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Earth Sciences Research Journal

Print version ISSN 1794-6190

Earth Sci. Res. J. vol.12 no.1 Bogotá Jan./June 2008

 

MATERIAL ASSIMILATION IN A SHALLOW DIAPIRIC FORCEFUL INTRUSION: EVIDENCE FROM MICROSTRUCTURES AND CSD ANALYSIS IN A PORPHYRITIC INTRUSIVE BODY, "LA LÍNEA" TUNNEL, CENTRAL CORDILLERA, COLOMBIA


Lorena Rayo and Carlos A. Zuluaga
Department of Geological Sciences, Universidad Nacional de Colombia, Edif. Manuel Ancizar, ofic. 301,
Ciudad Universitaria, Bogotá, Colombia

Manuscript received May 02, 2008. Accepted for publication June 10, 2008.


Abstract

The contact between the unit Porphyry Andesite and the Cajamarca Group is observed in the "Túnel de la Linea" section. The integration of petrographic, geochemical and textural (crystal size distribution, CSD) analysis allows description of physical and chemical processes that took place in the contact zone in order to propose a model for the intrusion. Material assimilation produced quartz enrichment towards pluton's boundaries associated to a simple process of melt injection. The difference between host rock and hot melt rheologies caused shear stress that produced crystal breaking, folding and foliation rotation.

Keywords: Cajamarca Complex, Cordillera Central, CSD, Igneous and metamorphic petrology, Andesite- Dacite porphyry, La Línea Tunnel.


Resumen

El contacto entre la unidad Porfido Andesitico y el Complejo Cajamarca es observado en la sección del "Tunel de la Linea". La integración de análisis petrograficos, geoquímicos y texturales (distribución de tamaño de cristales, CSD) permiten la caracterización de los procesos físicos y químicos que se dan en la zona de contacto
y que sirven como base para proponer un modelo de intrusión. La asimilación de material produjo enriquecimiento de cuarzo hacia los limites del pluton y esta asociada a un proceso simple de inyección de fundido. La diferencia de reología entre la roca encajante y el fundido caliente ocasionó cizallamiento que resultó en rompimiento de cristales, plegamiento y rotación de la foliación.

Palabras clave: Complejo Cajamarca, Cordillera Central, CSD, Petrología metamorfica e ignea, Porfido


Introduction

The Colombian Andes is a result of the interaction of several tectonic plates that have interacted since the Paleozoic; because of this, the orogen is an important record of all tectonic processes that have taken place in South America northwest corner from the Paleozoic to the present. The Central Cordillera is one of the most prominent geomorphologic features in the Colombian Andes and its central section consists of a set of metapelitic and metavolcanic rocks of Early Paleozoic age (Restrepo-Pace 1992), association that was intruded by Mesozoic and Cenozoic plutons probably related to subduction of oceanic lithosphere below the Colombian Andes (Aspden & McCourt 1986). In the axial zone of the Central Cordillera, between Calarca and Cajamarca towns, the digging of "La Linea" Tunnel (by "Instituto Nacional de Vias - INVIAS"), in both sides of the cordillera (Fig. 1), provided an excellent opportunity to have access to fresh rocks of the lithologic units present in the area.

This paper presents the study of the emplacement of an igneous body that involves juxtaposition of a hot and viscous liquid inmovement against a cold and stationary solid of a different composition in the section cut by "La Linea" Tunnel. The conjugation of contrasting material properties and relative movement produced characteristic structures and textures related to chemical and mechanical interactions in the contact zone as reported in similar diapiric intrusions (e.g., deflection of regional markers and evidence of stoping; see for example Miller and Patterson, 1999; Tikoff et al., 1999). The study presented here aims to a better understanding of the emplacement process of a small interpreted diapiric forceful intrusion at shallow crustal levels. With this purpose in mind, the use of traditional geochemical and petrographic techniques is complemented with a textural analysis of the porphyritic body to relate nucleation and growing rates with the emplacement process.

Geological setting

The eastern flank of the Central Cordillera, located inside the Central Andean Terrane (Restrepo& Toussaint 1988, Restrepo-Pace 1992), Chibcha Terrane (Toussaint 1993), or Cajamarca Terrane (Etayo-Serna et al., 1983), consists of polymetamorphic, low to medium pressure, metapelitic and metavolcanic rocks of continental and marine origin. The terrane is limited at the east by the Otú-Pericos Fault and at the west by the Romeral Fault System.

The area was first described by Botero (1946), but the work of Nelson 1962) was the first to identify that the area is characterized by igneous bodies mostly in tectonic contact wit metamorphic rocks, both lithologies covered by recent volcanic (Fig. 1). Metamorphic rocks are grouped into a unit known as Cajamarca Complex (Núñez 2001), and are characterized by a sequence of amphibole and graphite schist metamorphosed under the greenschist to amphibolite- epidote facies (Restrepo-Pace 1992). Mayor and trace element geochemistry indicates that the protolith were rocks related to an intraoceanic island arc and a continental margin (Restrepo-Pace 1992). Radiometric dating gives a wide spectrum of ages that range from Paleozoic to Paleogene (315± 15 Ma to 63±2.3 Ma), where the oldest ages could reflect the age of the protolith (Restrepo-Pace 1992), and the youngest may reflect isotopic resetting caused by overprinting of dynamothermal events. The protolith could be even older than the oldest age obtained by radiometric dating according to Silva et al. (2005) who argues a Neoproterozoic – Early Cambrian age based on C and O stable isotope analysis.

The metamorphic association was intruded by Mesozoic-Cenozoic plutons (e.g., Ibagué Batholith, Payandé and Dolores Stocks and minor associated intrusions); these rocks are predominantly of quartz-diorite composition (Nelson 1962, Alvarez 1979). Mojica & Kammer (1995) associated the smaller intrusions to discrete mesozone and epizone plutons intruded during Early and Middle Jurassic and associated with contact metamorphic aureoles, skarn zones, and copper and gold mineralizations. Small, porphyritic bodies are thought to be related to nearby intrusives of batholithic dimensions because most of the small bodies intrude the batholiths (Sillitoe et al., 1982).

Cause of magmatism is also a contentious issue, one view relates magmatism to evolution of a convergent margin where oceanic lithosphere is subducted below the Andes (Alvarez 1979, Aspden & McCourt 1986, Núñez 1986, 2001, Bayona et al. 1994); an alternative explanation is that magmatism is associated with distensive tectonics (rifting) caused by gradual continental separation across a paleorift (Mojica& Kammer 1995).

Structural styles in the region have a typical character of high angle inverse faults; seismic data indicates that these structures feed into a 20 km deep west-dipping décollement (Butler & Schamel 1988). Two of the most prominent faults of the region, the Chapetón-Pericos Fault and the Palestina Fault, separate different deformational styles. Between the two mentioned faults, the style is marked by isoclinals folds in all scales, while west of the Palestina fault and east of a third fault, the Aranzazu-Manizales Fault (La Soledad zone fault), a superimposed S-C fabric characterized the deformation style (Restrepo- Pace 1992). The Romeral Fault system is the main structure near La Linea Tunnel and it is also the main source of earthquakes; however, the 1999 Armenia'searthquake, showed the presence of NNW faults with recent activity (Monsalve &Vargas 2002). Additionally, there are several E-W systems that generate differential horizontal displacements and segment NNW faults (Vargas et al. 2008).

Methodology

Sample rocks were collected by a systematic way along the tunnel depending on the tunnel's walls covering. The contact zone between the metamorphic and the porphyritic body (abscissa K8+081 to K8+108, Fig. 1) was sampled with 2mseparation between samples to allow a detailed characterization of the zone. Beyond the contact zone, samples were taken with a separation of 10 m, 50 m and 200 m approximately. Sampling was accompanied by detailed structural characterization including data collection from joints, foliation, folds, cleavage and veins. A second auxiliary section was sampled in the surface in order to collect more information from the contact zone between the Cajamarca schist and the andesite porphyry. Nearly 200 structural data and 27 rock samples were collected (TPT-1 to TPT-24, QC-1 and QC-2). Thin sections of each one of the samples were obtained and three selected sections were polished for microprobe analysis. The petrographic characterization of the samples consisted of mineral identification, microstructure description and modal analysis (counting of 300 to 400 points). Rock microstructure descriptions include: grade of crystallinity, grain size, grain shape and crystal spatial relations. A FEI QUANTA 2000 scanning electron microscope, hosted at Universidad Nacional de Colombia – Bogotá, was used to obtained point analysis and backscattered electron images (BEI) of polished thin sections coated with a mix of Au-Pd (1:1 anode). Bulk rock chemical analysis were obtained from glass discs with the Universidad Nacional de Colombia – Bogotá MagixPro PW-2440 Philips X ray fluorescence spectrometer, fitted with a Rh tube, maximum power of 4 kW, and calibrated with international standards (MBH and NIST).

Crystal Size Distribution analysis was done for three pluton samples (TPT-16, TPT-20 and QC-2) at different scales, covering an average area of 2x3.5 cm2. Photomicrographs, taken in 6 to 10 fields in each sample, and scanned thin sections (1000 dpi resolution) were processed with a drawing program to trace the maximum length of each crystal. The minimum and maximum number of crystals measured in all sections was 317 and 1678, for a total of 4793 analyzed crystals. The data was then analyzed with the software CSD Correction 1.37 (Higgins 2002).

Porphyry andesite

The body is exposed in an area of approximately 5 km2, has an elongated N-NE shape geometry and its age is uncertain. It was probably originated by Neogene plutonism (see for example Aspden et al., 1987), but could also belong to the porphyry mineralized bodies associated to a Jurassic calc-alkaline suite described by McCourt et al. (1984). The main constituents are plagioclase, amphibole and quartz, with minor biotite, apatite, pyrite, chalcopyrite, sphene and ilmenite and chlorite, sericite, epidote and carbonates as alteration minerals. In the QAPF modal classification of volcanic rocks (Le Maitre et al., 2003) samples from this body felt in the basalt – andesite field (Fig. 2) and the rock is classified as a porphyry hornblende andesite. The quartz content increases towards the pluton´s boundary (most quartz-rich samples are located towards the pluton boundary) suggesting assimilation of material from the country rock.

The rock has prophyritic microstructure, is holocrystalline and contains phenocrysts of plagioclase up to 6 mm in diameter and hornblende from 0.5 mm to 2.5 mm in diameter. Micro-phenocrysts of plagioclase, hornblende and quartz with diameters of less than 0.4 mm are also present. The matrix (24 to 36%) is criptocrystaline; however, microlites of quartz and plagioclase were suspected under the petrographic microscope and confirmed by electron microscopy analysis. In general, crystals are subhedral to euhedral, but in some cases are highly fragmented due to deformation related to the intrusive process (e.g., TPT-21B; Fig. 3). Some samplesshow also microfaulting originated by post-emplacement processes as the microfractures also cut the country rock (Fig. 3).

Euhedral tabular plagioclase is the primary constituent of the rock (40 to 60%). Plagioclase composition ranges from andesine to oligoclase, phenocrysts range in composition from An26 to An43 and microphenocrysts range from An28 to An39 while matrix plagioclase has a composition of An31. Phenocrysts have inclusions of pyrite and amphibole and are partially or totally replaced by sericite (12 to 43%). Epidote and calcite are also present as secondary minerals within plagioclase probably originated by action of hydrothermal fluids. The presence of euhedral Fe-rich epidote associated with plagioclase is restricted to the contact zone (samples TPT-21B and QC-1) suggesting schist partial melting and assimilation of the country rock material in the melt. Olive green brown to green yellow amphibole (13 to 55%) in euhedral, prismatic, rarely twinned crystals is the second most abundant constituent of the rock. It contains inclusions of plagioclase, ilmenite and pyrite and is incipiently zoned. This amphibole is Ca-rich with intermediate to low Si content and relatively high Al, whose compositional classification ranges between pargasitic hornblende to ferrous pargasitic hornblende and to edenitic hornblende and silicic edenite. Most hornblende crystals are altered to chlorite and biotite along cleavage planes and occasionally they are completely replaced. Anhedral quartz is present in less than 10% modal proportion. It usually has rounded edges with reaction and corrosion bays, reaction textures that suggest disequilibrium of this mineral with the melt. Opaque minerals (up to 18% modal proportion) include pyrite, chalcopyrite, rutile, and intergrown titanite – ilmenite. Accessories phases (<3%) include euhedral lath-shaped and locally kinked biotite (0.7 mm), euhedral apatite (0.1-0.2 mm), and euhedral zircon.

Cajamarca Complex (Pzc)

Quartz-biotite-graphite schist: They consist mostly of quartz (34 - 54%), graphite (1 - 23%), biotite (2 -34%), and muscovite (1 - 14%), with minor plagioclase, calcite, actinolite and chlorite (all <10%). Accessory minerals include apatite, monazite, pyrite, chalcopyrite, ilmenite, rutile and titanite. These rocks have schistose microstructure with folded microlithons of plagioclase and biotite-quartz-graphite-muscovite(Fig. 3). The presence of biotite is indicative of the beginning of the Biotite Zone in the greenschist Facies. These rocks probably were originated from an impure psamitic to pelitic protolith consisting of thin interbeded sandstone and quartz claystone, very rich in organicmatter, with some proportion of carbonates. The presence of multiple foliations indicates at least two deformative events.

Mica-quartz schist: They are composed mainly of quartz (45-70%), muscovite (10-25%), biotite (10%), minor plagioclase (7-11%), calcite (6-17%), chlorite (4-13 %), and graphite (3%), and accessory apatite, titanite, pyrite and chalcopyrite. The protolith was a psamitic sequence with quartz sandstone and small proportions of claystones and limestones. The parageneses of quartz-chlorite-muscovite suggests a regional metamorphism in the greenschist facies.

Amphibole-epidote schist: It consists of hornblende (40-60%), epidote (9-67%), plagioclase (2-18%), and minor calcite, chlorite, titanite, zircon and opaques (<10%). They have a schistose microstructure marked by preferred orientation of hornblende and epidote. The parageneses talc-epidote-calcite indicates that these rocks were metamorphosed under regional metamorphism in the greenschist facies.

Quantitative analysis of Crystal Size Distribution (CSD)

Quantitative CSD analysis complements results from petrography and from chemical analysis to reveal the magmatic processes that affect the evolution of the body. This technique is based on textural analysis of rocks and considers the crystal content as a function of size, shape and orientation (Marsh 1998; Higgins 2002). Crystallization is mainly controlled by the rate of heat removal from the system, which results from interactions between kinetics, time and temperature; for example, high temperatures and large diffusion rates favored a few large crystals (Vernón 2004). The size of crystals is primarily the result of heterogeneous nucleation, where material is rapidly and continuous added to a crystal boundary (growing rate) during all stages of crystallization (Marsh 1998). The subsequent states of nucleation not only depend on the cooling rate but also in the process of growth of the nuclei initially formed. Therefore, changes in the rates of nucleation (N) and growth (G) are the result of the interplay of various factors such as temperature diffusion and time and are reflected in the crystal size population. The most important part of the CSD curves is their shape and not the absolute values in the graphs (Marsh 1998). A linear logarithmic CSD is basically originated from an exponential change of nucleation rate over time; thus, changes in the slope reflect changes in the relative rate of nucleation. Under stable conditions, the maximum size of crystals should increase systematically with the increase of crystallinity, so that a curved CSD clearly reflects the addition of natural crystals. For example, if a CSD suggests multiple states, then nucleation can be interpreted as induced by different thermal regimes. CSD is a statistical method and the frequency depends on the size of the crystals; thus, the analyzed samples must be large enough to get a statistically valid analysis, a minimum of 200 crystals must be measured to get a reasonably valid CSD (Mock and Jerram 2005). That is why samples selected for this study have a population of at least 317 measurements obtained at two different scales. Results CSD graphs for two crystalline phases (plagioclasehornblende) show a variable slope and concave shape (Fig. 4). The abnormal changes of the slope are interpreted as measurement errors and fall within the error bars that indicates wide dispersion of the data. The CSD curvature can be explained in two different ways. First, the shape could reflect two nucleation events (ΔN) with a super exponential increase in its final part that explains the higher frequency of small crystals. This increase in nucleation rate could be interpreted as a product of addition of country rock material, in agreement with variations in the slope of the CSD away from the edges of the intrusive, which becomes more linear, and with the interpretation of quartz and mica assimilation supported by rock compositional variations. Second, the shape of the curves could be caused by a growth rate dependent or proportional to size (ΔG) (Eberl et al. 2002), this is in discrepancy with mineral analysis that shows an overlap in the compositional range of phenocrysts, microphenocrysts and matrix crystals indicating that these may have nucleated simultaneously. However, an alternative explanation is that some nuclei may have begun to grow more rapidly than others, the larger crystals have lower surface energy and growth more at expenses of the smaller ones (Ostwald ripening) favoring the emergence of phenocrysts and the greater number of small crystals is favored by selective and concentrated growth of the larger crystals.

Discussion and conclusion

Since relationship between country rock and type of intruding magma is the governing factor for the type of generated contact, the characterization of the host rock is important to determine the effects of the approximating heat source in the pressure and temperature regime. For example, a type of relationship in forceful intrusions develops when magmatic fluids move into fractures opened in the country rock. These fracturing is created or enhanced by a momentum generated by the intrusive itself. In the case here, petrographic characterization and field evidence (Fig. 5) suggest that the contact between the Porphyry andesite and Cajamarca Group is intrusive, this contact was later affected by a deformation event causing a faulted contact in some parts of the intrusive (N to NNW predominant direction).

The suggested forceful intrusion is also characterized by assimilation of quartz in the igneous body and hydrothermal fluid exchange between country rock and the igneous body. The presence of Fe-rich epidote restricted to the contact zone support the interpretation of partial melting of the schist country rock and assimilation of that material into the melt and/or hydrothermal fluid exchange. CSD curves show an increase in nucleation rate that is interpreted here as a product of addition of country rock material, it is possible that the shape of the CSD was influenced by nucleation and growth, however geochemical evidence of country rock assimilation is in agreement with the interpretation of CSD affected by rock assimilation.

Structural data also support the interpretation of a forceful intrusion. The country rock near the contact has a different foliation orientation than the regional trend suggesting rotation of foliation that could be originated by the emplacement of the pluton (Fig. 6). The two lithologies have contrasting mechanical behavior, while the Cajamarca Group schistand quartzite have ductile behavior that is expressed in tight folds developed in two orientations (0/72 and 162/60) the intrusive body is affected by faults and microfaults (fragile deformation, Figs. 3 and 5) that also cut the country metamorphic rock suggesting a post-emplacement fragile deformation event. Additionally, crenulation cleavage that affects the foliation in several directions (Fig. 3e) and quartz, carbonate and sulfurs veins that cut in different directions the foliation suggests that other processes affect the rock after peak metamorphism. We suggest that the magma formed a gradational contact zone in a simple injection process, where fragments of the country rock were trapped into the magma and some of the fragments were not completely melted and formed xenoliths (see Fig. 5). However, a faulted contact between the intrusive and the country rock is observed in the area (Fig. 5), this faulted contact has a trend of N to NNE consistent with the regional trend of the structures such as the Otú Pericos Fault and Romeral Fault System. There is not field evidence in the form of dikes or sills that suggest pervasive invasion of melt, this is because of the characteristics of the country rock (quartz-rich schist) that acted as an impermeable unit.

Acknowledgments

This work had financial support from the Universidad Nacional de Colombia (project HERMES number 6214). The authors thank the Instituto Nacional de Vías (INVIAS) for their logistical support during field work within the "La Linea" tunnel. This project was initiated by the student group GEOYMCO. The authors thank Carlos Vargas and an anonymous reviewer for his comments on an earlier version of the manuscript.

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