Cld-St-And-Bearing Assemblages in the Central Southalpine Basement: Markers of an Evolving Thermal Regime during Variscan Convergence

Nicola Pigazzini,Valeria Diella,Marco Filippi,Jean-Marc Lardeaux,Maria Iole Spalla,Davide Zanoni

doi:10.3390/min11101124

Abstract

Multiscale structural analysis is carried out to explore the sequence of superposed pre-Alpine chloritoid–staurolite–andalusite metamorphic assemblages in the polydeformed Variscan basement of the upper Val Camonica, in the central Southalpine domain. The dominant fabric in the upper Val Camonica basement is the late-Variscan S2 foliation, marked by greenschist facies minerals and truncated by the base of Permian siliciclastic sequences. The intersection with the sedimentary strata defines a Permian age limit on the pre-Alpine tectono–metamorphic evolution and exhumation of the Variscan basement. The detailed structural survey revealed that the older S1 foliation was locally preserved in low-strained domains. S1 is a composite fabric resulting from combining S1a and S1b: in the metapelites, S1a was supported by chloritoid, garnet, and biotite and developed before S1b, which was marked by staurolite, garnet, and biotite. S1a and S1b developed at intermediate pressure amphibolite facies conditions during the Variscan convergence, S1a at T = 520–550 °C and P ≃ 0.8 GPa, S1b at T = 550–650 °C and P = 0.4–0.7 GPa. The special feature of the upper Val Camonica metapelites is andalusite, which formed between the late D1b and early D2 tectonic events. Andalusite developed at T = 520–580 °C and P = 0.2–0.4 GPa in pre-Permian times, after the peak of the Variscan collision and before the exhumation of the Variscan basement and the subsequent deposition of the Permian covers. It follows that the upper Val Camonica andalusite has a different age and tectonic significance as compared to that of other pre-Alpine andalusite occurrences in the Alps, where andalusite mostly developed during exhumation of high-temperature basement rocks in Permian–Triassic times.

Highlights

Andalusite is an indicator of high thermal regimes, such as those characterizing contact metamorphism or regional Buchan/Abukuma-type metamorphism in the metapelites of the continental crust [1,2]
High T/P ratios accountable for andalusite development are envisaged in various convergent and divergent settings, including late-orogenic thinning and lithospheric delamination in the mature stages of continental collision, as well as lithospheric thinning announcing continental rifting in post-orogenic settings [3,4]
Andalusite is quite common in the pre-Alpine continental crust of the Alps, even if the age of the andalusite-bearing assemblages and the geological context in which they formed are various (Figure 1, Table 1)

Summary

Introduction

Andalusite is an indicator of high thermal regimes, such as those characterizing contact metamorphism or regional Buchan/Abukuma-type metamorphism in the metapelites of the continental crust [1,2]. Andalusite is quite common in the pre-Alpine continental crust of the Alps, even if the age of the andalusite-bearing assemblages and the geological context in which they formed are various (Figure 1, Table 1). Andalusite-bearing rocks are associated with lateVariscan aplites, pegmatites, granitoids, and cordierite-bearing migmatites in the Helvetic. Pegmatites, granitoids, and cordierite-bearing migmatites in th2eoHf 2e0lvetic. AInlutshietePednenvienliocpBmriaennçtoanre envbliasBaagggaeesesdedmtDienieisnfnttfheittf,rehyeaeAinnntuAdggsauteltruosootslaoriPltogpeeaiicinrlsapmeliinaaniasnnetnddea–aScndTohdrurrioetaShnlsaoaostluelipocdtighnltiaeioctladhpalooisPnmesepteratmihidnnegiosar,sminwcfoathhtrihenearirsennm,dnawaanilnldhurgaeisslriuetaeesr[i1aedte2ne-d,vd1bee3easl]alo.cruprisnimbigteeeand-sbtseateomraegrbieenlntahgvgeeiasrs-sswemitholdteerstiPfyerinmgiaton–PCeramrbiaonn–iTferriaosussicalnitdhoasluphsietreicinthtihnneincognatraectdezsocnriebeodf ltaotgee-tVhearriwsciathn oinldterur sives [14P–e4r9m].iaInn–tChaerbSoonuitfehraolupsinaenddaolumsiatieni,natnhde acolunstaitcet zmoonsetolyf loatcec-uVrasriisncahniignhtr-utesmivepse[r1a4t–u4r9e].basemeInnt trhoeckSsotuhthatalipnicnleuddeomsialliinm, aannidtea,lucsoirtedimeroitsetl,yanodcc/uorrsspininheilgthh-taetmrepfelreacttuPreerbmasiaemn–eTnrtiassic. Bucrohcakns/Athbaut kinucmluad-etyspilelimeantaitme,ocroprhdiisemrit.eI,natnhde/uopr pseprinVeal lthCaatmreoflneicta,Paenrmdaialun–siTtreiaissiincstead fouBnudchinanc/hAlobruitkouimd–as-ttayuperomliteeta–mgaorrnpehti-sbme.arIinngthme eutpappeerliVteasl,Cwahmicohniscua,gagnedstalhuisgitheeirsPin/T- conditisotenasdafsoucnhdarianccthelroisrittiocido–fsBtaaurrroolvitiea–ng/aDrnaeltr-abdeaiarinngmmeteatampoelriptehsi,swmhi[c1h,2s]u.gTgheisst hcaigsheesrtPu/dTy rmepectmnrhoeteunsnsdeetrxineethitxposuhrneamussnmeaansaettitxcsoihocanneanrlpplaeexacanttctehethr,l,oilfserpftnoirpctmoooomprtfhtpBueotahnrVrteirautorynVvisiittcaaoyarnntii/osmccDioampannlrpvroarecovrdogveinaeeonnvocueumerrrgteuuoteannntmdhdceeeoerPrrsptsteoatrhanmintsdhimdianenig[n–1PTog,2erfi]orta.hmfsTestihihVacianeslri–ctVihsaTcoasraesrinpiassshbtcuseaaidrscnieyc-lbiathsoe-sphtherinicnitnhgin.

Geological Setting

Discussion and Conclusions