Model For Petrogenesis Research Articles

Miocene adakites in south Tibet: Partial melting of the thickened Lhasa juvenile mafic lower crust with the involvement of ancient Indian continental crust compositions

Abstract Although postcollisional adakitic rocks are widely distributed in the southern Lhasa subterrane, their petrogenesis remains controversial. Complex petrogenesis models, mainly including partial melting of subducted oceanic crust, partial melting of the Indian lower continental crust, and magma mixing, are pivotal in reconstruction of the postcollisional dynamic processes in south Tibet. In order to constrain the geodynamic processes, we present systemic geochronological and geochemical data for newly discovered adakitic dikes in the Xigaze area, southern Lhasa subterrane. Based on the K2O and Na2O contents, the Xigaze dikes can be divided into K-rich and Na-rich dikes. Zircon U-Pb dating for the Xigaze K- and Na-rich dikes yielded ages of ca. 10.31 Ma and 14.78–12.75 Ma, respectively. The K-rich dikes show porphyritic texture and are characterized by high SiO2 (68.91–69.59 wt%) and K2O (5.53–5.68 wt%) contents and low Na2O/K2O (0.48–0.60) ratios, with Al2O3/(CaO + Na2O + K2O) (=A/CNK) ratios of 1.07–1.23. They have lower MgO (0.63–0.64 wt%), Mg# (37–39), and Cr (18.56–26.62 ppm) and Ni (4.37–4.62) contents. In addition, the K-rich dikes display enriched ([La/Yb]N = 65–68) light rare earth elements (LREEs), low concentrations of heavy rare earth elements (HREEs) and Y (e.g., Yb = 0.83–0.86 ppm; Y = 10.56–11.55 ppm), and high Sr (841–923 ppm), with high Sr/Y (74–84) ratios, indicating geochemical characteristics of typical adakitic rocks. Compared with the K-rich dikes, the Na-rich dikes also display porphyritic texture, but they have lower SiO2 (59.14–64.87 wt%) and K2O (1.98–3.25 wt%) contents, and higher Na2O (4.43–5.64 wt%) and MgO (1.40–3.08 wt%) contents, Mg# (46–59), and Cr (22.62–82.93 ppm) and Ni (8.91–39.76 ppm) contents. The HREE abundances (e.g., Yb = 0.36–0.81 ppm; Y = 5.30–10.56 ppm) of the Na-rich dikes are generally lower than the K-rich dikes. These Na-rich dikes are also characterized by adakitic geochemical features with high Sr/Y (60–223) but low (La/Yb)N (15–40) ratios. Both the K-rich and Na-rich dikes display distinct whole-rock-element geochemistry and Sr-Nd isotopic composition, with (87Sr/86Sr)i = 0.7121, εNd(t) = –8.62 to –8.11 and (87Sr/86Sr)i = 0.7054–0.7086, εNd(t) = –7.55 to –1.23 for K-rich and Na-rich dikes, respectively, which indicate different magma sources for the two types of dikes. The K-rich dikes were most likely derived from partial melts of Lhasa juvenile mafic lower crust with significant involvement of Indian continental crust compositions, whereas the Na-rich dikes were generated in the same way with less input of Indian continental crust compositions. Moreover, the postcollisional adakites in the southern Lhasa subterrane display distinctive spatial variations in geochemistry along the strike of this subterrane, indicating that the magma sources were heterogeneous. In combination with previously published data, we therefore suggest that all these late Oligocene to Miocene adakitic rocks were most likely generated dominantly by partial melting of the Lhasa mafic lower crust with involvement of Indian continental crust components, which was probably triggered by the tearing of the subducting Indian plate.

Petrogenesis of Early Cretaceous granites and associated microgranular enclaves in the Xiabie Co area, central Tibet: Crust-derived magma mixing and melt extraction

Multiple petrogenesis models have been proposed for granitic rocks and the mafic microgranular enclaves (MMEs) hosted in them. The mantle and crust-derived magma mixing/mingling process was widely suggested for the formation of MMEs or the host granitoids. However, whether the mixing between magmas derived from different crustal sources may also form microgranular enclaves (MEs, not restricted to mafic) or the host granites remain unclear. Here, we report petrological, geochemical, and zircon in situ Hf-O isotopic data for MEs and their host granites in the northern Nyima area, southern Qiangtang terrane (SQT), central Tibet. The host granitic rocks consist of amphibole-bearing biotite granites and syenogranite, minor granodiorite and aplite veins, and MEs comprising diorite and granodiorite. Host rocks and MEs are contemporaneous and were generated in the Early Cretaceous (∼120 Ma). Host granitoid samples are high-K calc-alkaline rocks and have high SiO2 (66–78 wt%) and low MgO (<1.14 wt%). They have enriched whole-rock εNd(t) (−1.82 to −3.06), and variable zircon εHf(t) (+0.64 to +5.12) and δ18O (7.35 to 9.61‰) values. The host granodiorites contain some mafic mineral-rich clots and lower silica contents than the biotite granites. They exhibit distinctive geochemical patterns (such as high K2O > 5 wt%, high Zr >500 ppm with Zr/Hf ratios >45, and high Ba >400 ppm), implying biotite and zircon concentrations. Combining this information with embayed feldspars, curved biotites, and closely spaced feldspar grains, we suggest that liquid loss resulted from crystal compaction or filter pressing. Both the fine-grained syenogranites and aplite veins exhibit high SiO2 and Rb/Sr ratios in addition to low Zr/Hf ratios and Zr and Ba concentrations with remarkably negative Eu anomalies, indicating that they are highly differentiated melts extracted from felsic crystal mush. Such a crystal mush is possibly represented by the biotite granites. Gradual contact relationships between variable granitic phases indicate that melt extraction and crystal concentration were coeval. Residual melt extraction from the crystal mush was facilitated by mush remobilization resulting from magma replenishment. We suggest that for the host rocks, the biotite granites were mainly derived by partial melting of ancient crust, and the granodiorites most likely resulted from the magma mixing/mingling that might be accompanied with the loss of melts, the fine-grained syenogranites and aplites represent highly evolved residual melts. MEs are geochemically characterized by intermediate to felsic compositions (SiO2 = 56.3–67.8 wt%), with low MgO (<4 wt%) and Mg# (< 53) and low mantle compatible trace element (Cr and Ni) contents. They exhibit enriched whole-rock Nd (εNd(t) = −0.36 to −2.71), depleted zircon εHf(t) (+0.51 to +5.68) and high δ18O values (7.45 to 9.57‰), except for one sample with comparable whole-rock εNd(t) (−1.32), relatively high zircon εHf(t) (+5.05 to +7.11) and δ18O (6.09 to 7.10‰) values. MEs exhibit evidence for quenched magma crystallization, chemical exchange, mineral disequilibrium and mass transfer, indicating inefficient magma mixing/mingling. Mafic minerals from MEs were in equilibrium with evolved melts rather than mantle-derived primitive melts. Given the substantial absence of contemporaneous basaltic rocks from the SQT and the relatively high SiO2 and low MgO contents and crust-like zircon stable oxygen isotope of MEs, we propose that they most likely resulted from the mixing/mingling of diverse crust-derived magmas. Therefore, our study provides important evidence for crust-derived magma mixing and melt extraction from mush.

Rift melting of juvenile arc-derived crust: Geochemical evidence from Neoproterozoic volcanic and granitic rocks in the Jiangnan Orogen, South China

A combined study of zircon U–Pb and Lu–Hf isotopes, mineral O isotopes, whole-rock elements and Sr–Nd isotopes was carried out for Neoproterozoic volcanics and granites from the eastern part of the Jiangnan Orogen in South China. The results are used to test controversial models of petrogenesis (plume-rift, slab-arc and plate-rift) for similar ages of magmatic rocks in South China. Zircon U–Pb dating yields two groups of ages at ∼780 and ∼825 Ma, respectively, corresponding to syn-rift and pre-rift magmatic events in response to supercontinental rifting. Both volcanic and granitic rocks show trace element features similar to those of arc-derived igneous rocks, but with more significant enrichment in large ion lithospheric elements relative to oceanic arc basalts. They have positive ɛ Hf( t) values of 3.6–6.3 for zircons, with Hf model ages of 1.12–1.21 Ga. This indicates reworking of late Mesoproterozoic juvenile crust for the origin of the Neoproterozoic magmatic rocks. Thus, oceanic arc magmatism would occur in the late Mesoproterozoic, with remarkable production of juvenile crust at the southeastern margin of the Yangtze Block. Because of the tectonic collapse of arc–continent collision orogen in the pre-rift stage, S-type magmatic rocks were generated by burial and anatexis of juvenile arc-derived crust to form the pre-rift episode of granodiorites and volcanics. In the syn-rift stage, the volcanics formed by reworking of arc-derived sedimentary rocks whereas the granite was generated by melting of the pre-rift igneous rocks. Emplacement of evolved felsic magmas along the rift tectonic zone would cause subsolidus high- T meteoric-hydrothermal alteration, resulting in varying δ 18O values for minerals from the volcanic and granitic rocks. Therefore, the plate-rift model is advanced to account for petrogenesis of all Neoproterozoic magmatic rocks in South China, with lithospheric extsension as the driving force of supercontinental rifting. Partial melting due to arc-continent collision, orogenic collapse and supercontinental rifting is also proposed as a mechanism for the chemical differentiation of continental crust towards the felsic composition.

Lachlan Fold Belt granites revisited: High‐ and low‐temperature granites and their implications

Many of the granites in southeastern Australia possess compositional, petrographic, zircon age inheritance and other features that cannot be accounted for satisfactorily by the classical models of petrogenesis. The restite model was developed to account for these features and recognises that unmelted but magmatically equilibrated source material (restite) may be entrained in a partial melt, together comprising magma. Variation in the degree of separation of those two components, leading to differences in the ratio of melt to restite, is responsible for the variation in composition within many suites of granites. A popular alternative view, that variation within suites resulted from magma mixing or mingling, conflicts with simple observations of the rock compositions and cannot be sustained. Several strong arguments can be made against another alternative view that fractional crystallisation was the dominant process in producing variation within those suites. New and conclusive evidence against that process is provided by the fact that zircon age inheritance is present in most of these granites where that has been examined. That has shown that the S‐type and most of the I‐type granites formed at low magmatic temperatures and confirmed that the compositional variation within those suites must have resulted from restite fractionation. The Ordovician sedimentary rocks exposed in the Lachlan Fold Belt are not sufficiently feldspathic to have produced the voluminous S‐type granites and volcanic rocks of the Bullenbalong Supersuite. The view that those granites and volcanic rocks were derived from more feldspathic metasedimentary rocks is supported by much evidence and confirmed by the fact that the pelitic enclaves in those granites are relatively high in Ca. The presence of a once‐thick metasedimentary basement in those areas in which these S‐type granites occur is inferred. The I‐type granites of the Lachlan belt are not compositionally analogous to the more calcic and less potassic granites found in younger subduction‐related continental margins and mostly formed at low magmatic temperatures through the partial melting of pre‐existing quartzo‐feldspathic igneous crust. This implies that the petrological evolution of the belt during the Silurian and Devonian occurred dominantly by the vertical redistribution of the components of older crust through partial melting and movement of granite magmas. Evolution of the belt at that time was not related to active subduction.

Magnesium oxide‐iron oxide mass balance constraints and a more detailed model for the relationship between eucrites and diogenites

Abstract— According to a currently popular model for petrogenesis on the howardite, eucrite, and diogenite (HED) parent asteroid, the diogenites are not comagmatic with most eucrites but instead formed in separate orthopyroxenite‐dominated plutons. This model can be tested for consistency with mass balance for MgO and FeO, assuming the overall diogenite/(diogenite + eucrite) ratio, d, of the parent asteroid is at least comparable to the average d for the eucrite + diogenite dominated howardite regolith breccias. Average mg# (=MgO/[MgO + FeO]) is much lower for eucrites, especially noncumulate eucrites, than for diogenites. Unless the diogenite parent magmas eventually produced a large proportion of low‐mg# residual basalt and gabbro (RBG), the implied initial magma's mg# is vastly higher than that of any noncumulate eucrite. Starting from a source previously depleted by putative primary eucrite genesis, melt mg# can be estimated as a function of the exchange reaction KD and degree of melting. Using several very conservative assumptions (e.g., assuming that the total [MgO + FeO] concentration is nearly the same in the nascent melt as in the residual solids), the degree of melting required to yield a melt with mg# high enough to satisfy mass balance, without implying an RBG component that accounts for &gt;50% of all eucrites, is an implausibly high 60–80 wt%.The separate orthopyroxenitic plutons (SOP) model also seems inconsistent with the uniform density of melts across the diogenite‐eucrite compositional spectrum (2.77–2.82 g/cm3), which implies that diogenitic magmas should have been as capable as eucrites of extruding to form lavas. This difficulty cannot be reduced by simply assuming that later‐formed magmas were systematically both more plutonic and more MgO‐rich than earlier ones, because the plutonic cumulate eucrites equilibrated with melts systematically lower in mg# than noncumulate eucrites. Conceivably, the bulk mg# of the asteroid's silicate system was increased between primary‐melt eucrite genesis and SOP diogenite genesis by graphite‐fueled reduction of FeO. However, the graphite oxidation process generates a huge proportion of gas, which would have enhanced the buoyancy of the nascent diogenite‐parent magmas, thus exacerbating the difficulty of achieving the implied high degrees of partial melting.To avoid these difficulties but still form most eucrites as rapidly cooled extrusives, I propose the NERD (noncumulate eucrites as extruded residua of diogenites) model. In this model, the diogenites form as early cumulates from a large magma system (probably a global “magma ocean”) that yields a large proportion of eucritic melt as residuum. This residual melt zone undergoes relatively little crystallization during a period when it is episodically tapped to produce extrusions, dikes and sills of rapidly cooled noncumulate eucrites. Slight (∼5–10%) porosity in the nascent eucritic crust keeps it marginally buoyant over the residual melt zone. The common thermal metamorphism of noncumulate eucrites results from baking by superjacent flows during the episodic venting of the melt zone. The NERD model's greatest advantage is that it does not require implausibly high degrees of localized melting in the mature stages of igneous evolution of the HED asteroid.

Petrogenesis of lunar troctolites

Models of petrogenesis for lunar troctolites are severely constrained by the high Mg* (&gt;88) values of the most primitive members of the Mg‐rich suite. Melts derived by partial melting of olivine cumulates to the lunar magma ocean have high Mg* but are strongly undersaturated with respect to plagioclase. Assimilation of lunar crust cannot bring these melts to the plagioclase liquidus without unduly reducing the Mg* values of olivine. Partial melts derived by melting a Iherzolite primitive interior have higher normative plagioclase contents but require sources with Mg* &gt; 90. More terrestrial‐like Mg* (∼88) values are permitted if rising thermal plumes are capable of entraining the most Mg‐rich cumulates of the magma ocean. The Mg* values of lunar troctolites may, therefore, reflect a hybrid source composed of primitive mantle and Mg‐rich cumulates of the magma ocean. Higher Mg* values may also arise from the reduction of small amounts of FeO to Fe. Alternatively, the parent magmas to lunar troctolites may be impact melts of the anorthositic crust and the Mg‐rich dunite cumulates of the magma ocean; the latter were brought to the upper mantle during the overturn of the unstable cumulate pile of the magma ocean.

Trace element analysis of ureilites: New constraints on their petrogenesis

Six ureilites (ALHA77257, ALHA81101, ALH82130, PCA82506, Kenna, and Novo Urei) were analyzed using neutron activation analysis for Ca, Sc, Cr, Mn, Fe, Co, Ni, Zn, Ga, REE, W, Re, Os, Ir, and Au. We examined bulk samples as well as acid-treated samples. In the bulk samples the refractory siderophiles' concentrations range from approximately 0.1 to 1.0 times CI chondrites while the volatile siderophiles range from about 0.07 to 0.3 times CI chondrites. Rare earth elements (REEs) in ureilites are quite depleted and display light and heavy rare earth enrichments. The Antarctic meteorites display either much less pronounced v-shaped patterns or no enrichment in the light rare earths at all. In terms of the new trace-element results, ureilites do not fall into the coherent groups that other workers have defined by chemical or petrographic characteristics. Trace elements do provide additional constraints on the models for the petrogenesis of ureilites. In particular, the siderophile element abundances call for simplified models of chemical processing rather than the complex, multistage processing called for in silicate fractionation models. REE concentrations, on the other hand, imply multistage processing to produce the ureilites. None of the ureilite petrogenesis models extant account for the trace element data. These new data and the considerations of them with respect to the proposed ureilite petrogenesis models indicate that the direction of modelling should be toward contemplation of mixtures and how the components we observe in ureilites behave under such conditions.

Majorite fractionation and genesis of komatiites in the deep mantle

Abstract Melting experiments of the primitive mantle indicate that majorite fractionation is the key process to determine the geochemical nature of magmas generated in the deep upper mantle and the transition zone. A model of petrogenesis of komatiite magma is presented by using the melting relations of the primitive mantle and the partitioning data of lithophile elements between majorite and the liquid. Alumina-depleted komatiite, generally observed in the early Archean, can be generated by any degree of partial melting at depths from 450 to 650 km. This type of komatiite is also generated by a relatively low degree of partial melting at depths from 180 to 450 km. Alumina-enriched komatiite can be generated by remelting of the residual mantle after separation of alumina-depleted komatiite. Alumina-undepleted komatiite generally observed in the late Archean can not be formed by a low degree of partial melting of the primitive mantle at any depths. This type of komatiite can be generated only by a high degree of partial melting of the mantle at the depths shallower than 450 km. As an alternative model, the alumina-depleted komatiite can be formed by a high degree of partial melting of the Archean upper mantle depleted in majorite. Such heterogeneity in the Archean upper mantle was generated by a global melting event succeeding the accretion of the earth, and erased by convection by the later Archean. Hf-isotope data seem to favour the former model of the petrogenesis of komatiite, i.e., single stage melting of the primitive mantle at various depths.

Comments on a Paper by Jon. D. Woodhead Titled 'The Origin of Geochemical Variations in Mariana Lavas: a General Model for Petrogenesis in Intra-oceanic Island Arcs?'

Comments on a Paper by Jon. D. Woodhead Titled 'The Origin of Geochemical Variations in Mariana Lavas: a General Model for Petrogenesis in Intra-oceanic Island Arcs?'

The Origin of Geochemical Variations in Mariana Lavas: A General Model for Petrogenesis in Intra-Oceanic Island Arcs?

The Origin of Geochemical Variations in Mariana Lavas: A General Model for Petrogenesis in Intra-Oceanic Island Arcs?

Assimilation-fractional crystallization of Polvadera Group rocks in the northwestern Jemez Volcanic Field, New Mexico

Pliocene Polvadera Group rocks in the northwestern Rio Grande rift-marginal portion of the Jemez Volcanic Field record the rapid transition from weakly alkaline Lobato Basalt magmatism (48–52% SiO2; 7.9 Ma) through calc-alkaline Lobato andesite and dacite (53–64% SiO2) and Tschicoma dacite-rhyodacite magmatism (63–69% SiO2; 7.4 Ma). Petrologically, Lobato andesite and dacite and Tschicoma dacite-rhyodacite represent a cogenetic suite of differentiates (the La Grulla Plateau or LGP suite) distinctive from the bulk of Polvadera Group rocks including Tschicoma andesite. Increasing (87Sr/86Sr)O ratios with differentiation within the LGP suite from 0.7051 (54% SiO2) to 0.7064 (68% SiO2), trace element variations, and disequilibrium mineral assemblages suggest open system differentiation involving 87Sr-enriched upper crust. A likely parental magma is the voluminous Lobato Basalt ((87Sr/86)O= 0.7043–0.7050) which was erupted predominantly earlier and to the east toward the rift axis. The best model for petrogenesis involves bulk assimilation of locally wide-spread Proterozoic (1.4–1.6 Ga) upper crustal granite by fractionally crystallizing Lobato Basalt. Assimilation-fractional crystallization (AFC) modeling of Sr-isotope and trace element variation (DePaolo 1981) indicates that ∼40% crystallization of Lobato Basalt accompanied by ∼10% addition of granite reproduces the observed geochemical and Sr-isotopic diversity. Neither magma mixing, nor mafic recharge have complicated the magmatic evolution of the LGP suite. Crustal thickness and/or retarded tectonism could have facilitated conditions necessary for evolution by AFC to occur within the upper crust.

Volcanism of shoshonite to high-K andesite affinity in an Archean are environment, Oxford Lake, Manitoba

Volcanic rocks of the Archean Oxford Lake Group are characterized by a stratigraphic progression from mafic to felsic compositions, accompanied by a systematic decrease in incompatible element abundances. This, coupled with high abundances of Sr, Rb, K2O, and La, high (La/Yb)n, and unfractionated (flat to concave) heavy-REE (rare earth element) profiles, distinguishes these rocks as an Archean shoshonite to high-K andesite – dacite–rhyolite series, directly comparable to modern analogues formed in convergent tectonic settings. The trace-element data support a model of petrogenesis in which an Archean mantle source region was modified by volatiles rich in large-ion-lithophile elements, the modified mantle was subjected to partial melting forming a parental liquid of shoshonitic character, and this liquid principally underwent amphibole fractionation to form the evolved rock compositions. This process is envisaged as terminal magmatism during the final (senile) stage of activity in an arc setting. The Rb–Sr age (2650 ± 80 Ma) and initial-Sr ratio (0.70145 ± 38) of the Oxford Lake Group are in accord with this model; when coupled with the 2.7 Ga age for the underlying Hayes River Group volcanics, these data indicate a short interval (&lt;50 Ma?) for Archean crustal development in the area.