Volcanic facies, structure, and geochemistry of the marginal basin rocks of central Peru

Journal of South American Earth Sciences, Vol. 2, No. 3, pp. 241-261, 1989 Printed in Great Britain

0895-9811/89 $3.00+ 0.00 © 1989PergamonPress plc & Earth Sciencesand Resources Institute

Volcanic facies, structure, and g e o c h e m i s t r y of the marginal basin rocks of central Peru M. P. ATHERTON a n d S. WEBB Department of Earth Sciences, University of Liverpool, l,iverpool, L69 3BX, England (Received for publication February 1989)

Abstract--The western part of the Mesozoic Peruvian Trough, the ltuarmey Basin, has a till of pillow lavas, sheet lavas, hyaloclastites, tufts and minor cherts, siliceous and calcareous oozes. Maximum subsidence occurred in the Albian when 9000 meters of basin fill accumulated. This was later intruded by gabbros and dikes and then the Coastal Batholith. The basin is an extensional marginal basin continuous with other basins of similar age to the south. Facies analyses indicate that the basin was relatively deep, with no continental influx, and not dissimilar to spreading and off-axis systems on the ocean floor. Structures at the surface and at depth indicate the crust has split and the basin was floored by mantle material. The basin shows a marked polarity, with tholeiitic basaltic rocks at the center and hlgh-K acid rocks at the eastern margin, with intermediate types between. These changes in petrology and chemistry relate to lateral changes in source composition. Secular variations are also present and indicate a calcalkali source giving way to a more MORB-like source with a variable continental component. The basin is part of the major rifting event in the Cretaceous which affected the whole western margin of South America and was an important and necessary precursor to major batholith intrusion. Resumen--El sector occidental de la Fosa del occidente Peruano, la cuenca de ttuarmey, fue rellenada por lavas ahnohadilladas, mantos lavicos, hialoclastitas, tobas y subordinados fangos calcareos y siliceos. La ma.xima subsidencia ocurrio en el Albiano con la acumulacion de 9000 n: de relleno de cuenca. Esta secuencia rue mas tarde intruida por gabros y diques y a posteriori por el Batolito de la Costa. La cuenca es una cuenca marginal de extension que se continua con otras de similar edad hacia el sur. An~tlisisde facies indican que la cuenca rue relativamente profunda sin influencia continental y de comportamiento acorde con los sistemas de extension y off-axis en el medio oce.anico, l,as estructuras en superficie y en profundidad indican que la corteza fue fracturada y que la cuenca rue tapizada por material del manto. La cuenca muestra una marcada polaridad con basaltos toleiticos en el centro y roeas acidas ricas en potasio hacia el margen este, con tipos intermedios entre ambos extremos. Los cambios petrologicos y geoquimicos se relacionan con cambios composicionales laterales en la roca fuente. Variaciones seculares est~in tambien presentes e indiean una tiaente caieo-alcalina que dio lugar a una de basalto centro-oceanico con influencia continental variable. La cuenca es parte de un evento mayor de rifting en el Cretacieo que afecto todo el margen occidental de Sudamerica y el cual es un importante y necesario precursor de una intrusiOl batolitica de gran eseala.

INTRODUCTION THE HUARMEY BASIN of c e n t r a l P e r u l i e s i n t h e w e s t e r n p a r t of t h e M e s o z o i c P e r u v i a n T r o u g h ( W i l s o n , 1963), w h i c h C o b b i n g (1978) c o n s i d e r e d to r e p r e s e n t a c l a s s i c a l g e o s y n c l i n a l b i c o u p l e w uiz., a w e s t e r n v o l c a n i c e u g e o s y n c l i n e ( i n c l u d i n g the t l u a r m e y B a s i n ) a n d a s e d i m e n t a r y m i o g e o s y n c l i n e to the e a s t . A c c o r d i n g to M y e r s (1975), the H u a r m e y B a s i n is b o u n d e d to t h e e a s t by t h e T a p a c o c h a a x i s (Fig. 1), a s t e e p b e l t t h a t m a r k s t h e t r a n s i t i o n b e t w e e n the two p a r t s of t h e g e o s y n c l i n e , a n d to t h e west by the O u t e r S h e l f H i g h (OSH, Fig. 1), so t h a t m u c h of t h e b a s i n is offshore, i n c o m m o n w i t h t h e o t h e r b a s i n s of t h e t r o u g h , t h e H u a r m e y B a s i n was t h o u g h t to h a v e developed within a horst and graben megastructure m a d e u p of a s e r i e s of c o n n e c t e d b a s i n s p a r a l l e l to the c o a s t ( C o b b i n g et al., 1981). T h e t t u a r m e y b a s i n - f i l l is p r e d o m i n a n t l y m a d e u p of pillow l a v a s , s h e e t l a v a s , h y a l o c l a s t i t e s , w a t e r l a i n tufts, a n d s u b o r d i n a t e s e d i m e n t s ( c h e r t , l i m e s t o n e , tuff, s i i t s t o n e ) a n d h a s b e e n d e s c r i b e d b y R i v e r a et al. (1975) a n d C o b b i n g et al. (1981), as well

as by M y e r s (1980), W e b b (1976), B u s s e l l (1975), a n d others. Maximum subsidence took place in the A I b i a n w h e n up to 9000 m e t e r s a c c u m u l a t e d i n t h e d e e p e s t p a r t of t h e b a s i n n e a r T r u j i l l o ( B u s s e l l , 1975). T h i s a c c u m u l a t i o n r e p r e s e n t s a v e r y a c t i v e b u t r e l a t i v e l y s h o r t - l i v e d p h a s e of d e e p s e a v o l c a n i s m . T h e fill s e q u e n c e w a s g i v e n g r o u p s t a t u s b y M y e r s (1974) who d e s c r i b e d t h e s t r u c t u r e , s t r a t i g r a p h y , a n d p e t r o l o g y i n t h e C a s m a a r e a ( F i g . 1). T h i s d i s c u s s i o n will be c o n f i n e d to t h e s t r a t i f i e d r o c k s a n d d i k e s of the C a s m a G r o u p a s d e f i n e d o r i g i n a l l y by T r o t t e r e a u a n d O r t i z ( 1 9 6 3 ) , C o s s i o a n d J a e n (1962), M y e r s (1974), a n d W e b b (1975). D e t a i l s of t h e gabbro intrusions, burial metamorphism, and other a s p e c t s of t h e fill a r e g i v e n i n R e g a n ( 1 9 8 5 ) a n d A g u i r r e a n d Offler (1985), r e s p e c t i v e l y , a n d in t h e r e f e r e n c e s cited above. R e c e n t l y , A t h e r t o n et al. (1983) a n d A t h e r t o n et al. (1985b) h a v e r e i n t e r p r e t e d t h e t i u a r m e y B a s i n , a n d t h e r e f o r e the e u g e o s y n c l i n a l p a r t of t h e W e s t P e r u v i a n Trough, as an e x t e n s i o n a l m a r g i n a l b a s i n c o m p a r a b l e to t h a t of s i m i l a r a g e in C h i l e ( D a l z i e l , 1981). T h i s i n t e r p r e t a t i o n , if c o r r e c t , h a s i m p o r t a n c e

241

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M.P. ATtlERTON and S. WEBB

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Volcanic facies, structure, and geochemistry of the marginal basin rocks of central Peru in the study of the evolution of the western continental margin of South America, and also has implications for metaliogenesis and the development of the Coastal B a t h o l i t h , which a t t a i n s its m a x i m u m volume within the marginal basin (Fig. 1). In this paper, the evolution of the l t u a r m e y (northern) section of the marginal basin (Fig. l) is modelled using: (a) the volcanic facies characterizing the basin, and comparing these with modern spreading environments; (b) the surface structures and deep geophysical data, which indicate major crustal splitting; (c) the spatial and secular variations in chemistry of the volcanic fill; and (d) the source of the volcanic fill.

H U A R M E Y BASIN

The life of the Huarmey Basin extended from the Tithonian to the Albian (Cobbing, 1978), with maximum subsidence over a very short period in the Albian as indicated by the presence of only middle Albian fossils in most of the sequences (Myers, 1974). This period of very rapid subsidence and volcanic activity coincided with the main crustal extension. The Casma Group rocks that make up the fill of the H u a r m e y Basin extend from just north of Trujillo in north-central Peru, to near Lima (Fig. l) where a convenient marker for the base of the Casma Group is the top of the Atacongo Limestone (early Albian). There is no evidence that the Atacongo Limestone or other groups below the Casma Group extend beneath the Huarmey Basin itself (see also Cobbing, 1985), and it seems entirely possible that the Casma Group rocks rest directly on mantle (Atherton et al., 1983). The oldest observed volcanic rocks of the Huarmey Basin belong to the Berriasian and are represented in the Lima area by the Puente Piedra Group, some 2000 meters of pillow lavas and basic pyroclastics with intercalations of limestone and shale (Rivera et al., 1975). They represent magmatism at the initial stages of extension before the rapid subsidence of the Casma Basin. The stratigraphy of the Casma rocks of the Huarmey marginal basin has been studied in the Huarmey area (Myers, 1980) and in sections in the valleys of the Rio C u l e b r a s , t i u a r m e y , and Quebrada Pararin, as well as Tapacocha and Churin on the eastern margin (Webb, 1976, and Fig. 1}. Detailed descriptions of the volcanic and sedimentary successions are given by Myers (1980) and Webb (1976) and are not repeated here, but essential data pertinent to the argument are given diagrammatically for specific parts of the Pararin, Culebras, and Huarmey sections in order to typify the Casma rocks in terms of their facies (cf. Cas and Wright, 1987). This is then used to model the depositional environment of the basin. Sections through the western Casma Group rocks in the Huarmey Basin are shown in Fig. 2, as are the correlations and stratigraphic sequences of Webb (1976).

243

S T R A T I G R A P H Y AND D E P O S I T I O N A L E N V I R O N M E N T OF H U A R M E Y BASIN ROCKS

Stratigraphy of the Coastal Sections Lower and Upper Pillow Lava Formations. The stratigraphy of the two pillow lava formations in Quebrada Pararin and Huarmey are shown in Fig. 3. Notable are the thick, predominantly pillow lava sequences (up to 400 m), the difference between the eastern and western sections in Quebrada Pararin, and the axial differences between the Pararin and tluarmey sections (Fig. 3). The latter suggests there is no simple correlation on a detailed scale. The Lower" Pillow Lava Formation contains the oldest exposed rocks of the Casma Group, which clearly formed in a marine environment as indicated by their pillowed form, chilling, and u b i q u i t o u s association with submarine sediments. Pillow lava flow units, up to 20 meters thick, generally have a massive base, with a few isolated pillows resting directly on pillows of the previous flow; above and grading from it are well-formed budded, globule, or amoeboid type pillows, lnterpillow voids may be filled with a glassy green mesostasis, epidote, chert, or calcareous material. The upper part is often a breccia made up of a jumbled mass of pillows in a matrix of lava or tuff topped by a thin cap of scoriaceous lava. Commonly, the mechanism of pillow formation appears to have been by budding of subaqueous lava tubes (Jones, 1969). Another feature characteristic of budding is the presence of concentric zoning, a structure typical of solidified lava tubes (McDonald, 1953). Some pillows overlying tufts appear to have globulated passively and s e t t l e d gently; they show loading with squeezed-up t u f f material and have finely laminated sediment draped over their upper surfaces. Generally, lava lenses occur at the bases of pillow lava units, and massive lava flows grade laterally and vertically into pillow lavas (Fig. 3) and may act as feeders to the pillows. Individual pillow lava flows may be very extensive and lense out laterally, and the whole pillow lava pile is made up of these superimposed lenses extending for many kilometers north and south (Webb, 1976). Myers (1974) described coalescing ridges of pillow lavas, up to 400 meters high and 1000 meters long, flanked by pillow lava breccias and tufts, that parallel the present coast (i.e., approximately axial to the basin). Myers (1974) and Webb (1976) thought they were formed at linear, Andean-trending fissures along which volcanic centers may have been located. The lack of explosive fragmentation suggests a deep water environment, although it may also reflect the volatile content of the magma; however, the distal volcanogenic turbidites and finely laminated and occasionally graded tufts interbedded with the pillow lavas (Fig. 3) also indicate quiet, relatively deep, marine conditions in excess of 200 meters. Dikes intruding the Lower Tuff Formation while it was sill unconsolidated may have been feeders for the Upper

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Hyaloclastite Formation. The tlyaloclastite Formation is made up almost entirely of amygdaloidal and porphyritic lavas of similar petrology and composition to the rocks of the Pillow l,ava Formations, but in the form of autobrecciated and brecciated flows, lava breccias and pillow lavas (Fig. 4) occurring in 7- to 12-meter-thick units and lenses up to 18 meters thick. They are often hyaloclastic. The flows have amygdaloidal, brecciated bases and brecciated internal layers and flow banded vesicles. The autobrecciated lavas are not so much brecciated in an angular sense, but contain small irregular pillows, clasts, clots, and tongue-like forms often lobate and smooth with chilled or bleached margins - - the whole representing rapid eruption with the clots and small pillows being incorporated along in the flow, which is often less vesiculated than the clasts. Only a few clasts show a n g u l a r forms and these are usually parts of lobate masses r e m a i n i n g more or less adjacent to each other. Pillow lava breccias are relatively rare and grade up from true pillow lavas (Fig. 4a). They show a packed texture or have a hyaloclastic tuff matrix. In some cases pillow lava breccias may be slumped or sloughed consolidated pillow lava flows, but the major part of the pillow lava breccias are primary hyaloclastite breccia. The bulk of the formation is characteristic of quenching in sea water, with some fragmentation and/or autobrecciation, but in which pillow f r a g m e n t s c a n n o t be identified although fragments are of the same type of lava and some have chilled edges. Typical initial type hyaloclastite breccias occur - - i.e., isolated broken porphyritic pillows in a tuff matrix (Silvestri, 1963) - - but common hyaloclastites with small, irregular clasts in a tuff matrix predominate. The hyaloclastites are interbedded with pillow lava flows, finely banded silty tuft, and slumped a q u a g e n e tuffs showing typical upward sequences - - viz. pillow lavas--+hyaloclastite-~aquagene tuff, massive lava--~pillow lava (Fig. 4), confirming deposition under s u b m a r i n e conditions and their intimately related origin. In P a r a r i n and l t u a r m e y , the formations are more tuffaceous to the west. T u f f Formations. Rocks of the Lower Tuff Formation show some axial variation (Fig. 5a), with agglomerates and massive graded tuffs present only in the Rio I-Iuarmey section. Marine conditions obtained throughout and although extrusion of lava ceased temporarily, abundant volcanic debris was deposited mainly by turbidity currents. Instability is also indicated by slump structures and syndepositional faults, and is possibly associated with uplift as there is a slight disconformity at the base. Beds are made up of fragmental, fine-grained, aphanitic, porphyritic or amygdaloidal basic lavas probably dislodged from lava flows and agglomerates and redeposited by turbidity currents. Generally, they grade up from coarse lapilli tufts of gravely aspect to well sorted

parallel laminated tufts and silts. The general lack of glassy pumiceous debris, except at the base, indicates that the tufts were not derived quickly from large eruptions. The deposits probably originated from turbidity currents generated on the flanks of fault scarps (ridges) or volcanoes, and spread out from submarine canyons as fans on the seafloor where, when quiet conditions obtained, fine-grained laminated silts and sometimes calcareous mudstone accumulated. Occasional crystal tufts may have formed from an underwater pyroclastic flow. Toward the top of the formation in Quebrada Pararin (Fig. 5a), better sorting, c u r r e n t bedding, occasional ripple drift, and rounded ctasts in volcanic conglomerate may indicate a shallowing of the sea with more reworking and sorting of material. Interbedded turfs with accretionary lapilli and reversed grading may also indicate shallowing of the sea. The Upper Tuff Formation is made up of massive graded units fining upward to laminated, but rarely current bedded, tuffaceous sandstones and siltstones with planar bases, with interbeds of foraminiferal mudstone, tuff, and ash. The base of the massive units contain angular to s u b a n g u l a r f r a g m e n t s of allochthonous acid porphyries and fine-grained lava, as well as local tuffaceous sandstone, siltstone, and quartzite in a wackite texture. Isolated slabs (2 m × 1 m × 2 cm) of mudstone from the underlying Upper Sedimentary Formation occur parallel to bedding in these coarse-graded units. Some were well indurated, some folded, and hence plastic on incorporation into the unit. The beds of sand-grade tuff at the top of the units are often individually graded. Maximum thickness of a unit can reach 30 ineters, 20 meters for the massive unit, and 7-10 meters for the sandstones and silts, l,apilli in the tufts consists of glassy nonvesicular dacite and attenuated pumice, while the coarse ash consists of smaller chips of the same material and broken crystals of plagioclase and quartz. Progressively finer ash is composed of fragmented crystals and pumice shreds. The debris is mostly pumiceous or vitric inaterial, although some of the tufts are vitric crystal tufts and could result from sorting of the finer material from an underwater pyroclastic flow. In the lapilli tufts, most fragments of pumice are not oriented and the deposits are not welded. The vesicular nature of the graded units toward the top of the succession indicates ash-flow deposition and release of gas. The lack of internal stratification and general homogeneity and lack of welding indicate rapidly deposited, probably cool, flows. Derivation from large eruptions is indicated by the great volume of glassy, pumiceous debris, angularity of rock, and crystal fragments and fresh plagioclase crystals. The most common s e d i m e n t a r y structures are graded and contorted bedding due to slumping, while structures indicative of traction currents are rare - - all indicating deposition from turbidity currents in a low-energy environment below wave base on a slope. The source lay to the west, as

Volcanic facies, structure, and geochemistry of the marginal basin rocks of central Peru

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I%dc~ o'

E: HYALOCLASTITEFORMATION,QUEBRADAPARARIN.

Fig. 4. Stratigraphy ofthe HyaloclastiLeFormation,Quebrada Pararin (a(~.erWebb, 1976}. Typical upward sequencesare shown: a, pillow lava--*hyaloclastite---~aquagenetu(~, b, massive lava-*pi[low lava; c, tut~L-~hyaloclastite--*piliowlava; d, pillow lava---* massive lava. indicated by the slumping down the paleoslope to the southeast and fining and thinning of beds to the east. The massive graded beds (Fig. 5b) with the thinly bedded, finely laminated tuffaceous sandstones and siltstones are similar to submarine pyroclastic flow deposits described by Fiske and Matsuda (1964). In fact, the sequence has all the main characteristics of s u b m a r i n e nonwelded pyroclastic flows (Fisher, 1984; Yamada, 1984) - - i.e., (1)the rocks consist of lithic fragments, crystals, glass shards, pumice in varying proportions; (2)they contain shale rip-up fragments in the massive unit; (3) shattered crystals and glassy fragments formed on quenching; and (4) they are made up of a two-fold division into a massive, poorly sorted structureless, graded lower division with a sharp bottom and large lithic fragments and a thinly bedded subordinate upper division with

individual beds graded, the whole fining upward. C o n f i r m a t i o n of the s u b a q u e o u s n a t u r e of t h e sequence comes from the foraminiferal mudstone interbeds throughout the sequence. This doubly graded sequence is characteristic of violent eruption from a submarine vent (Fiske and Matsuda, 1964}. Initially, the bulk of the ejected material falls back to form a water-rich debris flow deposited as the thick lower division. As eruption waned, small intermittent turbidity currents entrained pumice and crystals to form the t h i n - g r a d e d beds of the upper division. Finer grained material from the water body settled on top to produce ash layers.

Sediments. The Upper Sedimentary Formation in Quebradas Pararin and Culebras marks an abrupt change from submarine pillow lava extrusion to

248

M.P. ATIIERTON and S. WEBB

Volcanic conglomerates; 20 cm dian~ter rounded pebbles of amygdaloidal and aphanitic lava, t u f f and siltstone; interbeds of laminated tuffaceous sandstones, siltstones and l a p i l l i tuffs with occasional reverse grading; ripple d r i f t and synsedimentary faults common.

Finely laminated black and grey siltstones and flaggy, fine ruffs. Laminated sandy tuffs and silts. Dolerite s i l l .

Massive fine to medium grained tuffaceous sandstones, laminated.

I,lassive green graded tuffs with s i l t y interbeds.

Tuffaceous sandstones, siltstones and mudstones, with current bedding, festoon bedding and ripple bedding loading and contorted bedding and occasional graded t u f f bands. Sandy tuffs and laminated s i l t y tuffs. Agglomerates, l a p i l l i tuffs and s i l t y tuffs. Massive tuffaceous, crudely laminated sandstones with channel structures, cross bedding; l a p i l l i turfs with interbeds of siltstones and mudstones with syndepositional faults.

Agglomerates, ] a p i l l i tuffs and well bedded tuffs.

Well bedded tuffs with finely laminated s i l t y ruffs, slumps.

Graded l a p i l l i tuffs, laminae parallel perfect to set boundaries, topmost sands graded, IOcm units; scours and channels common, and slumps often cut o f f by succeeding beds. Beds coarsen and thicken upwards; l a p i l l i of amygdaloidal and aphanitic lava, shale and tachylyte. Thin lavas and rhyolitic l a p i l l i crystal {hornblende) ruffs.

Lapilli tuffs contain shreds and l a p i l l i pumice, lavas, xenocrystal plagioclase and hornblende in mesostasis of acicular hornblende and microcrystalline quartz; pumice l a p i l l i , irregular, 5mm long.

LOWER TUFF FORMATION, QUEBRADAPARARIN.

i

10m

N

Crystal tuffs {hornblende), graded and s i l t y tuffs. Turfs and fine agglomerates.

Well bedded l a p i l l i turfs. Agglomerates.

Instability near base indicated by syndepositional faulting, slump structures; graded beds, laminations parallel to set boundaries; current bedding is uncommon.

LOWER TUFF FORMATION, RIO HUARMEY. l~'ig. 5a. Stratigraphy of the Lower TuffFormation m the Rio Huarmey and Quebrada Pararin (after Webb, 1976).

normal marine sedimentation - - viz. thinly bedded siltstones, lilac papery shales, and minor pebble conglomerates with acid volcanic fragments; these are followed by pyritic nodular" limestones, tuffaceous sandstones, siltstones, thin limestones, and minor rhyolite flows. In the Huarmey section, the formation is massively invaded by sills and is made up of coarse rhyolitic agglomerates, flaggy rhyolitic tufts, lavas, and thinly laminated graded andesitic tufts.

Agglomerates and Tufts. Lavas, agglomerates and tufts of the Green Agglomerate Formation (see Fig. 2) are interbedded with subaqueous tufts and lava flows. The pyroclastic rocks are well bedded agglomerates, with grain size in each unit decreasing upward to stratified laminated tufts that are made up of lapilli, and vitric and crystal variants with rare current bedding and fragment load and channel structures. Fragments are of aphanitic and porphyritic lavas - - fine-grained tufts and granodiorite. They may be massive or finely bedded (1-20 cm).

Individual beds were probably formed by eruption beginning with vent clearing, shattering of plugs, or lava domes producing lithic fragments with ash and crystal. Ash erupted later and settled on top of the breccias. The sequence coarsens upward as explosive intensity increased, becoming very coarse toward the top. This suggests that the sources were not more than several kilometers away and located in shallow water or were even subaerial, as evidenced by the predominance of explosive activity. The Brown Lapilli Tuff Formation contains flowbanded, basaltic, porphyritic lavas (2-13 m thick) with perlitic and brecciated bases, columnar jointing, and vesicular or pahoehoe top surfaces. They are interbedded with tufts, lapilli tufts, and graded lapilli tufts with lenses of agglomerate. The agglomerates and stratified lapilli tufts are composed of scoriaceous fragments and lapilli, and crystal tufts increase upward. These latter lense out rapidly, a characteristic of subaerial pyroclastic material deposited by fallout frmn eruption clouds.

Volcanic facies, structure, and geochemistry of the marginal basin rocks of central Peru

~S~

249

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i

UPPER TUFF FORMATION Turfs and foraminiferal mudstones. Laminated, current bedded wackites, graded pyroclastic tuff flows.

i....... ! I

:''

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r

,

...,,.....

Massive graded units with allochthonous acid porphyry, lava and mudstone slabs.

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UPPER SEDIMENTARYFORMATION Thin spherulitic acid flows and marly mudstones (E). Orange/brown thin bedded sandstone, s i l t s and cherts with black pyritic mudstones (W).

~

m

Rocks becomefiner grained and more thinly bedded to the east. UPPER TUFFAND SEDIMENTARYFORMATIONS,QUEBRADAPARARIN.

Fig. 5b. Stratigraphy ofthe UpperTuffandSedimentaryFormations,Quebrada Pararin(af~erWebb, 1976).

Chaotic Volcanic Melange On the coast near Culebras and south of lluarmey is a m61ange (>100 m thick) characterized by porphyritic lavas and orange crystal, vitric, bedded tufts forming breccias of huge, randomly oriented and folded blocks associated with pre-, syn,- and postdepositional dikes and intrusions. Slabs up to 40 meters across occur with very angular agglomeratic material between the fragments. Early basic dikes are also fragmented into 4-meter blocks. The breccias may be overlain by agglomerates, lapilli, and orange tufts - - similar to those in the m61ange. The agglomerate contains lava and rare fragments of quartzite and black siltstone, and loads into the finer vitric tufts with flame structures, sand dikes, and ball and pillow structures. Near Huarmey, the agglomerates are massively bedded and graded. Irregular intrusions and dikes are associated with the agglomerates and breccias, the dikes reaching up to 40 meters in width with chilled margins, but limited

in lateral extent (200-300 m). They are very variably oriented, from vertical to horizontal within short distances. Irregular and vesicular margins suggest they may have been intruded prior to consolidation. Webb (1976) thought the m61ange was due to sudden uplift of a volcanic pile lying to the west. Some of the volcanic m a t e r i a l may have been s u b a e r i a l , as amygdaloidal ropy lava and crystal vitric tufts were identified. Strata associated with the breccias also show sedimentary structures indicative of deposition in an unstable environment, p e r h a p s from submarine mudflows or avalanches. There is evidence of some blocks still being plastic during formation of the melange, as they are crumpled and show slump bedding and flow structures round other blocks. The whole association is related to active volcanism, as indicated by the material forming the matrix and the interbedded agglomerates and tufts. The age of the chaotic melange may possibly be correlated with the disruption of bedding in the Upper Sedimentary Formarion, where graded conglomerates and sandstones

250

M.P. ATHERTONand S. WEBB

above the disrupted black mudstones in Quebrada P a r a r i n (Fig. 5b), indicate successive eruptiongenerated volcanic avalanches that were redeposited as graded conglomerate-tuff units. The mdlange parallels the coast and was probably associated with axial rifting, as indicated by its position in the basin (see Fig. 1).

Rocks of the Eastern Margin of the Huarmey Basin The rocks of the eastern margin of the basin, marked by the Tapacocha axis of Myers (1974), belong to the Churin Group ]equated by Webb (1976) to the Albian Casma Group] and the Upper Cretaceous mainly pyroclastic Pararin Group (Fig. 6). in the north, the rocks of the eastern margin are similar to those in the west (submarine volcanic and volcaniclastic rocks). However, tufts, lavas, and agglomerates predominate to the south where the shallow water pyroclastic Churin Group is readily distinguishable from the deep western submarine facies. Unconformably overlying it is the Pararin Formation (Myers, 1974), which postdates the Albian folding. The Pararin Formation may extend to the west of the Coastal Batholith, but without fossils it is difficult to establish a correlation with the upper two formations of the Casma Group of similar composition and possibly similar age (Fig. 6). Two f o r m a t i o n s t h a t t o g e t h e r make up the Churin Group were distinguished at Churin by Webb (1976): the Paccho Tingo Formation, which consists of tufts, lavas, and agglomerates, and the Mirahuay Formation, which is made up of fine-grained sediments (Fig. 6). The former is a massive series of altered porphyritic lavas and well-bedded lapilli turfs. The lowermost beds are green, well-bedded lapilli, vitric crystal, lithic tufts, and chinastone ashes very similar to the rocks of the Lower Tuff Formation to the west. Laminated and graded beds (1.0-1.5 m) are common, with the graded beds being massive, vitric, crystal, lithic tufts at the base and becoming finer grained upward, with fine-grained ash at the top. Above are massive, poorly bedded lavas, which are succeeded by tufts and agglomerates

becoming thinly bedded toward the east. Lithic fragm e n t s include f i n e - g r a i n e d h y a l o p i l i t i c l a v a s , consisting mainly of devitrified glass, microcrystalline rhyolite, and hypocrystalline basic lava. The tufts show some evidence of sorting into line and coarse bands. The Mirahuay Formation rests conformably on the Paccho Tingo Formation and is made o[" finegrained, thinly bedded tufts and sediments, the latter made up of sandstones, siltstones, marbles, cherts, and limestones - - all very thinly bedded. The Pararin Group rests unconformably on the Churin Group rocks in the Churin region and is essentially made up of pyroclastic rocks and flows of andesitic and basic composition (Webb, 1976). At the top are coarse agglomerates.

Facies and Depositional Environment of the Casma Group Rocks The Casma Group sequences outlined above may be used to develop a facies model - - i.e., the type of volcanism associated with a specific basinal sequence (Cas and Wright, 1987). The lava forms are very similar to those seen at within-plate seamounts and at MOR (mid-ocean ridge) spreading centers - - i.e., sheet and pillow forms, one of which may predominate. They may form simultaneously and grade laterally into each other or may onlap one another (Ballard et al., 1979). Ballard et al. (1979) thought the transition within or between flows could reflect discharge rate and that sheet flows were analogous to modern unchannelled pahoehoe flows erupted at high discharge rates while pillow basalts were analogous to tube fed pahoehoe lavas erupted at much lower discharge rates. The sheet flows representing the early, short, voluminous stage while the pillows represent steady, sustained eruption after well-integrated plumbing s y s t e m s were developed. The sequence of s h e e t e d lavas followed by pillowed lavas is common in the Casma rocks (see Fig. 3), as is the lateral variation from lava flow through amoeboid lava form to pillow lava. This

East

West

Brown Lapilli Tuff Fm. Green Agglomerate Fm. ~

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Fig. 6. Idealized section across the exposed e a s t e r n part ol'the m a r g i n a l basin showing the e a s t e r n facies, made up of the C h u r i n and P a r a r i n Groups, which c o n t r a s t with the deeper C a s m a Group facies to the west. No clear correlation can be m a d e across the Coastal Batholith (after Webb, 1976). MF, M i r a h u a y Formation; other a b b r e v i a t i o n s relate to t b r m a t i o n s silown ill Fig. 2.

Volcanic facies, structure, and geochemistry of the marginal basin rocks of central Peru sequence is similar to that seen in mid-Atlantic ridge (MAR) cores from the Famous area. In general, the dominance of pillows over sheets indicates slow spreading without major fracturing and disruption of plumbing systems (cf. MAR) as opposed to the fast spreading ridges, such as Galapagos, where sheet flows dominate. The coalescing ridges of pillow lavas up to 400 meters high and 1000 meters long described by Myers {1974} are similar in morphology to those at slowly spreading MOR s y s t e m s where the neovolcanic zone lies within a well-defined rift valley zone and elongate, 1-4 km, 250-meter high discontinuous linear volcanoes produce predominantly pillow lavas. Increasing brecciation to the northwest may indicate a steepening sea floor and increased rifting (?) in that direction. These data indicate that the floor of the Casma Basin may have been similar in many respects to that of a slowly spreading mid-ocean ridge environment. The sediment associated with the massive lava flows in the Casma Basin is chert, siliceous ooze, and calcareous mud. Occasional thin tufts between pillow lavas are discontinuous and are made up of finely laminated silts and graded turfs, with bands of siliceous nodules. The association of thin cherts, calcareous muds, siliceous oozes, and nodules is characteristic of ocean floor sequences. In contrast to most MOR spreading centers, the C a s m a sequence c o n t a i n s m a j o r h y a l o c l a s t i t e sequences. However, off-axis drilling has revealed thick beds of h y a l o c l a s t i t e within ocean c r u s t (Schmincke et al., 1979). It may come from fissures and from seamounts (Lonsdale and Batiza, 1980) where it is associated with calcareous ooze and both sheet and pillow flows, the latter occurring near the summit of the seamount (Cas and Wright, 1987, Figs. 14,10). The hyaloclastites often form debris flows down the side of the seamount and "some of the largest may have debouched into ocean basins" (Cas and Wright, 1987) to produce the thick deposits seen in off-axis drilling. The upward sequence of pillow l a v a ~ h y a l o clastite-*aquagene tuff seen in Quebrada P a r a r i n and other places (see Fig. 4) is also reminiscent of the growth sequence on the flanks of marine stratovolcanoes which shows pillow lavas at the base followed by hyaloclastites and pyroclastic or epiclastic deposits. Indeed, hyaloclastites are most characteristic of volcanics in the off-axis fissural-type zone and are common at the outer m a r g i n s of the plate boundary zone of active tectonism (Lonsdale and Batiza, 1980). In general, the dominant control on hyaloclastite vs pillow lava formation appears to be viscosity at the vent which, at constant compositions, is determined by temperature and thus discharge rate. Magma of high viscosity is conducive to hyaloclastite formation and appears to mark the waning stages of growth of a tholeiitic volcano after pillow and sheet production. The structures in the two tuff formations emphasize the importance of instability and turbidity

251

currents in moving large masses of volcaniclastic material downslope in the basin. The absence of nonvolcanic material in these formations indicates the instability and source was within the newly forming basin, and presumably related to the rifting associated with spreading plus instability of the elongate volcanic fissure structures. The associated sediments, especially in the Upper Tuff, include calcareous and foraminiferal mudstones and testify to the relatively deep sea environment of the basin. From the vertical sequences seen in Figs. 3, 4, and 5, it is apparent that there are rapid changes in rock type along quebrada sections - - e.g., P a r a r i n and between quebradas, suggesting that the lateral continuity over large distances as implied by Webb (1976) is unlikely. This being so, it seems unlikely that the Lower and Upper Tuff Formations signify a basin-wide cessation of magma production. Rather, they are a part of a local sequence formed extremely rapidly on uplift, during rifting and spreading, near to the basin axis. In contrast to the deep marine lava sequence of the western facies, the eastern facies is noticeably more pyroclastic and acid in composition, and thinly bedded sedimentary intercalations of limestone and siltstones form an important part of the succession; deposition probably occurred in shallow water. The group thins dramatically near Churin, with facies changes that must relate to the western edge of the continental block, which was not e m e r g e n t d u r i n g the history of the Ituarmey Basin. Briefly, the volcanologic evolution of the basin during the Albian may be divided into three stages. An early stage of deep water volcanism produced pillow lavas and hyaloclastites and volcanigenic tufts deposited by turbidity currents, largely reflecting deposition in environments very similar to t h a t at present-day oceanic spreading and off-axis systems. Generally, the hyaloclastites are more important to the north and west while the pillow l a v a s predominate to the south and west in the central area of the basin, suggesting that lava eruption was more rapid in the north and west and slower in the south and east. This is compatible with basin opening and deepening to the n o r t h w e s t and n a r r o w i n g and shallowing to the southeast. Furthermore, the basin axis runs out to sea northward, so more off-axis, hyaloclastite-dominated volcanicity m i g h t be expected in that direction. There followed a period of marine sedimentation producing limestones, cherts, and silicified ashes, as well as quartzite and much volcanigenic material. The sequences are rather different in the sections studied and may not herald a basin-wide hiatus in volcanic activity. In the l l u a r m e y region, for example, the sequence was invaded by n u m e r o u s concordant basaltic sills and the whole sequence may related to a decrease in s p r e a d i n g r a t e and an increase in sagging rate of the basin, producing concordant sill bodies r a t h e r t h a n lavas w i t h i n an unconsolidated sedimentary pile (cf. Gulf of California). Instability in the upper part of this sequence,

252

M.P. ATIIERTONand S. WEBB

lstic flows Volcaniclastic mass flows from eroded volcanoes ~yaloclastites

Older mantellic basement b

~

v

~

v

.

.

.

.

.

.

.

.

.

.

dykes, flows

Fig. 7. A faciesmodelbased on the exposedrocksofthe internal Casma Basin, near to the axis ofthe basin. as shown by the chaotic m61ange, may herald the rift faulting responsible for the massive graded tufts of the Upper Tuff unit, which immediately preceded the major t e c t o n i s m producing the basin-wide late Albian folding episode - - the first compressional event in the basin development (see Fig. 2). The final stage of volcanic activity is represented by the agglomerates, tufts, and lavas of the Green Agglomerate and Brown Lapilli Tuff Formations. The former may relate to marine volcano growth on the basin floor, which eventually perhaps became subaerial as indicated by the lavas and tufts of the Brown Lapilli Tuff Formation. Thus, marine stratovolcanoes often show an upward sequence - - pillow lava--->pyroclastic and epiclastic rocks at the top (Cas and Wright, 1987). A facies model for the internal Casma Basin is shown in Fig. 7; this is not dissimilar in some respects from that shown by Cas and Wright (1987) for stratovolcanoes and environs and seamounts near the East Pacific Rise (Lonsdale and Batiza, 1980).

S T R U C T U R E S IN THE H U A R M E Y BASIN

Surface Structure An important feature of the Huarmey Basin is the presence of major fault systems parallel to the long axis of the basin. The e a s t e r n m o s t fault separates the basin from the shelf facies. Thus in the upper Huava Valley the eastern facies Churin Group rests on the Gollarisquizqua Group and is separated to the east from the shelf facies by a deep fault along which lie hot springs (see Fig. 6). Farther to the west a major fault separates the eastern facies Casma from the deeper marine facies of the basinal Casma rocks. Folds in the marginal basin trend subparallel to the coastline and cordillera (Andean), or are Andean normal (see Figs. 2 and 6). The Andean folds are ubiquitous with subvertical axial surfaces and gentle

plunges to the northwest and s o u t h e a s t (Myers, 1974). They are generally open with long wavelength. Along the Tapacocha axis, tight isoclinal folding with smaller amplitude is common, with similarly oriented axial surfaces and axial directions as in the more open folds to the west. A slaty cleavage and low-temperature metamorphism is usually developed. Away from the Tapacocha axis, Myers (1974) recognized other tight structures - - e.g., the Cafloas syncline with marked slaty cleavage in the muddy beds and, locally, an associated metamorphism, producing biotite and hornblende in the basic rocks. These features related to high heat flow up major fault zones. Webb (1976) also recognized the Cations structure and its relation to a major fault system downthrowing towards the west. M y e r s (1975) related sedimentation and strong, localized deformation to block movements, the surface expression representing high-level penetration of ductile deformation above steep b a s e m e n t s h e a r zones parallel to the present coast. We now consider these features and the volcanic sequences to be intimately related to the deep faulting that is associated with crustal splitting and marginal basin development. Andean normal folding is generally subdued, with an open style, and developed just after or during a late stage ofthe Andean folding (Myers, 1974}.

Deep Structure and Nature of Crust Previously, it was thought the Iiuarmey Basin rested on old, thick, block-faulted continental crust (Cobbing, 1978; Cobbing et al., 1981). C e r t a i n l y south of lama, part of the equivalent Caflete Basin rests unconformably on the Arequipa Massif and its Paleozoic cover (Shackleton et al., 1979). In northwest Peru (Fig. 1), schists and gneisses thought to be equivalent to the Arequipa Massif are a p p a r e n t l y linked by a structural ridge on the submarine shelf - - the Outer Shelf High (OSH; Thornburg and Kulm, 1981, and Fig. 1). Submarine drill cores and outcrops on offshore islands (e.g., Isla Hormigas and Pimentel;

Volcanic facies, structure, and geochemistry of the marginal basin rocks of central Peru Atherton et al., 1983) indicate that the rocks may be similar to the schists to the north. This ridge formed a positive structure during the Early Cretaceous and isolated the basin from the western ocean (Myers,1975; Rivera et al., 1975; Cobbing, 1978). Recent interpretations of the geophysical data and geochemistry has indicated that the t t u a r m e y Basin is not underlain by continental crust of the Arequipa type and extends westwards to the OSH (Atherton et al., 1983, 1985b), which is a complex s t r u c t u r e of basement ridges of Paleozoic and/or Precambrian age (Couch et al., 1981; Jones, 1981). Beneath this large basin is an arch-like structure of 3.0 g c m ~ rock considered by Jones (1981) to be indicative of crustal rupture and by Couch et aI. (1981) to be due to f r a c t u r i n g and i n s e r t i o n of material from the mantle (Fig. 8). The structure is absent near Arequipa (just off the bottom of the inset map, Fig. 1), but to the north it forms a dense wedge of rocks in continental crust that widens and shoals to the north, centered near the coast south of Lima and mainly offshore at 9°S (south of Trujillo). Thus, s p l i t t i n g of the c o n t i n e n t a l c r u s t i n c r e a s e d northward and 3.0 g c m ~ density material reached progressively higher levels in the same direction. This coincides with increasing subsidence northward in the basin - - i.e., 3000 meters (Webb, 1976), 6000 meters (Myers 1974), and 9000 meters (Bussel, 1975) near Casma. In the northernmost part of the exposed basin near Trujillo, extension measured by dike displacement is up to 50%. This is compatible with more extension in the north and the exposure of deeper levels in the basin bringing the major dike swarms to the surface. The dilation, with an axis of extension inclined to the west, is consistent with the center of the spreading system lying offshore in the north (see Fig. 1). The metamorphism throughout the basin relates to the high geothermal gradients (Aguirre et al., 1978; Aguirre and Offler, 1985) characteristic of such an environment. The formation of the t t u a r m e y Basin may be related to the Albian crustal e x t e n s i o n seen in

253

southern and central Chile (Dalziel, 1981), and may be considered to be an extreme type of a b o r t e d marginal basin (Aguirre and Offler, 1985). Dikes The Casma Group is intruded by numerous dikes of basalt to basaltic-andesite composition. These vary from 20 cm to 8 meters in thickness but are usually about 50 cm to 2 meters thick. Their attitude is vertical or near vertical, often east-dipping, and they form an intersecting network in the Upper Pillow Lava Formations. Generally, dikes are more a b u n d a n t in the Pillow Lava and H y a l o c l a s t i t e Formations. The dikes are oriented in either an Andean trend concentrating at NNW-SSE, 333-340 ° , or a less important E-W trend, 070-100 °. The dikes in the Culebras section are all Andean normal and are demonstrably the youngest magmatic event as they cut all the formations. They appear to be associated with the late cross-folding seen in the basin (Myers, 1974). The d i k e s a r e c o m p o s i t i o n a l l y a n d mineralogically similar to and i n d i s t i n g u i s h a b l e from the lavas and clearly relate to similar source(s}. In the H u a r m e y and P a r a r i n a r e a s t h e y a r e hornfelsed by the Coastal Batholith and hence are pre-Batholith in age. The penecontemporaneous character of the dikes is illustrated by their form and associations. Thus, they often bifurcate, converge, or tail out completely, especially in the t t y a l o c l a s t i t e Formation. Dike swarms are restricted to the pillow lavas and hyaloclastites, where they may be separated by only 1 meter of rock, and appear to have been channel ways for the lavas of these formations. The most convincing argument for near-simultaneous deposition and intrusion comes from dikes in the Lower T u f f Formation in Quebrada Pararin where buckling and drag of the tufts adjacent to the dikes can be seen. Similarly, dikes may have sinuous m a r g i n s when intruded into pillow lavas, suggesting the latter were not solid on dike intrusion. (Fig. 9).

G E O C H E M I S T R Y OF T H E C A S M A G R O U P iCasmatBasin'' ,

The chemistry and petrology of the Casma Group rocks has been previously discussed by Atherton et al. (1983, 1985b). We discuss here, with additional data, the east-west variation across the basin, the secular variation, and the source of the magmas.

20.

East-West Variation Across the t t u a r m e y Basin

km

T Tertiary M Mesozoic Pz Palaeozoic Pe Precambrian

~

0

km

./ 20,0

Fig. 8. S u b c r u s t a l a n d c r u s t a l cross-section at. 9°S, from g r a v i t y d a t a l a f t e r J o n e s , 1981 ; vertical e x a g g e r a t i o n 5:1.

In various oxide and normative classification diagrams, the rocks of the ttuarmey Basin are seen to be mostly basalts or basaltic andesites, with less abundant dacites and rhyolites, and there is a clear break between the western and eastern facies (Fig. 10). Thus, the western facies rocks are p r e d o m i n a n t l y basalt and basaltic andesite while the eastern facies

254

M. P ATHERTONand S WEBB

~ - ' ~ - ' - ~ \ ,~"'-----'----'~ .~ ~.~

-_.~l:~

~=1

Tuffs

V ) d ~ - ; , ~ ~ ! Y ~ ~, '//.,,/,~

rocks are dacites and rhyolites. The division between the two facies is clear, apart from one "basic" rock from Tapacocha (eastern facies) and those rocks from NelSena that belong to the western facies but which lie in the dacite or dacite/andesite field ( e . g . , Fig. 10). In many respects these latter rocks are intermediate in character (see Figs. 10-13). The basic western facies rocks tend to low-K (tholeiitic) types and the e a s t e r n facies to high-K types (Fig. 10), or sub-alkaline and a l k a l i n e on a Na20+K2) v s SiO 2 diagram, respectively (Atherton et a l . , 1985b, Figs. 6,8). This l at t er d i a g r a m also shows the predominantly high-A12•3 charact er of the rocks. Dikes in the basin lie on the boundary of the calc-alkali/low-K field or in the low-K field and are indistinguishable from the lavas in major e l e m e n t composition. In an AFM diagram (Fig. 11), about a half of the rocks are tholeiitic with some variation in the M/FM ratios. These are restricted entirely to the western facies. The more acid rocks, almost entirely from the eastern facies, are calc-alkali in c h a r a c t e r . C a • shows a clear decrease with increasing SiO2 (Fig. 12) across the whole compositional range, while N a 2 0 v s SiO2 shows a very distinctive grouping of the two facies and the lack of a typical calc-alkali trend (Fig. 13). REE patterns for western facies rocks vary from l,REE-depleted (CeN/YbN=0.68) t h r o u g h CeN/YbN values about 1, to LREE-enriched rocks with ratios up to 2.6 (Fig. 16). Rocks from the Eastern Casma at Churin and Tapacocha are characteristically much

Pararin

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~

,

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~.lP2'~%-~.~"=:~;,.,~'~;~:~_'~YA I i

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Fig. 9. Top: schematic diagram showing a chilled dike intruded into tufts, with drag features near to the dike margin, indicating intrusion into unconsolidated sediment. Bottom: dike with sinuous margin intruding an unsolidified pillow lave sequence (after Webb, 1976).

Basalt

Dacite

Andesite

Basaltic

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• •

Churin

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3

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cies

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o• I

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~

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so

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•

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55

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i

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70

l

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Si02% Fig. 10. K . ~ O u s S i O , z d i a g r a m l b r r o c k s o f t h e C a s m a ( ] r o u p f r o m t h e H u a r m e y B a s i n . Note the h i g h - K c h a r a c t e r o f t h e e a s t e r n compared to the western facies. T, rock from Tapacocha with intermediate character (also in Figs. 1 l- 13 ).

Volcanic facies, structure, and geochemistry of the marginal basin rocks of central Peru

255

F

00 •

""

12

•

•

CaO%

Ooe oOOoo •

OO• •

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Fig. 11. AFM diagram for rocks of the Casma Group showing the tholeiitic character of the western facies rocks and the calc-alkali affinity of the eastern facies rocks.

°4'o

I

I

50

6O

Easter~ ~aties 7o

SiO% more LREE enriched (CeN/YbN =6.8-7.1 ), due in part to fractionation as the most acid rocks show well developed europium anomalies. Notably the most basic rock from the eastern facies at Tapacocha is more evolved than any equivalent western facies rock (Fig. 17). These geographical polarities in composition and e l e m e n t variation p a t t e r n s i n d i c a t e there is no simple direct magmatic connection between the rocks, rather the differences relate to varying sources across the basin and greater fractionation of the eastern facies rocks.

•

N~

"i'.;'." '"

Na20% 3'

Fig. 12. CaO u s SlOe plot of the Casma Group rocks showing a clear decrease in CaO with increasing SiO., content, and also across the basin from west to east.

Secular Variations in Basin-Fill Composition Variation in composition of the magmas filling the Huarmey Basin are most clearly shown by the REE's. The change in shape of the REE profiles with stratigraphic height (time) is shown in Fig. 16. There is a marked change from LREE enrichment in rocks at the bottom of the sequence through near flat

" EasternFacies

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2 •

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Fig. 13. Na.20 vs SiO z plot of the Casma Group rocks showing tile distinctive character of the western and eastern facies, the poor correlation, and the lack of a talc-alkali trend.

SAES 2'3

D

Th

Ta

Fig. 14. tif-Ta-Th plot of rocks of the Casma Group showing a mainly destructive plate character witl~ some rocks showing MORB-type tendencies.

256

M.P. ATHERTONand S. WEBB

T!02

CASMA

MnO x 10

P205xlO

Fig. 15. TiO2-MnO-P~O 5 diagram showing rocks of the Casma Group lying within the field of'continental basalts and island arc tholeiites.

2° f Culebras Tortuga

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10

La ' C ePr ' Nd ' ' ' Sm ' ' Eu ' Gd Tb D y Ho ' E*r'Tm Yb ' ~ Fig. 16. REE plots of' rocks from the western Casma Group showing the change in form from LREE-enriched at the bottom of the sequence to LREE-depleted rocks at the top. Stratrigraphy as in Figs. 2 and 6. Key: L, lava; T, tuff, D, dike. Figures in brackets are SiO z contents.

profiles in the middle horizons to slightly LREEdepleted rocks at the top of the sequence. This consistent gradation is observed in rocks throughout the western facies. Incompatible trace elements show a stratigraphicaily related pattern similar to that shown by the REE's (Fig. 19). Thus Sr, K, Rb and Th decrease in abundance upwards - - all e l e m e n t s e n r i c h e d via aqueous fluids from the descending slab. HFS elements have values very similar to typical tholeiitic MORB in the Upper Pillow lavas, a p a r t from Ti which shows a slight depletion, ttowever, there appears to be a more primitive MORB component in the dike and the Lower Pillow Lava F o r m a t i o n rocks. There is a suggestion that the dike from Tortugas is the most primitive rock on the basis ofZr, Hf, and Sm (note also LREE depletion, Fig. 16). The upward decrease in Ce and P - - t o g e t h e r with that in St, K, Rb, Ba and Th, which are elements enriched in calc-alkali oceanic b a s a l t s - indicates a systematic decrease in the subduction component with decreasing age. The relatively high Ta, together with the high value relative to Zr and ttf, suggests a contribution 5"om e l e m e n t - e n r i c h e d mantle as seen in within plate basalts; however, the expected higher Zr and Hf r e l a t i v e to Y and Yb, although clear in the lavas 389 SW and possibly 7.112, is not seen in the dike. Although this component is likely to i n c r e a s e into t he c o n t i n e n t , splitting of the crust and subcontinenta] mantle may have plumbed sources with variable amounts of this component. The rocks from the topmost part of the sequence (Fig. 19b) include a tuff which, apart from K and Rb, appears to be very similar to the lavas and shows a subduction component defined by St, K, Rb, Th, Ce, and P (_+ Sin) contents and a m a n t l e c o m p o n e n t defined by Ta, Zr, tlf, Ti, Y and, Yb characteristic of a primitive type MORB (Pearce, 1982). The blocks of agglomerate from I I u a r m e y (Fig. 19c) have similar" trace element plots, although 7.32 seems to have lost K and have lower Ta, Zr, HI', Ti, and Y contents, suggesting a more p r i m i t i v e type MORB source. The high Cr and Ni values are due to the lack of marked fractionation as shown by the high modal clinopyroxene in these porphyritic rocks. Two turfs fr()m the t l y a l o c l a s t i t e and B r o w n l,apilli Tuff Formations (Fig. 19d) are very similar, although they come from t r a v e r s e s some d i s t a n c e apart (Quebrada P a r a r i n and n o r t h of H u a r m e y ) . Both have a well-defined subduction zone component and have typical tholeiitic MORB-type s i g n a t u r e s with Y,ttf, and Zr near one while the h i g h e r concentration of Ta relative to Zr and H f a n d the slightly higher Zr and [If relative to Y, as well as the general e n r i c h m e n t in incompatible e l e m e n t s , indicate a slight within-plate component. In conclusion, it appears that the rocks have a decreasing subduction com ponent with youn g in g , and different MORB-type sources - - either typical or more primitive, plus a variable w i t hi n-pl at e component.

Volcanic facies, structure, and geochemistry of the marginal basin rocks of central Peru

257

Typology and Source of the Basalts

10C

50

L. "o r. 10 0 e,. 0

V202 {69) ~/

7o 0 n.,.

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(Huarmey)

10

I

I

I

I

I

I

Pr Nd

I

i

I

I

I

I

,

i

i

Sm Eu Gd Tb Dy Ho Er Trn Yb Lu

Fig. 17. REE plots of acid rocks V200 and V202, and a basaltic rocks from the eastern (7.63) and western (389) facies. Bracketed figures as in Fig. 16.

Comparison with Recent Spreading Systems and the Southern Chile Marginal Basin

Primordial Mantle ~ 6 o % 1000 4 ~ 40%

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On immobile element plots, the basalts generally show good groupings in the destructive plate or IAT field, with some rocks showing MORB or incipient MORB-type character (see Fig. 14). They lie within the field of continental basalts on a TiO2-MnO-P205 diagram (see Fig. 15), but in the ocean floor field on a Ti-Zr-Y diagram (Atherton et al., 1985a). As a group they contrast markedly with the post-batholith Calipuy volcanic sequence, which has a very characteristic continental calc-alkali character (Atherton et al., 1985b). The quartz normative character of most of the basalts, and their high AI203 contents, are the result of partial melting of water saturated mantle pyrolite (Atherton et al., 1985b). Furthermore, there is little chemical evidence of fractionation from a single source composition. Only the eastern facies rocks show evidence that might indicate they were produced by extensive fractionation. It seems that the western facies rocks had little residence time in the crust and were produced as separate magma batches, although some of the slightly more evolved rocks may have been produced by plagioclase and clinopyroxene precipitation on ascent from more primitive basalt. The source is thought to be spinel-plagioclase-lhertzolite, with magmas produced by 5-36% melting of this source (see Fig. 18l, which has been variably enriched in LIL elements and depleted in HFS elements.

I

I

I

I

I

I I I I

100

Yppm Fig. 18. Cr v s Y plot of rocks of the Casma Group illustrating the possible melting parameters o f a lherzolite source (at),er Pearce, 1982). The continuous, large range shown by the rocks emphasizes the primitive character of some western facies rocks and the fractionated nature ofthe eastern t'acies rocks.

The similarity between the Casma basalts with flat REE traces and Dredge-24 basalts from the backarc spreading center of the East Scotia Sea (Saunders and Tarney, ] 979) is striking. They are also similar with respect to REE's to the basalts from the Gulf of California (Saunders et al., 1982), which also show slight decreases from Sm to La reminiscent of MORB, although CeN/YbN values are quite close to 1. The more evolved early Casma Basin rocks show a calc-alkali component (Fig. 19) with characteristic peaks at Ce and Sm and relatively high P in the trace element diagrams. These rocks may be compared to the Bransfield Straight lavas, which formed during the initial stages of back-arc spreading in an ensialic marginal basin behind the South S h e t l a n d Arc (CeN/YbN=2 for Deception and Bridgman Islands; Weaver et al., 1979). The rocks also resemble rocks from the narrowest part of the Chilean m a r g i n a l basin - - viz. Sarmiento, which Saunders et al. (1979) considered were derived from mantle enriched by fluids or melt in LII, elements. The basalts to the south of Sarmiento, where the basin is at its widest (Tortuga), and in the Gulf of California at the gulf mouth show depleted REE patterns akin to normal MORB. They are very similar to some of the C a s m a rocks, e s p e c i a l l y the youngest dike cutting of the top of sequence, from

258

M . P . ATHERTON a n d S. WEBB

10 7.105 Dyke (Tortugas) Rock

20

MORB

10

Rock 101 MORB

22'OykeCuebras7

/ #/\-I•

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•

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M-6E'1[ff Rock

•

.

o

_ _ _

389 SW Lower Pillow Lava Fm.

0.1L. .

Sr K RbBaThTa NbCe P ZrHf SmTi Y Yb ScCr Ni

.

.

.

.

.

.

.

.

.

.

.

.

I

.

I

I

Sr K RbBaTh Ta NbCe P Zr Hf SmTi Y Yb 3c Cr

b

a 40

• 7.113 H Fm '

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Agglomerate- Huarmey• 7-32 Amygdaloidalcoarse o 7.33 Finer grained

i~i

10

Tufts

Rock MORB

M--5"fi-~ Rock .

0.1

0'1 .

Sr K Rb BaThTa NbCe P Zr Hf SmTi Y YbScCr Ni C

.

.

.

.

j

,

,

,

,

,

,

,

,

,

,

,

,

Sr K RbBaThTa NbCe P Zr Hf SmTi Y YbScCr Ni

d

Fig. 19. MORB-normalized trace element plots, a) Four rocks in stratigraphic order from bottom to top of the Casma sequence. The LIL elements show a clear decrease with younging. Formations relate to the stratigraphy shown in Fig. 2. b)Three rocks from near the top of the Casma Group succession showing rather low I,II, and HFS element contents, c)Two variants from an agglomerate pipe, near tt uarmey, d I Two tufts from the Hyaloclastite(H) and Brown Lapilli Tuff( BLT/G A ) Formations.

T o r t u g a . T h i s rock also has the lowest LIL e l e m e n t or s u b d u c t i o n c o m p o n e n t and has low t | F S e l e m e n t c o n t e n t s (Fig. 19a). Sr, K, Rb, Ba, Th, a n d Sr a r e all lower in the y o u n g e s t rocks c o m p a r e d to the o l d e s t C a s m a r o c k s (cf., G u l f of C a l i f o r n i a , w h e r e g u l f m o u t h rocks h a v e lower v a l u e s t h a n the i n n e r b a s i n b a s a l t s ) . S t e r n (1980) o b s e r v e d a s i m i l a r difference

in CeN/YbN a n d also in K/Rb a n d Rb/Sr b e t w e e n the two C h i l e a n c o m p l e x e s a n d c o n s i d e r e d t h e r e was a l s o a s e c u l a r c h a n g e in c o m p o s i t i o n . T h u s , b a s a l t s with r e l a t i v e l y low K / R b = 2 5 0 , h i g h R b / S r = 0 . 0 7 5 , a n d C e N / Y b N = I . 7 were e m p l a c e d in the e a r l y s t a g e s of d e v e l o p m e n t , while r e l a t i v e l y high K / R b = 8 5 0 , low R b / S r = 0 . 0 2 0 , a n d low CeN/YbN=0.6 b a s a l t s , s i m i l a r

Volcanic facies, structure, and geochemistry of the marginal basin rocks of central Peru to ridge segment oceanic tholeiites, were emplaced during later stages of basin evolution. Values for the Casma rocks are comparable: K/Rb 330--*730, Rb/Sr 0.07--*0.02, CeN/YbN 2.4--*0.7 going from the bottom to the top of the sequence. Interpretation of the above data and comparison with modern spreading systems suggest the mantle source beneath the Casma Basin varied systematically. Initially at the onset of splitting, the source had a marked calc-alkaline chemistry - - i.e., high Rb, Sr, Ba, K, and Th contents and high CeN/YbN and Th/Hf ratios, as in the Gulf of California (Saunders et al., 1982) and southern Chile (Stern, 1980). As continental splitting and spreading continued, basalts with LREE and I,ILE enrichment gave way to rocks with LREE depletion and lower LIL element values. This change is similar to that seen in the Brans field Straight (Weaver et al., 1979). ttowever, unlike the latter and other back-arc basins in the Pacific, there is no evidence of an active arc to the west during the formation of the t t u a r m e y Basin. It seems more likely that there was no contemporaneous subduetion, and basin formation was related to a splitting of the crust by a spreading system similar to t h a t postulated for the Gulf of California by Dickinson and Snyder (1979). Ilere, and probably in Peru, the calc-alkaline component in the lowest basinal rocks was inherited from an earlier subduction event that enriched the subcontinental mantle, and this was later tapped by the spreading, rifting system. Continued spreading with splitting (Atherton et al., 1983) of the continental crust in Peru split the calc-alkaline, enriched sub-continental mantle, and the later magmas that welled up from below were less enriched and contained a lower calc-alkaline c o m p o n e n t and a more MORB-like c h a r a c t e r . Variations in the Zr, tlf, and Ta contents suggest subcontinental material was also variably involved, while variations in the MORB-type component indicate a complex history of the source. Notable perhaps is the contrast with marginal basins in oceanic settings, which evolve to more MORB-like compositions than those generated in sub-continental lithosphere.

259

The Casma rocks show a marked polarity across the basin (west to east): the eastern facies being shallower, more acidic, and generally more pyroelastic in character than the deeper, basic western facies. There is also an axial deepening of the basin to the north as indicated by the pillow lava/hyaloclastite ratio. Surface structures are consistent with extension in an Andean normal direction and are associated with high heat flow up major vertical fault systems that lie parallel to the coast. The deep structure beneath the basin connects with the surface structures and indicates that splitting of continental crust produced the basin, with extension and upwelling of 3.0 gcm-3 mantle derived material from depth increasing northward. The major dilation seen in the north is consistent with this, as is the high heat flow consequent on the upwelling of the high density material flooring the basin and the massive basic magma extrusion. The chemistry of the Casma m a g m a s shows secular and Andean normal variations. The basin polarity is well seen with low-K tholeiitic basalts in the west and high-K dacites/rhyolites in the east, with intermediate types (calc-alkaline dacites) in between in an off-axis position. These variations related to lateral changes in source composition. Secular variations are well seen in the REE's and trace elements and indicate a calc-alkali source giving way on splitting to a more MORB-like source with a variable continental component. The source of the magmas appears to be spinel-plagioclase lhertzolite variably melted and enriched, with some fractionation occurring on ascent. The basin development in Peru is part of a major extension and/or rifting event in the Cretaceous that affected the whole western margin of South America south of Colombia (Dalziel, 1981; Atherton et al., 1983; Aguirre and Offler, 1985) and was a precursor for the major batholiths of the Andes which immediately followed basin formation (e.g., Coastal Batholith of Peru). This tectonic scenario of an extensional spreading volcanic system followed by massive batholith intrusion along and above it is no coincidence, and is a cipher for continental growth at Andean-type cordilleran continental margins.

CONCLUSIONS Facies analyses of the rocks of the l t u a r m e y Basin indicate a relatively deep sea environment with virtually no continental material deposited. Eruption of lavas along the rifting basin floor was akin to relatively slow spreading MORS and/or offaxis systems. Instability along Andean-trending fault scarps and ridges within the basin produced thick volcanielastic sequences deposited by turbidity currents and a elastic volcanic m61ange, emphasizing the contemporaneous nature of magmatism and deposition.

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260

M . P . ATIIERTON a n d S. W E B B

Atherton, M. P., Sanderson, L. M., Warden, V., and McCourt, W. J., 1985a. The volcanic cover: Chemical composition and origin of the m a g m a s of the Calipuy Group. In: Magmatism at a Plate Edge, the Peruvian Andes IEdited by W. S. Pitcher, M. P. Atherton, E. d. Cobbing, and R. B. Beckinsale}. Blackie Halstead Press, Glasgow, 273~284.

Fiske, R. S., and Matsuda, T., 1964. Submarine equivalents of ash flows in the Tokiwa Formation, ,Japan. A m e r i c a n J o u r n a l o f Science 262,76-106.

Atherton, M. P., Warden, V., Sanderson, L. M., 1985b. The Mesozoic marginal basin of central Peru: A geochemical study of within-plate-edge volcanism. In: Magmatism at a Plate Edge, the Peruvian Andes (Edited by W. S. Pitcher, M.P. Atherton, E. J. Cobbing, and R. B. Beckinsale~. Blackie tlalstead Press, Glasgow, 47-58.

Jones, P. R., 1981. Crustal structures of the Peru c o n t i n e n t a l margin and adjacent Nazca Plate, 9"S latitude. Geological Society of A merica , Memoir 154,423-444.

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