Emplacement of the Las Chacritas pluton (Sierras de Humaya, NW Argentina): multiple material transfer processes at mid-crustal level

LARROVERE, MARIANO A.; ALASINO, PABLO H.; DE LOS HOYOS, CAMILO R.; WILLNER, ARNE P.

General Roca

Simposio; XVI Reunión de Tectónica; 2015

Universidad Nacional de Río Negro

The Las Chacritas pluton (LCP) is an elongated, nearly elliptical body in map-view exposed in the northern region of the Sierra de Humaya, which intruded into the medium grade metamorphic rocks of the El Portezuelo Metamorphic?Igneous Complex (EPMIC). The pluton is about 4 x 1.5 km, with a long axis trending NNW. The main plutonic unit is a light gray, equigranular, medium-grained two-mica granodiorite to tonalite. NNW-striking pluton wall contacts dip 25°-66° to the west, except for the NE part of the pluton where they dip to the east. On the margin of the pluton, a subordinate igneous unit is formed by leucocratic equigranular medium- to coarse-grained muscovite-bearing syenogranite to granodiorite. This unit occurs as concordant meter-scale sheets within the host rock, or as parallel to sub-parallel meter-scale border zones in sharp and straight internal contact with the main igneous unit of the pluton. The host rocks are two-mica schists with a metamorphic foliation (S2) defined by a differentiated layering that strikes NNW-SSE. At map scale, the LCP boundary is generally concordant with S2, which roughly drapes around the pluton, although in some parts the host rock fabric in map view is truncated by the pluton. Based on downward pluton projections, the relationships between pluton and S2 are supposed to crusscut. Two structural domains were documented: (1) the ?LCP domain? restricted to the aureole of the host rock of the pluton and (2) the ?regional domain? which includes the northern region of the Sierra de Humaya, outside the LCP domain. In the LCP domain, one fold set was mapped wherein the NNW-striking foliations S2 dip toward the ENE and WSW, suggesting hectometre to kilometre scale local folding of S2. Fold axis trends lie nearly parallel to the strikes of S2. It is important to note that W-dipping and folding of S2 in the northern region of Sierra de Humaya are restricted to zones near the LCP (i.e. the LCP domain). In contrast, the regional domain shows a more uniform E-dipping S2 with an average strike and dip of 0°, 36°E, respectively. The average N-S strike of S2 in the regional domain slightly differs from the average NNW-SSE strike in the LCP domain. The contacts between the LCP and the wall rock are intrusive, i.e. injected-type. They grade from undisturbed host rock passing through a zone defined by an increasing number of concordant sheets to the plutonic rock containing host rock xenoliths, i.e. a sheeted border zone. Locally, discordant contacts are observed. In the central-eastern part of the border zone some concordant sheets show the same deflection as the folded host rock foliation. Some irregular branch-like discordant granitic dykes cross-cut S2 through fractures and generate local meter-scale gentle folds. Symmetric lens-shaped apparent boudins of leucocratic composition are developed in the host at high angles with S2 and the pluton-wall rock contact. Xenoliths of wall rock occur within the pluton, mainly near the contacts. Larger xenoliths (tens of meters) were recognized. Some rotated wall rock xenoliths with angular shapes are present in the leucocratic sheets and within the pluton near the contact. Disaggregation of the metasedimentary rocks is observed in the wall rock and xenoliths.Based on field relationships between metamorphic rocks of the Sierra de Humaya and barometric calculations on migmatites of the EPMIC from nearby areas, a rough estimation of the emplacement level for the LCP is deduced. Structural continuity of the metamorphic foliation from two-mica schists into migmatites allows us to estimate depth levels. The highest calculated regional pressures of ~ 5 kbar in migmatites of the region (Larrovere et al. 2011) are consistent with metamorphism at about ~ 19 km of depth. Assuming this depth level for migmatites of the Sierra de Humaya, lower pressures (depth) would correspond for the two-mica schist levels. Supposing similar vertical pressure gradients, the measured crustal width between two points with parallel metamorphic foliation in migmatites and schists is 4.5 km. This means the current exposure of the LCP represents a maximum emplacement depth of ~ 14.5 km (~ 3.8 kbar) consistent with the upper part of the middle crust. Brittle and ductile deformation mechanisms in the wall rock were active during the pluton emplacement, as evidenced by field observations. Localization of late magmatic intrusions (i.e. leucocratic sheets) and xenoliths on the pluton-host rock contact suggests that these sectors are the lateral wall zones of the LCP. Intrusions associated with structures formed in the aureole (i.e. folded sheets) suggest that ductile deformation in the aureole formed prior to final solidification of the chamber (e.g. Paterson y Farris 2008).Although magmatic fabrics provide information about emplacement in field-based studies, it is believed that the main clues for Material Transfer Processes (MTPs) come from examining the host rock around the plutons, i.e., the structural aureoles (Buddington 1959; Paterson y Fowler 1993). Documented changes in the attitude of host rock markers around the LCP (but not recorded from regional orientations) indicate a strong relationship between host rock ductile deformation and pluton emplacement. Examples include (1) local folding of the host rock layering, (2) draping of S2 around the pluton, and (3) minor strike deflection of S2 (NNW-SSE in LCP structural domain and N-S in regional domain). The most obvious evidence of the emplacement of the LCP is the ductile deformation observed in the structural aureole, i.e. rim folds. The lateral and upward deflections of the beds, synclines/anticlines associations, major and minor folds with sub-horizontal fold axes trending parallel to the general pluton-host rock contact, and sub-vertical fold axial planes in the structural aureole indicate that host rocks were displaced laterally with respect to the pluton. These structural features are consistent with a lateral shortening mechanism as a result of magma emplacement. Lateral transport around rising/expanding magma chambers requires compensating vertical transport, and ductile downward host rock transport relative to pluton roofs is a common process during the rise of magma (Paterson y Farris 2008). Therefore, at levels below the present exposure, downward ductile flow along pluton walls could have compensated the vertical host rock displacements during emplacement of the LCP. Because it has been widely argued that magma emplacement in the crust involves multiple MTPs (Buddington 1959; Paterson y Fowler 1993; Miller et al. 2009), even for a single pluton, we cannot exclude that additional emplacement mechanisms may have contributed to create sufficient room for the emplacement of the LCP. In order to evaluate the amount of displaced host rock material by lateral shortening, longitudinal strain has been estimated for the central contact aureole. The extension (e) values are -0.43 (43%) on the east side and -0.22 (22%) on the west side. Thus, total shortening by folding in the structural aureole is 65%. This total aureole shortening accommodates ~ 40% of the width of the pluton, evidencing that additional MTPs are necessary to generate the remaining amount of room for the emplacement of the pluton.Preserved wall-rock xenoliths in the contact zone, partial local truncations of S2 and fold traces, and inferred cutting relationships between pluton and folds from upward pluton projections, are consistent with magmatic stoping. Because aureole ductile strains are insufficient (~ 40%) to accommodate the emplaced magma, we suggest that stoping could be a complementary mechanism to create the necessary space. Most of the missing host rock material from the space now occupied by part of the pluton must have been transported downward to levels below the present exposure (e.g. Paterson y Farris 2008; Miller et al. 2009). Whole-rock geochemistry supports assimilation of crustal material during magma evolution (Larrovere et al. 2015). Although P-T conditions at the structural level where the LCP was emplaced are compatible with dominant ductile deformation mechanisms, it has been recognized that magmatic stoping may occur at all crustal levels (Miller et al. 2009). This assumption agrees with the observed structures that support concomitant fracturing and ductile deformation. Even if field observations suggest that stoping was a relatively late process with respect to aureole ductile deformation, we cannot rule out the idea that stoping has operated from early stages of the magma intrusion.