Water-assisted production of late-orogenic trondhjemites at magmatic and subsolidus conditions

Abstract

Peraluminous granites and trondhjemites make up small plutonic bodies intruded into high-grade paragneisses in the Peloritani Mountains, marking the beginning of late Variscan granitoid magmatism in southernmost Italy. The granites range from low-Ca monzogranites to alkali feldspar granites, while the trondhjemites vary from trondhjemites s.s. to low-Ca trondhjemites. Relatively high radiogenic (87Sr/86Sr)i ratios (mostly from 0.7073 to 0.7125) and negative εNd values (mostly from −5.66 to −8.73) point to crustal sources for all the granitoids. Major and trace element compositions indicate an absence of genetic relationships between the trondhjemites s.s. and the low-Ca granitoids, but possible relationships between the low-Ca trondhjemites and the granites. All of the studied granitoids have near-pure melts compositions, consistent with H2O-fluxed and dehydration melting of metasediments for the trondhjemites and the granites, respectively. However, the unusual compositions of the low-Ca trondhjemites and microstructural evidence in these rocks for pervasive subsolidus replacement of magmatic feldspars by secondary sodic plagioclase indicate that they were derived instead from metasomatic alteration of the granites. Thus, water may be involved in the production of trondhjemites in two different ways, driving water-fluxed melting in the magma source and driving alkali metasomatism at the sites of granite emplacement in the upper crust.

Metasedimentary Metatexites with Trondhjemitic Leucosomes from NE Sicily: Another Example of Prograde Water-fluxed Melting in Collisional Belts

“…The Variscan orogeny was responsible for the formation of most of the CPO basement rocks and was marked by voluminous post-collisional granitoid magmatism. The late Variscan magmatism gave rise first to small, weakly peraluminous trondhjemite plutons, emplaced in the southernmost CPO (Aspromonte Massif and Peloritani Mountains) at c. 314 Ma [15,19,20], then to strongly peraluminous leucogranodiorite-leucogranite plutons scattered throughout the CPO, at c. 304-300 Ma [20,29,30] and, finally, to the composite Serre and Sila batholiths in central and northern Calabria, at c. 297-292 Ma ( [21,[31][32][33][34] and references therein). Granitoid magmatism was followed by late-to post-orogenic rhyolitic to andesitic subvolcanic magmatism [35,36], later evolving to the early breakup of Pangea, as documented by the production of sodic-alkaline to tholeiitic Triassic basalts in northern Calabria [37] as well as in central-western Sicily [38,39].…”

“…The late Variscan granitoids of the Aspromonte Unit occur as isolated plutons of a few km 2 in size, typically emplaced into upper amphibolite facies rocks. They range from weakly peraluminous trondhjemites, produced by water-fluxed melting of metagreywackes, to strongly peraluminous leucogranodiorites-leucogranites derived from fluid-absent melting of similar, but more pelitic paragneiss sources [15,19,20,30,54,55]. P-T estimates for the Variscan metamorphism in the Aspromonte Unit are in the range of ~0.4 GPa at ~550-680 °C, according to [56][57][58].…”

“…However, before trondhjemitic rocks are used to draw any possible conclusions, it is necessary to verify that their typical mineral assemblages and geochemical features genuinely reflect crystallization from an original trondhjemitic magma. In fact, in late-to post-orogenic settings, trondhjemitic compositions can be achieved also by the loss of evolved, K 2 O-rich, granitic melt after the accumulation of early-crystallized plagioclase and quartz [8][9][10][11][12], as well as by the subsolidus replacement of magmatic K-feldspar by a secondary plagioclase during infiltration metasomatism of original granites (e.g., [13][14][15][16][17]).…”

“…This study aimed to investigate in more detail the origin of the above trondhjemitic leucosomes, taking into account a more complete geochemical dataset including the major, trace and rare earth elements of both leucosomes, mesosomes and melanosomes, in a significantly updated geo-petrological background. In fact, subsequent studies have provided a more comprehensive geochronological framework for the timing of anatexis and granitoid magmatism in the southernmost Calabria-Peloritani Orogen (e.g., [20][21][22][23]) and evidence for the generation of metasomatic trondhjemites in the same region of the studied leucosomes [15,19]. Furthermore, the present study is framed in the context of the recently revitalized debate on the relevance and role of water-fluxed melting in the evolution of the continental crust (e.g., [24][25][26][27]), aiming to provide a constructive contribution to this major subject.…”

Poly-Orogenic Melting of Metasedimentary Crust From a Granite Geochemistry and Inherited Zircon Perspective (Southern Calabria-Peloritani Orogen, Italy)

“…After the thickening-related baric peak, scattered small late Carboniferous (∼314-300 Ma) early post-collisional plutons of weakly to strongly peraluminous leucotonalite to leucogranite were emplaced (Rottura et al, 1993;Graessner et al, 2000;Fiannacca et al, 2005Fiannacca et al, , 2008Fiannacca et al, , 2019. These were followed by batholith-scale magmatism at 300-290 Ma (Rottura et al, 1990;Del Moro et al, 2000b;Langone et al, 2014;Fiannacca et al, 2015Fiannacca et al, , 2017 and finally by late-to post-orogenic rhyolitic to andesitic dykes (Festa et al, 2010;Romano et al, 2011Romano et al, , 2012, heralding the early breakup of Pangea (Barca et al, 2010;Cirrincione et al, 2014Cirrincione et al, , 2016.…”