Using zircon petrochronology to assess the evolution of the early Andean Cordillera in northern Chile and its relationship with IOA and IOCG metallogenesis

Abstract

early Andes of northern Chile represents the initial development stage of the Southern Andes, one of the most remarkablecontinental arcs in the world [1]. Here we use a multi-proxy geochemical approach to determine the participations of mantle-crust-slabsedimentsto the source of plutonic complexes to better understand the evolution of the early Andean magmatism [2] and itsrelationship with the metallogenesis of Andean IOA and IOCG deposits.TECTONO-MAGMATIC EVOLUTION OF THE EARLY ANDESOur results show that plutonism occurred over a period of ~120 My with six distinct episodes. The first and second ones between 215–203 and 200–185 Ma, when continental crust underwent continuous thinning from ~30 to ~22 km. Minor sediments and crustalcontributions are also noted. Consequently, magmas became increasingly depleted, displaying a signature similar to N-MORB andrelatively low oxidation states. This suggests that plutonism occurred at an attenuated margin [3] in a transtensional regime withincreasing plate decoupling, consistent with structural observations in the forearc [4] and an isotopic ‘pull-up’ of its igneous units [5].This period ended with an arc-normal extension with La Negra volcanic event (~180–155 Ma) [4].During the third episode (160–145 Ma) crustal thickness reached ~20 km and crystallization temperatures were ~850 °C, suggesting amagmatism led by decompressional melting [6]. This is reflected in a highly depleted and reduced signature with whole-rock and zirconratios consistently near or below N-MORB references. This is consistent with: (i) strong arc-normal extension and shallow plutonicemplacement [4,7]; and (ii) negative to neutral, trench normal absolute and relative convergence rates [8].The fourth and fifth episodes occurred between 138–121 and 120–108 Ma, respectively. During them crustal thickness reached ~35km. Slightly lower crystallization temperatures and higher fluid and sediment contributions are also recorded. Magmas became moreenriched, hydrated and oxidized, which could be explained by: (i) transition to an oblique transtensional regime [4,7,9]; and (ii) increasedcoupling between plates associated with higher convergence rates [4,8].The last episode occurred in late Albian to Cenomanian (103–94 Ma). A crustal thickness of >35 km and increasing slab-derived fluidsand lesser sediments contributions, as well as a depleted mantle source, are best explained by an extensional to transtensional settingwith high convergence rates and relative decoupling of plates [4]. This is also supported by relatively high oxygen fugacity markers andlow crystallization temperatures. These findings are concordant with the geological record in the back-arc [10] and changes inconvergence rates [8].IMPLICATIONS FOR THE METALLOGENESIS OF ANDEAN IOA/IOCG DEPOSITSThe early Andes of northern Chile hosts some relevant IOA and IOCG deposits [11]. However, its plutonic complexes are representativeof the generality of intrusive units in most continental arcs in rather thinned crusts [12]. Therefore, it is hard to correlate the genesis ofthese mineral systems with specific magmatic conditions. Nevertheless, our petrogenetic indicators give us some insights about therole that tectono-magmatic changes and magmatic conditions could have played in the formation of andean IOA and IOCG deposits.Most andean IOA deposits were formed between 132 and 128 Ma [11], concurrent or after the transition from extensional totranstensional settings [4,7,9,11]. This suggests that magmatism not only contributed to their formation as a heat source, but probablyas the main source of the mineralizing fluids and metals in these systems. All the elements needed to generate IOA and IOCG depositsin the basin model [13] were in place before this tectonic change [7,9], but no significant ones were formed during that time span. Prior,magmatism was depleted, reduced and dehydrated [5,6,14], does not have the proper conditions to mobilize siderophile and chalcophileelements characteristic of IOA and IOCG deposits.In contrast, the tectonic shift brought slight but growing sediment and fluid additions to the magmatic factory, generating moreenriched, hydrated and oxidized magmas during the fourth plutonic event. Though, not sufficient to scavenge key elements in IOCGdeposits but enough to trigger the genesis of IOA systems. Consequently, a few IOCG deposits were generated between 138 and 120Ma, being the most relevant ones formed after 120 Ma [11]. The continuous addition of sediments and slab-derived fluids during morethan 15 My produced more enriched, hydrated and oxidized magmas able to incorporate siderophile and chalcophile elements sincevolatile ligands were increasingly available [11]. This may have help to enhance the formation of IOCG deposits between 120 and 110MaKeywords: zircon petrochronology, early Andes, metallogenesis, IOA and IOCG deposits.