Single Parental Magma Research Articles

Geochemistry of granitic rocks from the Hercynian Sardinia-Corsica batholith: Implication for magma genesis

The Sardinia-Corsica batholith was structured in the late stage of the Hercynian orogenesis. The granitoids intrude mainly metamorphic complexes grading from zeolite up to amphibolite facies. The batholith is heterogeneous consisting of complexes with different affinity, chemical composition, age and degree of deformation. The present paper reports major-and trace-element data for selected samples coming only from Sardinian outcrops. The rocks range from gabbro-diorite to tonalite, monzogranite and leucogranite. The two latter lithologies are the most abundant, gabbrodiorites and tonalites occurring in minor amounts and mainly in northern-central Sardinia. Over 75% of the granitoids contain microgranular enclaves of magmatic origin. The age of the rocks falls in the interval between 307 and 281 Ma. Sr isotope initial ratios are high, ranging between 0.7083 and 0.7107. REE, Rb, Sr, Ba, Zr, Th, Ta, Hf, Co and Sc abundances were determined on selected samples. All elements follow three types of trends vs. CaO, which is used as differentiation index. Two trends show positive and negative correlations while the third one shows a bell-shaped pattern. LREE have different degrees of enrichment (La = 20−120× ch) and HREE show variable fractionation with prevailing ( Tb Yb ) n <1 . The two peraluminous samples have very different geochemical characteristics. From the geochemical point of view all the rocks coming from the Sardinian segment of the batholith display a typical calc-alkaline chemical character showing the imprint of both “normal and mature” continental arc geodynamic environments. Geochemical trends suggest some petrogenetic constraints. The complete sequence of differentiation can be neither the product of crystal/liquid fractionation processes starting from a single basic parent magma nor the product of an AFC process. On the contrary, a two-stage model can be proposed. In the first stage a mafic melt of subcrustal origin interacted with monzogranitic magmas derived from 25–35% degree of melting of a crustal biotite amphibolitic source. Such a mixing process acted together with a crystal/liquid fractionation process to give tonalites and granodiorites. In the second stage lesser degrees of melting of the same crustal source could give the late-stage leucogranitic masses. A possible scenario, able to take into account field and geochemical data, can be suggested for the genesis of this suite and we propose it as a working model for future investigations.

Sm-Nd and Rb-Sr isotopic and geochemical systematics in Phanerozoic granulites from Fiordland, southwest New Zealand

Sm-Nd and Rb-Sr isotopic analyses are reported for granulite facies orthogneisses from Fiordland southwest New Zealand. Whole-rock samples define a Rb-Sr isochron age of 120±15 Ma and an initial 87Sr/86Sr ratio of 0.70391±4. ɛNd values (at 120 Ma) show a relatively wide range of from −0.4 to 2.7 indicating decoupling of Sr-Nd isotope systems. Associated ultramafic rocks have initial 87Sr/86Sr ratios of from 0.70380 to 0.70430 and ɛNd values of from 0.1 to 3.0. The different initial ratios suggest that the various intrusions, although contemporaneous, were not derived through fractionation of a single parent magma. A metasedimentary enclave incorporated during emplacement of the granulitic rocks preserves a Proterozoic isotopic signature with a measured ɛNd(0) value of −10.2, 87Sr/86Sr ratio of 0.73679 and a TNd provenance age of 1490 Ma. The Rb-Sr whole rock age of the granulites is the same as obtained from recent U-Pb zircon dating (Mattinson et al. 1986) and is interpreted as the time of magmatic emplacement and essentially contemporaneous granulite facies metamorphism. Rb-Sr and Sm-Nd analyses of mineral systems indicate that the terrain had cooled below ∼300° C by ∼100 Ma providing further evidence that high grade metamorphism was of exceptionally short duration.

The petrology and geochemistry of massif anorthosites and gabbros of the Bavali fault zone, North Kerala

The massif anorthosites and gabbros occurring in the north Kerala region are localised along the Bavali fault. Several other bodies of granophyres and syeites also occupy this fault. All these bodies are considered to be a continuous graduating igneous series. This paper aims at delineating the field relations, petrochemistry and origin of massif anorthosites and gabbros occurring in the vicinity of the fault.Anorthosites (An 70-55) are nonlayered and highly deformed with no igneous structure present. Gabbros show relict poikilitic texture with a mineralogy of plagioclase, clinopyroxene, hornblende and biotite. Geochemical study suggests dominantly calc-alkaline characteristics for both anorthosites and gabbros. However, their chemical variation is not transitional to indicate evolution by continuous differentiation from a single parent magma. The rock types seem to be chemically unrelated and appear to have evolved independently from discreet magma sources and the magma migration might have only been promoted by the Bavali lineament.

Pristine highland clasts in Consortium Breccia 14305: Petrology and geochemistry

The Apollo 14 site continues to provide pristine non mare rocks that contrast sharply with those from Apollo 16, the archetypical highland site. We characterize six small (&lt;1 cm) samples of pristine highland crust that occur as clasts in polymict lunar breccia 14305. One is an alkali anorthosite; the others include troctolite, norite, and anorthosite of the Mg‐suite. The alkali anorthosite (,400) is a plagioclase adcumulate with minor intercumulus augite. Cerian fluorapatite with LREE‐contents ≅30,000 x chondrite occurs as an inclusion in plagioclase. Enstatite troctolite, 358 and magnesian anorthosite, 361 have trace element characteristics similar to “eastern” Mg‐suite rocks, highlighting the importance of local variations in geochemical systematics that are superimposed on moon‐wide, longitudinal variations. The highly fractionated trace element abundances and highly magnesian mineral compositions of olivine orthopyroxenite, 389 suggest a new, previously unknown magma‐type related to the Mg‐suite. This magma may have formed in equilibrium with garnet. Alkali anorthosites and magnesian anorthosites‐troctolites cannot be related to a single parent magma by fractional crystallization or by variable KREEP assimilation. These suites represent separate parent magmas, each of which may have been modified by mixing with KREEP.

Petrological characteristics of the Damiao anorthosite complex in Hebei Province, China

The Damiao anorthosite complex occurs in the high-grade metamorphic Precambrian rocks in the axial part of the Nei Mongol (Inner Mongolia) Anticline. It is a monoclinal, layered intrusion, with distinguishable layers of anorthosite, monzonite and quartz-monzonite from bottom to top. Gradation evidence can be noticed between these layers. Accumulation structure is obvious in anorthositic rocks. In the anorthositic rocks feldspars, mainly antiperthite and plagioclase, are hosted with An 44–49, while in the acid rock facies, perthite and plagioclase are present. The complex investigated can be assigned to andesine-type rocks. Both the Damiao anorthosites and the Miyun rapakivi granites are calc-alkaline in terms of petrochemical characteristics, and the latter exhibits a very close petrochemical similarity, i.e., high in Al and K, with the acid members in the upper layers of the complex at Damiao. In view of the close temporal-spatial relationship and the resemblance of rock structure, it is thought that the two suites are resulted from a single parent magma under approximate tectonic environments, and that gravitative fractionation may have played an important role in their evolution. Calculations show that the parent magma responsible for the suites bears obvious resemblance to quartz diorite and quartz monzonite in composition, which is in agreement with the experiments by T. H. Green. The Damiao anorthosite complex is characterized by deep source and shallow emplacement as evidenced by high-pressure and high-temperature experiments, geothermometry and petrochemical features as well as the occurrence of both pigeonite and primary “Al-pyroxene”.

Petrology and geochemistry of hawaiite lavas from Crater Flat, Nevada

Hawaiite-type lavas were erupted in three cycles (3.7, 1.2, and 0.3 M.y.) at Crater Flat, Nevada. The compositions of all three cycles, considered together, form a “straddling” alkalic series as defined by Miyashiro, in which the less evolved basalts plot near the normative olivine-diopside divide and the more evolved basalts project into hypersthene or nepheline fields. Fractionation modeling based on the oldest cycle allows the removal of olivine, clinopyroxene, and amphibole to arrive at the more evolved hawaiite compositions. In general, fractionation of phlogo-pite or feldspar is limited by the fractionation modeling and by Eu/REE relations. In detail, all hawaiites within one cycle (3.7 M.y.) need not be derived from a single parent magma. Varied parentage is more evident between cycles although all cycles are consistently of hawaiite composition. Basalts of the youngest two cycles are generally enriched in trace elements. Superimposed on this enrichment is a lack of Rb variation, leading to Rb/Sr ratios far lower than required to generate the high 87Sr/86Sr ratio (0.707) typical of basalts in this region. The very low Rb/Sr ratios limit processes that may lead to trace-element enrichment during magma evolution (cyclic recharge of a fractionating magma chamber). Decreased fractions of mantle melting leaving phlogopite in the residuum or an earlier event of metasomatic transport from phlogopite-bearing mantle rocks into a phlogopite-absent mantle assemblage might explain the observed trace-element enrichment with low Rb/Sr.

Gabbroic rocks from the southern Malay Peninsula and their relation to similar rocks of other orogenic zones

The Linden and Gombak gabbroic stocks outcrop as two discrete bodies in the southern tip of the Malay Peninsula. They may be referred to as batholithic gabbros. which occur as relatively minor components within orogenic belts in association with large. intermediate to acidic batholiths. The Linden stock is mainly of cumulate-textured olivine eucrite and allivalite. They are characterised by an entirely unzoned anorthite (An 92). The Gombak stock is predominantly of medium grained hypidiomorphic granular norite. which is invariably olivine-free and contains both augite and Ca-poor pyroxene (either primary orthopyroxene or inverted pigeonite). This is only the second reported occurrence of inverted pigeonite in batholithic gabbros. Systematic mineralogical. textural and petrochemical variations demonstrate that the gabbroic rocks of the Linden and Gombak stocks constitute a differentiated series derived from a single parental magma. Allivalite and olivine eucrite of the Linden stock represent crystal cumulates resulting from high level fractionation of a high-alumina. low-potash. basalt parental magma which subsequently crystallised apparently non-cumulate Gombak norite. Ca-poor pyroxene of the Linden-Gombak suite demonstrates a change in texture from ophitic in olivine-bearing Linden cumulates to granular in olivine-free Gombak norite. A similar textural change involving Ca-rich or Ca-poor pyroxene. on the cessation of olivine crystallisation. is apparently common amongst gabbroic associations and more important than previously considered. The Linden-Gombak gabbroic association and similar batholithic gabbro suites of other orogenic belts are interpreted as unroofed magma chambers representing the root zones of palaeo calc-alkaline volcanoes by virtue of the similarity between these gabbroic rocks and the cognate plutonic xenoliths of calc-alkaline volcanics.

Rare earth element patterns of rocks from the centre 3 igneous complex, Ardnamurchan, Argyllshire

Systematic, differences in absolute abundancies and distribution patterns of rare earth elements are shown to exist for the main rock types of the third and final phase of the Tertiary igneous complex of Ardnamurchan. Published partition coefficients of the rare earth elements between crystals and host magmas of extrusive rocks have been used, together with modes of the Ardnamurchan rocks, to estimate the rare earth element patterns of the parent magmas. The results confirm that the basic rocks formed by crystal fractionation but that continued crystal fractionation from a single parent magma could not have formed the intermediate rocks.

Strontium isotopic geochemistry of the volcanic rocks and associated megacrysts and inclusions from Ross Island and vicinity, Antarctica

Twelve whole-rock samples of volcanic rocks and a composite of 11 basanitoid samples from Ross Island and vicinity, Antarctica show a narrow range of 87Sr/86Sr ratios from 0.70305 to 0.70339. This range is consistent with a model of differentiation from a single parent magma, but the data allow a 30% variation in the 87Rb/86Sr ratio in the source region if the average ratio is less than 0.057 and if the source region has existed as a closed system for 1.5 b.y. Megacrysts of titaniferous augite, kaersutite, and anorthoclase are isotopically indistinguishable from the host volcanic rocks and therefore are probably cogenetic with the volcanic sequence. A single trachyte sample is isotopically distinct from the rest of the volcanic rocks and probably was contaminated with crustal strontium.

Rare-earth elements, Co, Sc and Hf in the Steens Mountain basalts

The concentrations of the rare-earth elements (REE) in 52 of 70 consecutive lava flows from Steens Mountain, Oregon, are reported. The concentrations of Co, Sc and Hf were measured in 17 of the flows. Logarithmic partitioning theory is used to correlate the concentrations of the trace elements with the major element and mineralogical compositions of the samples. Production of the basalts by partial melting requires a parent material that has concentrations of the REE that are several times those of chondrites. Those samples enriched in alumina have positive Eu anomalies compared to chondrites and those depleted in alumina have negative Eu anomalies. Production of these samples by partial melting is unlikely because the Eu anomalies would require that plagioclase was stable under the conditions of melting. All of the samples can be related to a single parent magma by fractional crystallization processes. The concentrations of the trace elements are controlled by the addition or removal of plagioclase and the removal of clinopyroxene. Samples with high concentrations of alumina are interpreted as plagioclase cumulates while those with low concentrations of alumina are residual liquids produced by crystallization of plagioclase. During fractional crystallization, the concentrations of the light REE are increased selectively in the residual liquid. The material that must be crystallized and removed during fractional crystallization has characteristics that are similar to those reported for gabbros.

An alternative model for the anorthositic and associated rocks of the Nain massif, Labrador

The two rock series identified in the Naim massif by de Waard & Wheeler ( Lithos 4, 1971) differ fundamentally in their silica saturation and potassium content. It is suggested that the two series instead of having a single parent magma, may represent two parents, one essentially low-K, high-alumina basalt with FeO:MgO near 1, and the other essentially andesitic. The apparent continuity of trends between the two rock series may be fortuitous in that the basic differentiates of the more acid magma show mineralogical and chemical trends toward the critically undersaturated suite because of their common P-T environment of crystallization.

The structural control and genesis of alkaline rocks in central Kenya

In the Nairobi area and the adjacent region of the Gregory Rift Valley of Kenya two series of Cainozoic alkaline volcanic rocks, of mildly and strongly alkaline type respectively, are considered to have been derived from a single parental magma of alkali olivine basalt composition. Vulcanicity is genetically associated with tectonic movements attendant upon doming and rifting and distances from the rift margin decrease with crystal fractionation, the most acid differentiates being located at the maximum elevation of the rift floor. An early period of nepheline-bearing lava extrusion associated with central vulcanism is followed by a later period of welded tuff — trachyte — rhyolite fissure eruption. Caldera formation associated with central vulcanism within the Rift accompanies the later volcanic phase.

Trends of differentiation in basaltic magmas

Kennedy disputes the idea of a single basaltic parent magma, reasoning that there are two series, olivine basalt and tholeiite, each with its own liquid line of descent. He briefly mentions Hawaii as being characterized by the olivine basalt type.