Jan Hertogen

Contrasting origin of post-collisional high-K calc-alkaline and shoshonitic versus alkaline and peralkaline granitoids. The use of sliding normalization

Abundant high-K calc-alkaline (HKCA) magmatism appears to be post-collisional and often shifts to shoshonitic or alkaline–peralkaline compositions in the final stages of orogeny. The nature and the causes of this transition are studied on the basis of 308 major element and of 86 unpublished trace element (including REE) analyses of the Pan-African granitoids from the Tuareg shield (Adrar des Iforas, Mali and Air, Niger). This database covers a wide variety of magmas from subduction-related to intraplate-type including abundant HKCA batholiths. Literature data from geodynamically well-constrained cases are also included. In addition to a conventional geochemical approach of the studied magmatism, the sliding normalization method is proposed. This tool aims at comparing magmatic series: each studied rock is normalized to the interpolated composition of the reference series that has the same SiO2 content as the sample. This method amplifies differences in sources and in fractionation processes and allows comparison of rocks from basic to acid composition. Two distinct juvenile sources are proposed: a previously enriched phlogopite-K richterite bearing lithospheric mantle or a lower juvenile crustal equivalent for HKCA-shoshonitic magmas, and a lowest lithospheric-upper asthenospheric OIB-type mantle for alkaline-peralkaline magmatism. The first source is melted only shortly after its generation when the lithosphere was still hot, which restricts HKCA magmatism mainly to post-collisional settings. The second asthenospheric/lowest lithosphere source is by definition close to its melting temperature and can generate magma ubiquitously both in space and time. The main melting triggers are lithospheric major structures which are not only operative in a post-collisional setting but also in other environments such as intraplate setting. Geochemistry thus gives indications about the nature of the source and on geotectonic settings. However, the latter is a second rank information, which is partly model-dependant. The post-collisional period differs from other settings by a propensity to generate large amounts of magma of various kinds, among which HKCA magmatism is volumetrically the most prominent.

Redistribution of rare earth elements, thorium, and uranium over accessory minerals in the course of amphibolite to granulite facies metamorphism: The role of apatite and monazite in orthogneisses from southwestern Norway

The amphibolite to granulite facies transition has been studied in a high-K calc-alkaline, hornblende-biotite, K-feldspar megacrystic augen gneiss series from the Rogaland-Vest-Agder (Rog-VA) sector of the Sveconorwegian province (southwest Norway). Hornblende begins to break down mainly to clinopyroxene at the Cpx-in isograd and biotite mainly to orthopyroxene at the Opx-in isograd (abbreviations for minerals following Kretz, 1983). The magmatic accessory mineral association comprises titanite and allanite, which begin to break down before the Cpx-in isograd. Titanite is preserved as relict inclusions in other minerals at higher grade. Monazite and thorite formed in the breakdown of allanite. Monazite abundance reaches a maximum between the Cpx-in and Opx-in isograds. The middle and heavy rare earth element (M-HREEs, except Eu) content of apatite steeply increases with increasing metamorphic grade and is directly correlated to the decrease of the modal abundances of titanite, hornblende, and biotite. The U contents of apatite are low and do not increase with metamorphic grade. The light rare earth elements (LREEs) and Th content of apatite are not correlated to the breakdown of allanite around the Cpx-in isograd but increase around the Opx-in isograd. A simplified equation is proposed for monazite crystallisation in the vicinity of the Cpx-in isograd that accounts for the allanite, titanite, and hornblende breakdowns and the M-HREE substitution in apatite: 3 (M-HREE)2O3in hornblende and litanite+3(LREE)2in allaniteO3+2Ca5(PO4)3apatite(F,OH)+6SiO2quartz⇔6(LREE)PmonaziteO4+2Ca2(M-HREE)3(SiO4)3(F,OH)lessingite in apatite+6CaOin plagioclase. At the Opx-in isograd, the increase of LREEs and Th content of apatite results either from the breakdown of some monazite: 3(LREE)PO4monazite+3SiO2quartz+4CaOin plagioclase+(F2,H2O)fluid⇔Ca5(PO4)3(F,OH)+apatiteCa2(LREE)3(SiO4)3(F,OH)less ingite in apatite. or the breakdown of the remaining allanite: 3(LREE)2O3in allanite+6SiO2quartz+4CaOin plagioclase+(F2,H2O)fluid⇔2Ca2(LREE)3(SiO4)3(F,OH)lessingite in apatite. The release of fluorine from the breakdown of biotite at the Opx-in isograd may increase apatite stability relatively to monazite in the granulite facies. when compared to amphibolite facies, the granulite facies augen gneisses do not show any U or Th depletion. This means that changes in the accessory mineral associations and the simultaneous breakdown of hydrous minerals at the amphibolite-granulite facies transition do not inevitably result in depleted granulite facies rocks but rather to an isochemical element redistribution. The coexistence of small amounts of metamorphic monazite and of relict inclusions of titanite in upper amphibolite facies augen gneisses suggests that (high-K) calc-alkaline orthogneisses are a suitable material to date (with the U-Pb method) prograde path of amphibolite facies regional metamorphism on monazite and the cooling path on titanite.

Sr and O isotope constraints on source and crustal contamination in the high-K calc-alkaline and shoshonitic neogene volcanic rocks of SE Spain

The Neogene volcanic province of SE Spain NVPS is characterized by calc-alkaline CA , high-K calc-alkaline KCA , . . . shoshonitic SH , ultrapotassic UP , and alkaline basaltic AB volcanic series. All these series, except the AB, have high LILErLREE, LILErHFSE and BrBe ratios and high but variable Sr, Pb and O isotope compositions. The KCA and SH lavas contain metapelitic xenoliths whose mineralogical and chemical composition are typical of anatectic restites. The geochemical characteristics of CA, KCA, SH and UP series suggest that they originated from the lithospheric mantle, previously contaminated by fluids derived from pelagic sediments. Additionally, the presence of restite xenoliths in the KCA and SH lavas indicates some sort of interaction between the mantle-derived magmas and the continental crust. Trace element and isotope modeling for the KCA and SH lavas and the restites, point towards the existence of two mixing stages. During the first stage, the lithospheric mantle was contaminated by 1-5% of fluids derived from pelagic sediments, which produced

U-series, SrNdPb isotope and trace-element systematics across an active island arc-continent collision zone: Implications for element transfer at the slab-wedge interface

We present U-series, SrNdz.sbnd;Pb isotope and trace-element results of a regional study of geochemical systematics across an island arc-continent collision zone in the East Sunda Arc of Indonesia. Samples from four active volcanoes exhibit a striking compositional range from low-K tholeiitic to ultrapotassic, but all are characterised by high (0.7053–0.7067), radiogenic lead isotope ratios ( = 18.99–19.15), low (0.66–0.85), and low (0.51255–0.51272), except for high (>0.51286) at the volcanic front. Low ratios are also found in terrigenous sediments in front of the arc, which, in combination with Srz.sbnd;Ndz.sbnd;Pb isotopic constraints, indicates that subducted continental material contributes to magma sources in this arc sector. The volcanoes close to the trench show a large excess of 238U over . 230Th (up to 80%) and of 226Ra over 230Th (up to 800%). In addition, they are enriched in elements thought to be mobile in hydrous fluids during slab-wedge transfer, such as Ba, Pb, and Sr. In contrast, Uz.sbnd;Thz.sbnd;Ra systematics are close to equilibrium in the volcanoes behind the front. Abundance patterns of incompatible trace elements in these rocks are similar to those of the terrigenous sediments, so that, in comparison with the arc-front lavas, they possess low Ba/La, Ba/Th, La/Th, Pb/Ce, and Zr/Nb. Higher concentration levels and less interelement fractionation form conspicuous differences with the front volcanics. Our combined isotopic and trace element data are consistent with three-component mixing whereby a slab-derived hydrous fluid and a siliceous melt are both added to the sub-arc mantle source. The hydrous fluid largely controls the input in the shallow part of the subduction zone, whereas the siliceous melt dominates the flux at deeper levels. Sedimentary material is considered to be the primary source of both. The large U-Th-Ra disequilibria at the front suggests that element transfer is a currently active process associated with present-day subduction of continental material.

Calculation of trace-element fractionation during partial melting

Abstract The mathematical expressions of trace element fractionation during partial melting as presented in a paper by Shaw (Geochim. Cosmochim. Acta34, 237–243, 1970) are modified and extended to fulfil three requirements: 1. (i) to overcome the restriction that no phase may be used up in the course of the melting process 2. (ii) to account for variations of partition coefficients and melting proportions during melting 3. (iii) to treat incongruent melting processes. Examples are given to illustrate these principles.

Rare earth element geochemistry and strontium isotopic composition of a massif-type anorthositic-charnockitic body: the Hidra Massif (Rogaland, SW Norway)

Abstract The Hidra Massif (Rogaland Complex, SW Norway) mainly consists of plagioclase cumulates (anorthosites and leuconorites), which grade progressively into a fine-grained (200 μm). locally porphyritic, jotunitic rock towards the contact with the granulite facies gneisses. The massif is cross-cut by thin (10 cm up to 1 m) charnockitic dykes. The petrographical and geochemical evolution of the Hidra Massif can be explained by fractional crystallization of a jotunitic parental magma. Major and trace element constraints indicate that mafic phases are underabundant in the exposed levels of the massif, most likely as a result of plagioclase flotation in the early stages of solidification. Partitioning into the cumulate minerals (mainly plagioclase and orthopyroxene) governs the trace element contents of the leuconoritic adcumulates. However, the trace element geochemistry of the apparently early formed anorthositic orthocumulates largely depends upon the amount of a trapped intercumulus liquid. On the basis of trace element abundances (high REE, Rb, Th, U; negative Eu anomalies) the silicic charnockitic dykes can be considered as the residual liquid of the anorthositic fractionation trend. The higher initial 87 Sr 86 Sr ratios (0.7086 ± 0.0006 vs 0.7055 ± 0.0004 for the plagioclase cumulates and jotunites) point to contamination of the charnockitic liquids by surrounding gneissic material.

Petrogenesis and paleotectonic history of the Wild Bight Group, an Ordovician rifted island arc in central Newfoundland

The Wild Bight Group (WBG) is a sequence of early and middle Ordovician volcanic, subvolcanic and epiclastic rocks, part of the Dunnage Tectonostratigraphic Zone of the Newfoundland Appalachians. A detailed geochemical and Nd-isotopic study of the volcanic and subvolcanic rocks has been carried out to determine the geochemical characteristics of the rocks, interpret their palcotectonic environments and constrain their petrogenetic history. The lower and central stratigraphic levels of the WBG contain mafic volcanic rocks with island-arc geochemical signatures, including LREE-enriched are tholeiites with εNd(t)=-0.1 to +2.2 (type A-I), LREE-depleted arc tholeiites with εNd(t)=+5.6 to +7.1 (type A-II) and an unusual suite of strongly incompatible-element depleted tholeiites in which εNd(t) ranges from-0.9 to +4.6 and is negatively correlated with147Sm/144Nd (type A-III). High-silica, low-K rhyolites occur locally in the central part of the stratigraphy, associated with mafic rocks of arc affinity, and have εNd(t)=+4.7 to +5.4. The upper stratigraphic levels of the WBG dominantly contain rocks with non-arc geochemical signatures, including alkalic basalts with εNd(t)=+4.6 to +5.5 (type N-I), strongly LREE- and incompatible element-enriched tholeiites that are transitional between alkalic and non-alkalic rocks with εNd(t)=+4.4 to +7.0 (type N-II) and rocks with flat to slightly LREE-enriched patterns and εNd(t)=+5.1 to +7.4 (type N-III). Rocks with non-arc and arc signatures are locally interbedded near the stratigraphic type of the WBG. Nd-isotopic data in the type A-I and A-II rocks are generally compatible with mixing/partial melting models involving depleted mantle, variably contaminated by a subducted crustally-derived sediment. The petrogenesis of type A-III rocks must involve source mixing and multi-stage partial melting, but the details are not clear. The geochemistry and Nd isotope data for types N-I, N-II and N-III rocks are compatible with petrogenetic models involving variable partial melting of a source similar to that postulated for modern oceanic island basalts. Comparison of the WBG with modern analogues suggests a 3-stage developmental model: stage 1) island-arc volcanism (eruption of type mafic volcancs); stage 2) arc-rifting (continued eruption of type A-I, A-I, eruption of types A-II and A-III mafic volcanics and high-silica, low-K rhyolites); and stage 3) back-arc basin volcanism (continued minor eruption of type A-I basalts, eruption of types N-I, N-II, N-III basalts). Stages 1 and 2 volcanism involved partial melting of subduction contaminated mantle, while stage 3 volcanism utilized depleted-mantle sources not affected by the subducting slab. This model provides a basis for interpreting coeval sequences in central Newfoundland and a comparative framework for some early Paleozoic oceanic volcanic sequences elsewhere in the Appalachian orogen.

Rare-earth data on monzonoritic rocks related to anorthosites and their bearing on the nature of the parental magma of the anorthositic series

Abstract Major and trace elements have been determined in monzonoritic rocks (hypersthene-monzodiorite or jotunite) from two intrusions belonging to the South Rogaland anorthositic complex (Norway). The rare-earth abundance pattern reveals no Eu anomaly, or only a very small one. This fact together with field observations suggest that these rocks represent the parental magma of the anorthositic suite. High Ti and P abundances, low Si content, high Fe/Mg and K 2 O/SiO 2 ratios are characteristics of the major element geochemistry. Absolute amounts of some trace elements abundances vary distinctly between the two intrusions. K/Rb ratios as high as 1700 are observed. Partial fusion of upper mantle kaersutite is proposed as a possible mechanism of magma generation. Partition coefficients between plagioclase phenocrysts and liquid are determined.

Monzonorites from Rogaland (Southwest Norway): a series of rocks coeval but not comagmatic with massif-type anorthosites

Duchesne, J.C., Wilmart, E., Demaiffe, D. and Hertogen, J., 1989. Monzonorites from Rogaland (Southwest Norway ): a series of rocks coeval but not comagmatic with massif-type anorthosites. Precambrian Res., 45:111-128. Monzonorites, members of the anorthositic suite of rocks, are usually considered as residual after the formation of massif-type anorthosites. In Rogaland (S.W. Norway) they occur as large dykes, intrusions and chilled margins to differentiated massifs, emplaced during and soon after the main anorthosite massifs, in granulite facies conditions. Ti, P and Fe are enriched in monzonorites and steadily decrease towards quartz mangerites, the FeO/FeO + MgO ratios varying slightly during alkali enrichment (Bowen trend). Trace element spidergrams show deep troughs in Rb, Th, Nb-Ta, Sr, Zr-Hf and Ti. The REE slightly decrease in the evolution with (La/Yb)S ratios about 9 and neutral to positive Eu anomalies. Several occurrences, however, show highly contrasted trace element features. Sr isotope ratios (/st) show a wide interval of variation (0.704-0.710) between the various dykes and intrusions without any correlation with the elements indicative of crustal contamination. In the Tellnes main dyke, evolution towards acidic rocks occurs without contamination and variation in Isr. Fractional crystallization with subtraction of apatite-bearing noritic cumulates can account for the major and trace element evolution from monzonorite to quartz mangerite, but is unable to explain the large differences between occurrences. It is concluded that monzonorites cannot be comagmatic (though coeval) with massif-type anorthosites. They result from the crystallization of distinct magma batches, possibly formed through partial melting of basic to intermediate rocks in the lower crust.

Magmatic evolution of the Karmøy Ophiolite Complex, SW Norway: relationships between MORB-IAT-boninitic-calc-alkaline and alkaline magmatism

The polyphasal magmatic evolution of the Caledonian Karmøy Ophiolite Complex includes: (1) formation of an axis sequence from island-arc tholeiitic (IAT) and more MORB-like magmas (493+7/-4 Ma); (2) intrusion of magmas of boninitic affinity (485±2 Ma); (3) intrusion of MORB- and IAT-like magmas; (4) intrusion and extrusion of calc-alkaline magmas (470+9/-5 Ma); (5) intrusion and extrusion of basalts with alkaline trace-element affinity. Repeated intrusion of MORB and IAT-like magmas may be explained by intermittent magmatism involving magma-chamber solidification and remelting of a source characterized by initial ɛNd of approximately +6.5. The boninitic rocks may have formed from two LREE-depleted sources: the primary source of the axis-sequence magmas and the residual source left after extraction of these magmas. These sources have been enriched in LREE, Th and Zr from subducted material exhibiting a continental Nd-isotope signature with initial ɛNd less than-8. Covariation between ɛNd and Th, Zr, Nd, Y and Yb may be explained by metasomatic enrichment of a LREE-depleted mantle source by a LREE-enriched subduction component, followed by partial melting during which the degree of melting of the metasomatized mantle source increased linearly with the amount of subduction component added to the mantle source. The calc-alkaline magmas may have formed by remelting of a highly depleted source, which became enriched in some trace elements derived from the source of the subsequent alkaline magmatism. The geology and geochemistry of the Karmøy Ophiolite Complex suggest growth of an island-arc upon newly-formed oceanic crust, followed by arc-splitting and the development of a new basin.