WR) Rbue5f8Sr “ages” on Norwegian eclogites range from 1200 to < 400 Ma. Many samples show marked isotopic disequilibrium and the detailed significance of these “ages” is debatable. However, a five-point mineral isochron on one kyanite eclogite gives an isochron age of 398±1 Ma, with ISr = 0.7034. The eclogite-forming event in western Norway was apparently part of theCaledonian continent-continent collision, in which the Western Norwegian Gneiss terrane was overridden by the Greenland plate. The Wenlockian Smue5f8Nd mineral ages may refer to the beginning of uplift after this collision, while the Rbue5f8Sr mineral ages probably reflect a later stage of the uplift-cooling cycle.

Structure of the Havallah sequence, Golconda allochthon, Nevada: Evidence for prolonged evolution in an accretionary prism

The Golconda allochthon of northern and central Nevada contains the Havallah sequence and correlative units of latest Devonian to early Late Permian age. The Havallah sequence is dominated by radiolarian ribbon chert and argillite associated with variable, but generally subordinate, siliciclastic, cal-carenitic, and volcaniclastic turbidites and slump deposits. Cherts of all ages (except Pennsylvanian?) locally rest depositionally on tholeiitic basalts. Some were altered and mineralized by ridge-type hydrothermal systems, suggesting deposition in an ocean basin containing active spreading center(s) for much of the upper Paleozoic.nnThe Havallah sequence was thrust eastward along the basal Golconda thrust onto coeval sediments of the western North American shelf during the latest Paleozoic-earliest Mesozoic Sonoma orogeny. Detailed studies of the Havallah sequence in the Sonoma and Tobin Ranges and in Battle Mountain indicate the presence of complex diagenetic and structural fabrics that developed prior to the obduction of the allochthon. A large number of thrusts, composite thrusts, and shear zones slice the Havallah into numerous tectonic packets of contrasting lithology and/or internal structural style. Internal structures in chert packets include bedding-parallel and bedding-normal solution cleavage, solution boudins, three or more sets of east-verging folds of variable geometry, and features associated with high pore-fluid pressures such as crack-seal fractures, dilation breccias, clastic dikes and sills, and clastic intrusions along thrust surfaces. The fabrics suggest a general progressive evolution from deformation characterized by bedding-normal compression and slight east-west extension (D1) to deformation dominated by bedding-parallel, east-directed shear (D2). Local chert packets may have suffered alternating episodes of high and low pore-fluid pressure. The degree of development and the styles and orientations of various structural elements vary, sometimes radically, from packet to packet.nnThe pre-Golconda thrust fabrics can be modeled as the result of imbrication and deformation of successive batches of ocean-floor sediments into the toe of an accretionary prism in front of an east-facing arc. Relatively undeformed Permian calcarenite units, interpreted as being trench-slope deposits, suggest that this prism was well developed by Permian time. Coarse slump deposits containing chert clasts with pre-depositional structural fabrics suggest a prism that was active long enough to recycle previously tectonized cherts. If we are correct, the classical Sonoma orogeny (D3) marked the culmination of a protracted structural evolution that may have spanned much of the upper Paleozoic.

A Pan African origin and uplift for the gneisses and peridotites of Zabargad Island, Red Sea: A Nd, Sr, Pb, and Os isotope study

A Sr, Nd, Pb, and Os isotopic study of peridotites and granulite-facies gneisses from Zabargad Island in the Red Sea suggests that the tectonothermal, petrogenetic, and geochemical evolution of these rocks occurred largely during the Pan African Orogeny rather than the recent opening of the Red Sea. Sm-Nd model ages and whole rock errorchrons indicate that spinel Iherzolites and gneisses differentiated from a common depleted mantle source about 700 Ma. The Iherzolites were mylonitized, metasomatized, and amphibolitized during a structural event that juxtaposed the peridotites with the gneiss complex and uplifted the gneiss/peridotite complex to relatively shallow crustal levels. Most radiometric dating schemes suggest a Pan African age for this event. The gneisses generally have lower 143Nd/144Nd, 87Sr/86Sr, 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios than the peridotites. They extend linear trends defined by the spinel and amphibole peridotites on Sr-Nd, Sm-Nd, and Pb-Pb diagrams, suggesting the gneisses were either the source or buffering medium for the Pan African metasomatism. Only one post-Pan African event had a significant effect on the geochemistry of the gneiss/peridotite complex: shallow level metasomatism by ultrahot (750–900°C) hypersaline solutions with high 87Sr/86Sr (≈0.710) ratio led to the development of gem-quality olivine crystals as well as low-pressure mineral assemblages in the peridotites, gneisses and younger rocks. Plagioclase-rich assemblages with apparent igneous textures (“troctolites”) that are most common in the southern peridotite body may have formed by interaction of these fluids with peridotite (i.e., are “pseudo-troctolites”). Metasomatism changed the 87Sr/86Sr, Sm/Nd, and Re/Os ratios of the plagioclase peridotites making them unsuitable representatives of the Pan African mantle.

Diagenetic controls on the structural evolution of siliceous sediments in the golconda allochthon, Nevada, U.S.A.

Abstract A model is proposed that links the diagenesis of siliceous sedimentary rocks with deformation to explain the heterogeneous structural fabric of radiolarian cherts in the upper Paleozoic Golconda allochthon of Nevada, U.S.A. Numerous thrust faults slice the cherts into packets, each with a unique set of internal structures. Fold geometries, boudin profiles, pressure-solution features, etc. vary from packet-to-packet and layer-to-layer. Analogous variations in the diagenetically mixed siliceous sediments of the Miocene Monterey Formation. California, suggest the cherts of the Golconda allochthon were similarly mixed when they were deformed. Radiolarian sediments composed of biogenic silica (opal-A) developed ductile structures and pressure-solution features because of their high porosity, weak lithification and the high solubility of disordered silica. Lowporosity, strongly-lithified, relatively-insoluble quartz cherts, the final product of silica diagenesis, deformed in a brittle fashion and developed fewer pressure-solution features. Diagenetically intermediate CT-cherts and CT-porcelanites exhibited transitional behavior. Mixed diagenetic zones produced structures with layer-by-layer changes in structural style. The dehydration of opal-A and opal-CT helped create the excess pore fluid pressures responsible for thrust faults, hydraulic fractures, dilation breccias and elastic intrusions. The presence of ductile structures and numerous pressure solution features in the oldest cherts of the Havallah sequence suggests that these cherts were deformed while diagenetically immature, possibly within a long-lived upper Paleozoic accretionary prism.

Fracture-zone tectonics at Zabargad Island, Red Sea (Egypt)

Abstract Zabargad Island, which lies along the western margin of the Red Sea rift, is a remarkable place because it provides fresh exposures of undepleted mantle peridotite. How this peridotite came to be exposed on Zabargad remains unclear. Our field mapping indicates that most of the contacts between peridotite and the adjacent bodies of Pan-African gneiss and Cretaceous(?) Zabargad Formation on the island are now high-angle brittle faults. Zabargad Formation strata have been complexly folded, partly in response to this faulting. Overall, the array of high-angle faults and associated folds on the island resembles those found in cross-rift transfer zones. We suggest, therefore, that the Zabargad fracture zone, a band of submarine escarpments on the floor of the Red Sea north of the island, crosses Zabargad Island and has actively resolved differential movement between the central Red Sea rift and the northern Red Sea rift. The final stage of uplift that brought the unusual peridotite to the earths surface is related to shallow crustal transpression, which may have inverted an earlier transtensional regime.

“Mantle” Rb/Sr and 87Sr/86Sr ratios for clinopyroxenes from Norwegian garnet peridotites and pyroxenites*

Abstract Clinopyroxenes separated from garnetiferous ultramafic rocks in the core zone of the Norwegian Caledonides have rubidium concentrations of 0.008 to 0.064 ppm, strontium concentrations of 23.5 to 421 ppm, and 87Sr/86Sr ratios of 0.7011 to 0.7029. The very low Rb/Sr ratios of the clinopyroxenes (less than 0.0004) suggest that their 87Sr/86Sr values have not varied significantly over geologic time and may approximate the initial 87Sr/86Sr of the eclogite-facies ultramafic mineral assemblages at their time of formation. The ultramafic rocks occur in a basement complex that yields Rb-Sr whole-rock and U-Pb zircon ages of about 1800 m.y. Garnetiferous ultramafic rocks are apparently lacking in younger (Sveconorwegian or Caledonian) sialic sequences, raising the possibility that the eclogite-facies metamorphism may have occurred at least 1800 m.y. ago. The Rb/Sr and 87Sr/86Sr ratios of the clinopyroxenes are as predicted for the ancient upper mantle under most evolutionary models. However, the data do not preclude the possibility that the eclogite-facies metamorphism occurred in the crust. The garnetiferous ultramafic rocks are generally enclosed by large volumes of dunite which could have shielded the eclogite-facies assemblages from contamination by fluids from the country rock during metamorphism.

An Orphaned Baltic Terrane in the Greenland Caledonides: A Sm-Nd and Detrital Zircon Study of a High-Pressure/Ultrahigh-Pressure Complex in Liverpool Land

Liverpool Land, at the southern tip of the Greenland Caledonides, exposes a composite metamorphic terrane: the midcrustal granulite-facies Jaettedal Complex tectonically juxtaposed against the eclogite-facies, peridotite-bearing Tvaerdal Complex. The Jaettedal Complex is a Laurentian terrane, whereas the Tvaerdal Complex was proposed by earlier investigators to be a Baltic terrane. PT estimates (880°–920°C at 35–40 kbar) and Sm-Nd mineral isochrons from Tvaerdal eclogites indicate that recrystallization occurred under ultrahigh-pressure (UHP) metamorphic conditions ≈400 m.yr. ago, the same time and under similar conditions as the Western Gneiss Complex of the Norwegian Caledonides. Detrital zircons from the Tvaerdal Complex, analyzed for U-Pb, Lu-Hf, and trace elements by laser ablation inductively coupled plasma mass spectrometry, give concordant Mesoproterozoic ages but not the Archean and ≈1.8 Ga Proterozoic ages characteristic of Laurentian terranes. Most remaining concordant U-Pb ages are 411–375 Ma (i.e., Scandian), which contrast with older (≈460–410 Ma) zircon ages from the Jaettedal Complex as well as other Laurentian terranes. Both the Precambrian and the Scandian age sets confirm the Tvaerdal Complex as an orphaned Baltic terrane. The Jaettedal Complex underwent a lengthy Caledonian history as part of a continental arc system during the closure of Iapetus, whereas the Tvaerdal Complex was a fragment of the approaching Baltic passive margin. UHP metamorphism occurred when this margin subducted into the mantle beneath Laurentia. We propose that the Tvaerdal Complex separated from Baltica and rose through the hot mantle wedge to the base of the overriding Laurentian crust by diapirism, a process that may explain its abundant anatectic granitoid intrusions.