Unconformities as mineralogical breaks in the burial metamorphism of the Andes

Contrib. Mineral. Petrol. 66, 361 366 (1978) Contributions to Mineralogy and Petrology �9 by Springer-Verlag 1978

Unconformities as Mineralogical Breaks in the Burial Metamorphism of the Andes

Luis Aguirre i, Beatriz Levi 2, and Robin Offler 3

1 Department of Geology, University of Liverpool, Liverpool, Great Britain 2 Geologiska Institutionen, Stockholms Universitet, Stockholm, Sweden 3 Geology Department, University of Newcastle, N.S.W., Australia

Abstract. Stratigraphical-structural units separated by regional unconformities in the Andes of Peru and Chile, display a pattern of low grade burial metamorphism. Each stratigraphical-structural unit shows a particular facies series covering part or all the range between the zeolite and the greenschist facies. These facies series were episodically generated as part of the geological evolution of each unit prior to its own folding. Mineralogical breaks are found to coincide with the regional unconformities and often cases of higher grade assemblages on top of lower grade ones occur. This pattern may be explained by a process o f " sealing" of each unit after its particular metamorphic episode took place. Porosity and permeability conditioning Pf, as demonstrated for individual lava flows, are the significant controlling factors in the production of the metamorphic assemblages.

Introduction

The purpose of this paper is to show that a pattern of burial metamorphism present in the Andes of Cen- tral Chile which is characterized by the coincidence of mineralogical and structural breaks marked by unconformities (Levi, 1970) is also present in Central Peru (Offler et al. in prep.). Furthermore, the model of episodic metamorphic events postulated by Levi (1970) for the Chilean rocks is thought to be an ade- quate explanation for the Peruvian segment. The combined analysis of these areas enhances the model concerning the role played by the different variables conditioning this type of metamorphic processes. Moreover, it shows that burial metamorphism took place in each of the complex geotectonic cycles which repeatedly took place between orogenic phases in this segment of the Andean Cordillera.

The Mesozoic-Cenozoic Andean belt in Central Southern Peru and North Central Chile is of ensialic nature with simple and mainly non-compressional structures and huge volumes of effusives and granitoids. There is no regional deformational metamorphism nor any ultrabasic rocks. This Andean segment is approximately 3000 km long and 200 km wide and trends parallel to the Pacific coast line. The main structural feature in this segment is a north to north- west trending synclinorium. The materials which formed this belt are sediments and volcanics, partly shallow marine and partly continental, and were de- posited in elongate, narrow overlapping troughs, during the Mesozoic and Cenozoic. The volcanics are subalkaline and are partially transformed to spilitic- keratophyric rocks by low-grade burial metamorphism. Their extrusion was closely associated in time and space with emplacement of high level granitoids through a caldera-cauldron mechanism (Aguirre etal. , 1974; Aubouin etal . , 1973; Cobbing and Pitcher, 1972; Cobbing et al., in prep.; Levi, 1970; M6gard, 1973; Myers, 1974, 1975; Offler et al., in prep., Pitcher, 1978).

Axes of depositional basins and folding structures and the loci of volcanic and plutonic centres have been amazingly parallel in trend troughout the whole evolution of this narrow Andean belt.

Excellent exposures in a high-relief, dissected land- scape, and good stratigraphic control, permit reliable three-dimensional observations.

Stratigraphical-Structural Units and Unconformities

As a result of mapping, several stratigraphical-structural units separated by unconformities have been found in Peru (Myers, 1974; Webb, 1976; Cobbing et al. in prep.) and in Chile (Aguirre et al., 1974; Aubouin et al., 1973; Levi, 1970; Ruiz et al., 1965). Each such unit records a history of a continuous process where vulcani- city, sedimentation, subsidence, folding, granitoid intrusion and

0010-7999/78/0066/0361/$01.20

362 L. Aguirre et al. : Mineralogical Breaks in the Burial Metamorphism of the Andes

uplift closely follow each other or overlap in time (Aguirre et al., 1974; Charrier, 1973; Cobbing etal., in prep.; Pitcher, 1978). Lateral shifting of the depositional troughs and volcanic feeders and small scale vertical oscillations recorded as disconformities, occurred in each unit (Howell and Molloy, 1960; Levi, I973; Myers, 1974; Vergara and Munizaga, 1974; Webb, 1976 ; Zentilli, 1974).

The unconformities are expressions of orogenic phases (folding and/or uplifting) which episodically closed the history of each stratigraphical-structurai unit. These orogenic phases and their corre- sponding unconformities are regional, although not always coeval. They can be traced for long distances in Chile and some of them may be correlated over thousand of kilometres to the north with equivalent folding events in Peru, e.g. the Subhercynian (middle Cretaceous) orogeny from Central Chile to Central-Coastal Peru (Aguirre, 1976).

The deposition of a younger stratigraphical-structural unit on top of the unconformity was preceded by erosion of the older underlying unit; the inferred eroded thicknesses varying from 1500 to more than 4000 m after the Subhercynian phase in Central Chile (Aguirre and Thomas, 1964; Levi, 1968, p. 66). The new deposition and magmatism took place in a new trough overlapping the area occupied by the preceding unit (Clark et al., 1976; Cobbing and Garayar, 1973; Farrar et al., 1970; Levi, 1973; Levi and Corva- 15_n, 1968; Myers, 1974; Vergara, 1969). Measured thicknesses of each unit are generally of the order of several thousand metres. However, these "thicknesses"-and still less the total added value reaching up to 35 km in Chile- do not represent significant figures. Vertical accumulation at any place is always less than the one obtained by simple addition because of the lateral shifting of troughs and the volcanic loci (Levi and Corvalfin, 1968; Clark et al., 1976).

Burial Metamorphic Facies Series Within Each Stratigraphical-Structural Unit

Detailed studies in Central Chile (Levi, 1968, 1969, 1970) and in Central Peru (Offler et al., in prep.), mainly on basic and inter- mediate volcanic rocks, have shown that each stratigraphical-structural unit displays a regional low-grade metamorphic facies series covering part or all the range from zeolite to greenschist facies. This pattern is quite constant through outcrop distances of thousand of kilometres in correlatable units along the trend of the belt. The metamorphic grade increases with stratigraphic depth, the regional facies boundaries being parallel to subparallel to bed- ding and not to the contact of the later granitoids. The metamorphism is non-deformative: primary fabrics are completely preserved and schistosity generally is absent. Metamorphic minerals occur in volcanic and sedimentary rocks as cement, veinlets, amygdules and replacing crystals, crystal fragments and ground- mass. Replacement is commonly partial and is positively correlated with original porosity and/or permeability. Metamorphic facies as used in the Chilean and Peruvian studies have been defined on the most transformed, amygdaloidal part of each sampled flow.

The above described characteristics of the metamorphic facies series found in each stratigraphical-structural unit conform to Coomb's definition of burial metamorphism (Coombs, 1961, p. 214).

Unconformities and Mineralogical Breaks

Breaks in the mineralogical assemblages of the metamorphic facies series are always found in this Andean

segment when crossing unconformities separating stratigraphical-structural units (Fig. 1). Thus, structural unconformities are also mineralogical unconformities. Moreover, higher-grade metamorphic assemblages at the basal part of a younger stratigraphical-structural unit overlie lower-grade assemblages at the top of the underlying older unit in several cases (Levi, 1968, 1969, 1970; Offler et al., in prep.). Examples based on rocks of similar basic composition show that: (a) at the younger unconformable boundary of the Subhercynian phase in Peru (Fig. 1 B) albite-chlorite-calcite-quartz-actinolite-(biotite) bearing rocks overlie chlorite-epidote-calcite-pumpellyite- chalcedony-mixed layer clay or albite-chlorite-epidote- calcite-white mica-quartz-wairakite-prehnite-pumpellyite-mixed layer clay bearing rocks; (b) at the older unconformably boundary of the Subhercynian phase in Peru (Fig. 1B) albite-actinolite-mixed layer clay- biotite bearing rocks overlie albite-epidote-chlorite- calcite-white mica-quar tz-zeolite-prehnite-pumpellyite-mixed layer clay bearing rocks ; (c) at some places along the unconformable boundary of the Sub- hercynian phase in Chile (Fig. 1A) albite-epidote- actinolite-chlorite-calcite-sphene-quartz bearing rocks overlie albite-pumpellyite-prehnite-calcite-chlorite- laumontite bearing rocks; (d) at the unconformable boundary of the Araucanian phase in Chile (Fig. 1 A) albite-actinolite-quartz-chlorite-calcite-epidote- sphene bearing rocks overlie albite-chlorite-epidote- calcite-prehnite-pumpellyite-quartz bearing rocks.

Burial metamorphic assemblages characterized by facies series increasing in grade with stratigraphic depth have been described from many parts of the world. However, to the authors ' knowledge, the existence of metamorphic mineralogical breaks linked to unconformities has only been reported f rom the Andes of Chile and Peru and from the New Zealand geosyncline (Landis and Coombs, 1967).

Mineralogical Breaks and Inferred Metamorphic Episodes

Repetition of metamorphic facies series (Fig. 1) is consistent with a history of several episodes of burial metamorphism each of which took place before the folding phase of each stratigraphical-structural unit. This hypothesis has already been proposed for the Central Andes of Chile (Levi, 1970). Burial metamorphic episodes separated by local unconformities have been also described by Landis and Coombs (1967) for the New Zealand geosyncline. The existence of these episodes in New Zealand is illustrated by the presence of pebbles and sand grains derived from earlier members that had suffered metamorphism be-

L. Aguirre et al. : Mineralogical Breaks in the Burial Metamorphism of the Andes 363

~ Zeolite facies

~ Zeolite ~ Prehnite - Pumpellyite ( transitional facies)

~ Prehnite- Pumpellyite facies

~ Prehnite - Pumpellyite ~ Greenschist ( transitional facies )

r ~ Greenschist facies

S J .

\

Laramian phase

- \ -Subhercynian phase

j

, ,r e 2

phase Lara ia Araucanlan phase o~s \ / ~-$ubhercynian phase

Fig. 1. Stratigraphical-structural units, metamorphic facies series and unconformities in the Andes. (A) Central Chile, (B) Central Peru

fore their incorporation in a host sediment of lower metamorphic grade. A similar example has been described by Levi (1969) in conglomerates of upper Cre- taceous age directly overlying the unconformity marking the Subhercynian folding phase (Fig. 1 A) in Cen- tral Chile. In the Tanzawa Mountains, Japan (Seki et al., 1969), sedimentary beds metamorphosed under zeolite facies conditions contain clasts of higher grade metamorphic rocks. However, isograds separating mineralogical zones are not parallel here to the stratigraphic horizons and no coincidence of mineralogical and structural breaks exists. The Tanzawa Moun- tains metamorphism as described by Seki et al. is part of a continuous tectonic history thus differing with the episodical hypothesis inferred for the Andes.

To explain the Andean mineralogical breaks, in- cluding those cases of higher grade assemblages overlying lower grade ones we invoke a process of "sealing" produced in each stratigraphical-structnral unit when its own burial metamorphism took place. The "sealing" is considered to be the result of a drastic decrease in porosity and/or permeability caused by the generation of new minerals which completely filled open spaces. As the existence of open voids seems to be a sine qua non condition for the generation of new mineral phases in a non-deformative, low- grade metamorphic environment (Browne and Ellis,

1970; Levi, 1969; Otfilora, 1964; Steiner, 1953; Zen and Thompson, 1974) the metamorphic episode which took place later in the immediately partially overlying stratigraphical-structural unit would not have met the conditions necessary to affect the already metamorphosed underlying unit (see section on conditioning parametres). Still older, sealed units, were at the time further away from the realm in which burial metamorphism was taking place, so a prograde, new metamorphic episode, never took place. Subsequent metamorphic episodes could have influenced pre-existing metamorphosed units only if they were accompanied by penetrative deformation. This would have created channelways and thus allowed the influx of fluids which would have helped to accelerate metamorphic reactions. It appears significant in this respect that greenschist-to amphibolite-facies assemblages have been developed only locally in narrow and intensely deformed belts associated with deep faulting of regional extension (Myers, 1974).

This model of burial metamorphism has been cri- ticized by Zen (1974) and by Zen and Thompson (1974) who argue that metamorphic mineral phases such as zeolites should be susceptible to transforma- tion into new phases under the load pressure generated by the total accumulated pile. This criticism is based however on the assumption that "the entire


section was at one time complete and flat lying" (Zen, 1974, p. 449) a situation never reached in the geological evolution of the Andean belt. The suggestion by Thompson (1971a), also quoted by Zen (1974), that the unconformities in Chile may actually be large- scale low-angle thrust faults is disproved by detailed mapping at different scales in several regions of that country (Aguirre, 1960; Aguirre and Thomas, 1964; Carter, 1963 ; Jarog and Zelman, 1969 ; Klohn, 1960; Levi, 1968; Thomas, 1958 and many 1:50,000 quad- rangle maps by different authors). Moreover, detailed mapping in Peru (Cossio and Ja6n, 1967; Myers, 1974; Webb, 1976) has demonstrated that the structural discontinuities where mineralogical breaks were found (Offler et al., in prep.) are regional unconformities and not faults. Criteria used to stablish these unconformities were, among others, the existence of ancient erosional surfaces, paleosoils, basal conglomerates containing pebbles of rocks and fossils from older units and angular relationships at the contacts.

Parametres Conditioning the Burial Metamorphism of the Andean Rocks

Discussion concerning the role of the different parametres is here based on general field and laboratory observations.

Load pressure (PI) has been commonly taken as the main factor producing burial metamorphic pat- terns. However, in regions such as Central Peru, where there is evidence that subsidence was restricted and deposition limited because of continuous vertical oscillations (Cobbing et al., in prep.), extremely thin units (less than 500 m) display a metamorphic grade ranging from zeolite to greenschist facies (Fig. 1 B). Within these thin units wairakite has been found by one of the authors (R.O.). Its presence in the Peruvian assemblages plus the absence of lawsonite and of the pumpellyite-actinolite facies (Hashimoto, 1966; Seki, 1969) both in the Peruvian and the Chilean sequences, show that low-pressure/high temperature gradients existed during the burial metamorphic episodes (see also Coombs, 1971, p. 318). Some of these gradients could have been similar to those existing in present day geothermal fields.

These high geothermal gradients demonstrate that temperature (7) is a more important parametre than PI as already pointed out for other areas (Zen, 1974; Zen and Thompson, 1974). A main source of heat responsible for these gradients is thought to be magma at depth before its ascent to shallower levels of the crust (Aguirre and Levi, 1977; Cobbing et al., in prep. ; Offler et al., in prep. ; Pitcher, 1978).

The main difference in burial metamorphic pattern between Peru and Chile is that a metamorphic

series covering the same range in grade is displayed within a much thinner sequence in Peru than in Chile. This implies a higher temperature-depth gradient dT/ dD which in turn suggests that the magma, before its emplacement, was closer to the stratified rocks in the Peruvian region. A similar case of telescoping of a metamorphic facies series in a narrow section in Puerto Rico was interpreted by Jolly (1970) as produced by a steep geothermal gradient, mainly the result of "a sudden surge of relatively shallow plutonic intrusion from depth". In the volcanic rocks an additional source of heat may come from the hydration of the primary phases since the metamorphic reactions which produce low-grade hydrous minerals from initial anhydrous phases are exothermic and self- accelerating (Turner, 1968; Thompson, 1971 ; Zen and Thompson, 1974). Temperatures might be even more augmented in those places where freely circulating fluids allow both the transport of ions and heat.

Reactions leading to assemblages of the zeolite and related facies probably are influenced by fluid- pressure factors (Pf, XH2O, Xco2, etc.) even more than by Pl and T. In each flow T and P1 must have had similar values across its thickness of about 20 m. The fact that the upper, porous part, has been totally metamorphosed while the lower, non-porous part, is completely unmetamorphosed (Ch~ivez and Nister- enko, 1974; Levi, 1969) and that the general trend within each amygdule is characterized by an increasing grade from rim to core (Levi, 1969), plus the similarity of the metamorphic associations of small amygdules with those found in the outer zones of the bigger ones within each rock (Offler et al., in prep.) strongly underline the role of porosity in relation to metamorphic grade in these low-grade processes.

The importance of porosity and pore solutions has been repeatedly stressed both in reference to the extent and/or the grade in processes such as diagenesis, low-grade burial metamorphism and hydrothermal alteration. It has been explained as the result of fluid pressure being less-up to one th i rd- than P~ in the upper part of the crust where pores and other open spaces are abundantly present. Under this situation the reaction boundaries are displaced drastically to lower temperatures (Boles and Coombs, 1975, 1977; Coombs, 1961, 1971; Coombs etal., 1959; Jolly, 1970, 1972; Jolly and Smith, 1972; Landis and Coombs, 1967; Levi, 1969; Norris and Henley, 1976; Packham and Crook, 1960; Zen, 1974). The combined effect of this pressure mechanism plus the rise in T due to hydration reactions and hot fluid circulation may explain an observed increase in metamorphic grade within each flow towards the more porous parts. This same combined process can explain the mineralogical breaks between stratigraphical-struc-

L. Aguirre et al. : Mineralogical Breaks in the Burial Metamorphism of the Andes 365

rural units where higher grade assemblages overlie lower grade ones.

It has not been possible to evaluate the roles of g C O 2 , gO2, gSiO2, PIt and E h in the present study. However, calcite is abundant and ubiquitous in the metamorphic assemblages as well as in veinlets cutting other metamorphic minerals (Offler et al., in prep.); this suggests significant values of Xco2 in the pore fluids. The coexistence of calcite with minerals such as zeolites, prehnite and pumpellyite would suggest that the role of Pco2 should be specially studied (see Thompson, 1971b). The role of salinity is also unknown in the areas described although an anomalous increase in metamorphic grade has been observed by one of the authors (B.L.) in Chile in terrestrial rocks when they laterally pass into brackish-water deposits. This change in grade may be due to the lowering of gH20 by the increase in salinity in the brackish- water deposits, an effect equivalent to an increase in temperature (Coombs, 1971).

Each complete burial metamorphic episode generated by the combined effect of subsidence, heat from and underlying magma, exothermic hydration reactions and hot pore fluid circulation would conform to the possible geological setting envisaged by Val- lance (1969) as an explanation for the occurrence of secondary processes in igneous rocks. This setting is transitional between one in which there is extensive geothermal' activity and one in which alteration of the burial metamorphic type involves pore fluid in a regionally controlled hydrostatic environment (Val- lance, 1969, p. 22).

Acknowledgements. The authors are deeply indebted to M.P. Ather- ton, A. Beach, J.E. Cobbing, D.S. Coombs, W.S. Fyfe, J.S. Myers, W.S. Pitcher, Y. Seki, A.B. Thompson, F.J. Turner, C. Vidal, and E-an Zen for their constructive criticisms of this paper. J. Lynch kindly prepared the illustration.

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Burial Metamorphism and Geologic History

Burial metamorphism in the Central Chile-Central Peru segment is episodic. Each metamorphic episode is just one chapter within a more complex, dynamic sequence of events making a cycle (Aguirre et al., 1974; Charrier, 1973). Each of these cycles has an average total duration of 4 • 107 years and comprises: (a) lateral shifting of volcanic activity and deposition accompanied by subsidence and vertical oscillatory movements; (b) burial metamorphism produced under a culminating thermal gradient; (c) short period of regional compression in which burial isograds have been folded, closely followed by; (d) ascent of granitoid magmas and production of narrow contact metamorphic aureoles; (e) uplift followed by erosion1.

Time and space relationships between burial metamorphism, immediately subsequent folding, and ascent of high level granitoids could suggest that each episode of burial metamorphism was short-lived. This phenomenon might have occurred in a sort of gigantic regional geothermal system in which folding could have been produced by the pulsatory movement of big masses of magma and/or by the decrease in vol- ume undergone during the higher-grade, dehydrating stage, of burial metamorphism. Still another possibil- ity is that burial metamorphism, folding and magma ascent are all three related to a yet unknown common cause.

1 A detailed account of the plutonic events in the Coastal Batho- lith of Peru has been given by Pitcher (1978). The role and the place of metallogenesis in this cycle has been dealt with for the present Andean segment among others by Chfivez and Nisterenko, 1974; Clark et al., 1976; Levi, 1970; Losert, 1974; Sillitoe, 1976; Zentilli, 1974

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Received March 16, 1978; Accepted April 2, 1978

Documents

Unconformities as mineralogical breaks in the burial metamorphism of the Andes