Carboniferous mafic metavolcanic rocks in the Northern Gemeric Unit: Petrogenesis, geochemistry, isotope composition and tectonic implication (original) (raw)
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Proceedings of the Ocean Drilling Program, 153 Scientific Results
and 923A were drilled into crust of the Mid-Atlantic Ridge near the Kane Fracture Zone (MARK) area during Leg 153 of the Ocean Drilling Program. Holes 920B and 920D were drilled into an ultramafic massif and Sites 921, 922, and 923 were drilled into a gabbroic massif, both of which are located on the western rift valley wall of the Mid-Atlantic Ridge south of the Kane Transform. Bulk-rock major-, trace-, and rare-earth element (REE) analyses of diabasic, gabbroic, and ultramafic rocks recovered from these holes are reported here. These bulk analyses are augmented by the results of mineral chemistry studies on a subset of the same samples. Large ranges in bulk-rock and mineral chemistry are documented from all rock types. Ultramafic rocks in Holes 920B and 920D are interpreted to be dominantly residual mantle, but they include variably fractionated ultramafic and mafic cumulates that have intrusive contacts with the residual mantle harzburgites. Bulk-rock major-and compatible trace-element abundances, as well as petrographic data for residual harzburgites, indicate that a fertile MORB mantle was depleted by-15% to 20% partial melting or 10%-15% if a more depleted mantle source, such as Tinaquillo Lherzolite, is chosen. The mean extent of melting is likely to have been approximately half of the maximum value computed based on the residuum. Incompatible traceelement data show, however, that this residuum may have been part of an open system and refertilized at late stages by melts flowing through a locally porous matrix and later by more channelized melts (veins) as the residuum became part of the mechanical lithosphere. The crystallization products of these late melts include disseminated magmatic clinopyroxene and narrow veins or composite veins of dunite, wehrlite, pyroxenite, and gabbroic rocks. Ultramafic vein samples are variably depleted to enriched in incompatible elements and span a wide range of fractionation extents based on bulk-rock and mineral chemistry. Melts calculated to have been in equilibrium with clinopyroxene in ultramafic and mafic samples from Site 920 vary widely. They are dominantly ultradepleted, but include some samples that are enriched in incompatible elements (Na and Ti) with respect to MARK basalts, glasses, and Leg 153 diabases. The range in composition cannot simply be explained by crystal fractionation of a single parental magma, but requires a broad range of parental melts or their derivatives to be in equilibrium with clinopyroxene. Bulk-rock and mineral chemistry studies of residual and cumulate ultramafic rocks support the notion of an open-system, near-fractional mantle melting column. The residual peridotites were also cut by late-stage, variably altered, high-MgO (13-15 wt%) diabase dikes with quenched margins. Gabbroic samples from Sites 921, 922, and 923 drilled within the gabbroic massif likewise cover a broad spectrum of lithologies and compositions, and include troctolites, olivine gabbros, gabbros, oxide gabbros, felsic diorites, and quartz diorites. Melt compositions calculated to be in equilibrium with gabbroic clinopyroxene include melts that range from those that are significantly more fractionated to less fractionated than basaltic glasses from the MARK area, but also show a smaller range of parental melts in gabbroic samples when compared to the range documented in Site 920 ultramafic and mafic samples. Hole 923A, in which recovery was high, shows clear evidence of downhole cryptic chemical variation consistent with recharge and magma mixing within subaxial magma chambers. In addition, bulk-rock REE abundances in gabbroic samples show both enriched and depleted light REE (LREE) patterns. The LREE abundances range from less than 1 X chondrite to >IOO × chondrite in gabbroic samples. MARK basaltic rocks cover a much narrower range, from 6 to 24 X chondrite. The chondritenormalized La/Yb ratios of plutonic rocks vary from 0.3 to 2.3, whereas MARK basalts have a narrower range, from 0.5 to 1.2. Extended REE patterns for plutonic rocks tend to be spiked with Sr and Eu anomalies for all samples, and both positive and negative Zr and Ti anomalies for primitive and evolved samples, respectively. The range of incompatible trace-element depletions and enrichments in plutonic samples, in part, reflects modal variations in clinopyroxene and Plagioclase, the abundance of trapped intercumulus melt, and a range of parental melt compositions input into the subaxial plumbing system. Ultramafic and plutonic rock compositions are interpreted to indicate that MORB erupted at the surface in the MARK region represents the more homogeneous hybrid derived by mixing of a diverse set of liquids generated within the mantle melting column or that evolved in plutonic environments in the crust or mantle.
Geochemical correlation of metabasic rocks from Central and
2007
Abstract. Metaigneous rocks, including metagabbros and amphibolites, are well known elements of the Variegated Formations from Central and East Rhodopes. A comparison between the geochemical features of these rocks from Madan-Davidkovo Antiform (Central Rhodopes) along with the Avren Synform, and Bela Reka Antiform (East Rhodopes) is the aim of this contribution. The chemical composition of metabasic rocks from both regions, e.g., high MgO, and low high field strength elements (HFSE) contents, indicates boninite and arc tholeiite affinities. Such affinities are also reflected by the values of specific ratios such as CaO/Al2O3, CaO/TiO2, Al2O3/TiO2, La/Sm. Plotted on discrimination diagrams, the metabasic rocks from Variegated Formations in both regions fall mainly in the fields of Phanerozoic boninites and arc tholeiites. Three types of REE patterns are observed in the metabasites from these regions: U-shaped, flat to LREE enriched pattern, and LREE depleted pattern found in the roc...
Trace elements and Sr-isotopes in some mantle-derived hydrous minerals and their significance
Geochimica et Cosmochimica Acta, 1978
New analyses of K, Rb, Sr and Ba contents and the 87Sr/86Sr ratios of eight amphiboles, one phlogopite, two diopsides, and one host alkalic basalt for an amphibole are reported: The samples are mostly inclusions in alkalic basalts and occur in association with peridotite inclusions. Two of the samples are from alpine-type peridotite bodies -one from the Etang de Lhers massif in the French Pyrenees and the other from the Finer0 massif in the Ivrea zone in northern-Italy.
Proceedings of the Ocean Drilling Program, 153 Scientific Results
Drilling on the western wall of the Mid-Atlantic Ridge south of the Kane Fracture Zone at MARK (23°20'N and 23°30'N) recovered plutonic rocks <l Ma in age. A selection of these rocks has been analyzed for their Sr-, Nd-, Pb-, and O-isotope compositions. O-isotope analyses of gabbro mineral separates have near primary magmatic values. δ 18 θ cpx vs. δ 18 θ plag systematics support only limited fluid-rock interaction, predominantly at moderate to low temperatures (200°-300°C). Combined Sr-and O-isotope data indicate that, even where fluids have found ingress into the gabbroic crust, they have had limited compositional consequences, presumably because seawater was rapidly modified by interaction with the overlying basalt crust. Pb-and Srisotope compositions of leachates further demonstrate that, for most samples, seawater was the principal component in hydrothermal fluids; in a few cases, however, sediments were also involved. Sr-, Nd-, and Pb-isotope systematics of leached gabbro mineral separates indicate that, to a first-order approximation, the sub-MARK mantle is isotopically homogeneous from near the middle of the spreading cell (Sites 921-924) to its boundary in the south (Site 920). Trace-element concentrations of Sr, Nd, and Pb (and, by implication, other incompatible trace elements) must also be homogeneous. Thus, variations in trace-element concentrations in gabbros and calculated parental magmas must be the product of the melting regime and subsequent fractionation processes in the magma chamber(s) and not variations in the mantle source composition. Superimposed on this homogeneity is smaller scale, within-sample, isotope disequilibrium between coexisting Plagioclase and clinopyroxene. Such heterogeneities provide evidence of complex magma chamber processes and are most likely to be preserved at the beginning of magmatic cycles. Unusual isotopically enriched compositions are found in the MARK area as diabase and amphibolitized microgabbros that crosscut the peridotite section. Restriction of these distinctive compositions in the peridotite sections at Site 920 may result from the lower magma supply rate expected near the boundary of a spreading cell. The average isotopic composition within the MARK area has changed over a period of <l m.y., becoming more radiogenic in its Pb-isotope composition but less radiogenic in its Nd composition, implying that the source is currently less depleted than that available 750,000 yr earlier.
Earth and Planetary Science Letters, 1988
A series of compositionally diverse, Late Archean rocks (2.74-2.79 Ga old) from the eastern Beartooth Mountains, Montana and Wyoming, U.S.A., have the same initial Pb, Sr, and Nd isotopic ratios. Lead and Sr initial ratios are higher and Nd initial ratios lower than would be expected for rocks derived from model mantle sources and strongly indicate the involvement of an older crustal reservoir in the genesis of these rocks. Crustal contamination during emplacement can be ruled out for a variety of reasons. Instead a model involving subduction of continental detritus and contamination of the overlying mantle as is often proposed for modern subduction environments is preferred. This contaminated mantle would have all the isotopic characteristics of mantle enriched by internal mantle metasomatism but would require no long-term growth or changes in parent to daughter element ratios. This contaminated mantle would make a good source for some of the Cenozoic mafic volcanics of the Columbia River, Snake River Plain, and Yellowstone volcanic fields that are proposed to come from ancient, enriched lithospheric mantle. The isotopic characteristics of the 2.70 Ga old Stillwater Complex are a perfect match for the proposed contaminated mantle which provides an alternative to crustal contamination during emplacement. The Pb isotopic characteristics of the Late Archean rocks of the eastern Beartooth Mountains are similar to those of other Late Archean rocks of the Wyoming Province and suggest that Early Archean, upper crustal rocks were common in this terrane. The isotopic signatures of Late Archean rocks in the Wyoming Province are distinctive from those of other Archean cratons in North America which are dominated by a MORB-Iike, Archean mantle source (Superior Province) and/or a long-term depleted crustal source (Greenland).
Economic Geology, 2016
Iron oxide-apatite and iron oxide-copper-gold deposits occur within ~1.48 to 1.47 Ga volcanic rocks of the St. Francois Mountains terrane near a regional boundary separating crustal blocks having contrasting depletedmantle Sm-Nd model ages (TDM). Major and trace element analyses and Nd and Pb isotope data were obtained to characterize the Pea Ridge deposit, improve identification of exploration targets, and better understand the regional distribution of mineralization with respect to crustal blocks. The Pea Ridge deposit is spatially associated with felsic volcanic rocks and plutons. Mafic to intermediate-composition rocks are volumetrically minor. Data for major element variations are commonly scattered and strongly suggest element mobility. Ratios of relatively immobile elements indicate that the felsic rocks are evolved subalkaline dacite and rhyolite; the mafic rocks are basalt to basaltic andesite. Granites and rhyolites display geochemical features typical of rocks produced by subduction. Rare earth element (REE) variations for the rhyolites are diagnostic of rocks affected by hydrothermal alteration and associated REE mineralization. The magnetite-rich rocks and REE-rich breccias show similar REE and mantle-normalized trace element patterns. Nd isotope compositions (age corrected) show that: (1) host rhyolites have eNd from 3.44 to 4.25 and TDM from 1.51 to 1.59 Ga; (2) magnetite ore and specular hematite rocks display eNd from 3.04 to 4.21and TDM from 1.6 to 1.51 Ga, and eNd from 2.23 to 2.81, respectively; (3) REE-rich breccias have eNd from 3.04 to 4.11 and TDM from 1.6 to 1.51 Ga; and (4) mafic to intermediate-composition rocks range in eNd from 2.35 to 3.66 and in TDM from 1.66 to 1.56. The eNd values of the magnetite and specular hematite samples show that the REE mineralization is magmatic; no evidence exists for major overprinting by younger, crustal meteoric fluids, or by externally derived Nd. Host rocks, breccias, and magnetite ore shared a common origin from a similar source. Lead isotope ratios are diverse: (1) host rhyolite has 206 Pb/ 204 Pb from 24.261 to 50.091; (2) Pea Ridge and regional galenas have 206 Pb/ 204 Pb from 16.030 to 33.548; (3) REE-rich breccia, magnetite ore, and specular hematite rock are more radiogenic than galena; (4) REE-rich breccias have high 206 Pb/ 204 Pb (38.122-1277.61) compared to host rhyolites; and (5) REE-rich breccias are more radiogenic than magnetite ore and specularhematite rock, having 206 Pb/ 204 Pb up to 230.65. Radiogenic 207 Pb/ 206 Pb age estimates suggest the following: (1) rhyolitic host rocks have ages of ~1.50 Ga, (2) magnetite ore is ~1.44 Ga, and (3) REE-rich breccias are ~1.48 Ga. These estimates are broadly consistent and genetically link the host rhyolite, REE-rich breccia, and magnetite ore as being contemporaneous. Alteration style and mineralogical or textural distinctions among the magnetite-rich rocks and REE-rich breccias do not correlate with different isotopic sources. In our model, magmatic fluids leached metals from the coeval felsic rocks (rhyolites), which provided the metal source reflected in the compositions of the REE-rich breccias and mineralized rocks. This model allows for the likelihood of contributions from other genetically related felsic and intermediate to more mafic rocks stored deeper in the crust. The deposit thus records an origin as a magmatichydrothermal system that was not affected by Nd and Pb remobilization processes, particularly if these processes also triggered mixing with externally sourced metal-bearing fluids. The Pea Ridge deposit was part of a single, widespread, homogeneous mixing system that produced a uniform isotopic composition, thus representing an excellent example of an igneous-dominated system that generated coeval magmatism and REE mineralization. Geochemical features suggest that components in the Pea Ridge deposit originated from sources in an orogenic margin. Basaltic magmatism produced by mantle decompression melting provided heat for extracting melts from the middle or lower crust. Continual addition of mafic magmas to the base of the subcontinental lithosphere, in a back-arc setting, remelted calc-alkaline rocks enriched in metals that were stored in the crust. The St. Francois Mountains terrane is adjacent to the regional TDM line (defined at a value of 1.55 Ga) that separates ~1600 Ma basement to the west, from younger basements to the east. Data for Pea Ridge straddle the TDM values proposed for the line. The Sm-Nd isotope system has been closed since formation of the deposit and the original igneous signatures have not been affected by cycles of alteration or superimposed mineralizing events. No evidence exists for externally derived Nd or Sm. The source region for metals within the Pea Ridge deposit had a moderate compositional variation and the REE-rich breccias and mineralized rocks are generally isotopically homogeneous. The Pea Ridge deposit thus constitutes a distinctive isotopic target for use as a model in identifying other mineralized systems that may share the same metal source in the St. Francois Mountains terrane and elsewhere in the eastern Granite-Rhyolite province.
Lithos, 2014
Orogenic garnet peridotites of diverse origins and histories in the Bohemian Massif attest to a variety of mantle processes, including partial melting, cryptic metasomatism, and modal metasomatism (refertilization), all of which are recorded by Saxothuringian garnet peridotite from the T-7 borehole in northern Bohemia. The T-7 peridotite consists of interlayered garnet lherzolite, harzburgite, and phlogopite-garnet pyroxenite lenses that yield peak temperatures and pressures of 1030-1150°C and 36.1-48.0 kbar. Olivine crystallographic preferred orientations exhibit [axial](010) slip, corresponding to a pure shear component of deformation under relatively low flow stress conditions. Some lherzolite samples are fertile, resembling primitive mantle in major and trace element composition, but other lherzolites are slightly depleted in incompatible major elements, HREE, and HFSE, and slightly enriched in LREE. Harzburgite is depleted in incompatible major elements, HREE, and HFSE, but enriched in LREE. Harzburgite adjacent to pyroxenite has been refertilized, containing phlogopite, less olivine, more orthopyroxene, and more garnet than distal harzburgite. The T-7 peridotite compositions are the result of variable degrees of partial melting in the spinel stability field, followed by cryptic metasomatism and modal metasomatism by transient basaltic melts in the garnet field. Trace elements, Sr and Nd isotopes, and occurrence of phlogopite reflect a subduction component in the metasomatising melts. Partial melting of the T-7 peridotite was a Proterozoic event, as indicated by Rhenium depletion model ages (T RD ); the age of cryptic and modal metasomatism is unconstrained, but is thought to be related to Variscan subduction and amalgamation of the Bohemian Massif.
Journal of Petrology, 2014
The Central Atlantic Magmatic Province (CAMP) is one of the largest igneous provinces on Earth, with an areal extent exceeding 10 7 km 2. Here we document the geochemical characteristics of CAMP basalts fromTriassic^Jurassic basins in northeastern USA and Nova Scotia (Canada). The CAMP rocks occur as lava flows, sills and dykes. All of our analysed samples show chemical characteristics typical of CAMP basalts with low titanium content, which include enrichment in the most incompatible elements and negative Nb anomalies. All the basalts also show enriched Sr^Nd^Pb initial (t ¼ 201 Ma) isotopic compositions (206 Pb/ 204 Pb ini. ¼ 18•1551 8•691, 207 Pb/ 204 Pb ini. ¼ 15•616^15•668, 208 Pb/ 204 Pb ini. ¼ 38•160^38•616, 143 Nd/ 144 Nd ini. ¼ 0•512169^0•512499). On the basis of stratigraphy, rare earth element (REE) chemistry and SrN d^Pb isotope composition, three chemical groups are defined. The Hook Mountain group, with the lowest La/Yb ratios, initial 206 Pb/ 204 Pb ini. 418•5 and 143 Nd/ 144 Nd ini. 40•51238, comprises all the lastest and upper stratigraphic units. The Preakness group, with intermediate La/Yb ratios, 206 Pb/ 204 Pb ini. 418•5 and
Archean Metaterrigenous Rocks: Major Geochemical Constraints
The paper summarizes data on the geochemistry of metaterrigenous rocks from 26 reference Archean territories: the Pilbara and Yilgarn blocks; Isua and Akilia complexes; Wittwatersrand, Swaziland, Pongola, and Yellowknife supergroups; Khapchanskaya and Gimol'skaya groups; Kan, Sharyzhalgai, Chupa, Slyudyanka, and Onot complexes; etc. The general sets of data points and the calculated median values of the concentrations of trace elements and their ratios are compared to those of Archean and post-Archean shales. In Ce/Ni-Cr, and some other diagrams, the fields in which the most data points of Archean metaterrigenous rocks group are outlined. The results of this research indicate that there are no values of geochemical parameters that are inherent only in Archean or only in post-Archean fine-grained terrigenous rocks. Within 80-85% confidence levels, most individual compositions of Archean metaterrigenous rocks are characterized by the following geochemical parameters: (1) Th/Sc < 0.6-0.7, (2) Ce/Cr < 0.6, and (3) Eu/Eu* > 0.70-0.75. If the median values are used, these ranges can be further constrained to (i) Th/Sc < 0.55, (ii) Ce/Cr < 0.4, (iii) Cr/Th > 25, and (iv) Th < 12 ppm. Compared to PAAS, Archean metaterrigenous rocks are characterized by higher median concentrations of Cr and Ni and the Eu/Eu*, Sc/Th, Cr/Th, and Co/Hf ratios, whereas the Nb, La, Ce, Yb, Hf, Th, and U concentrations and the La/Sm and Ce/Cr ratios of PAAS are, conversely, lower. The median values of the La N / Yb N ratios of reference Archean terranes can be either higher or lower than in PAAS, likely depending on the proportions of various rock types in the sources of the terrigenous material. The medians of the Gd N / Yb N ratios of ~60% of the reference Archean metaterrigenous terranes in our databank are slightly higher than the Gd N / Yb N ratios of PAAS. The median values of the La N / Sm N ratios of Archean terrigenous rocks are mostly slightly lower than the typical PAAS ratios.