D. F. STRONG Department of Geology, Memorial University of Newfoundland, St. John's, Newfound- land, Canada Petrology of the Isknd of Moheli, Western Indian Ocean ABSTRACT the first petrographic descriptions and chemi- cal analyses of rocks from the Comores. De Three phases of volcanic activity, older, Saint Ours (1960) showed that the topography middle, and younger, are represented on and reef development of each island (Fig. 1) Moheli. The island was originally a large cen- indicate a progressive westward decrease in age, tral-type shield volcano, made up of ancient from deeply dissected Mayotte to the undis- lava flows, but it presently has an elongate sected and still active volcanos on Grande form reflecting fissure eruptions of intermediate Comore. phase lavas. Esson and others (1970), in a preliminary Silica-undersaturated lava, alkalic basalt, geological description of the archipelago, inter- basanite and melanephelinite, with lesser preted the linear topography and alignment of amounts of nephelinite, trachyte, and phonolite pyroclastic cones as indicating that volcanism are the main rock types found on Moheli. In- in the archipelago was controlled by two trusive equivalents occur in the form of alkali principal fissure systems. They indicated that gabbro, theralite, and ijolite. Inclusions of the dominant petrological association through- ultramafic rocks are common, and rare sand- out the archipelago is alkali basalt, basanite, stone inclusions are also present. nephelinite, trachyte, and phonolite, although Two separate petrochemical variation trends quartz-bearing gabbro on Anjouan is a notable are proposed, namely a low-pressure trend exception. from melanephelinite through nephelinite and Inclusions of sandstone are found in lava on trachyte to phonolite, caused by fractionation three of the islands, indicating the presence of of olivine, clinopyroxene, and (later) feldspar. underlying sandstone through which the lava A high-pressure trend from alkali basalt was erupted. For a discussion of the tectonic through basanite to melanephelinite probably significance of these sandstone inclusions, see results from variable (low) degrees of partial Strong and Flower (1969), Wright and Mc- melting of upper mantle material. A striking Curry (1970), and Strong (1970). low potassium content and consequent low The present report is the first detailed study K:Rb ratios in the basanite are tentatively of Moheli to be published, although it still explained as resulting from fractionation of must be regarded as preliminary in nature. amphibole. More detailed accounts for Anjouan (Flower, 1971) and Grande Comore (Strong, 1972) will INTRODUCTION soon be available. Location and Geological Setting GEOLOGY OF MOHELI The Comores Archipelago is situated at the northern entrance to the Mozambique Chan- Topography nel, between 11° and 13° S. and 43° and 46° E. Moheli, the smallest of the four main Comores (Fig. 1). The four main islands are, from east to Islands, lies 40 km southeast of Grand Comore west, Mayotte, Anjouan, Moheli, and Grand and 45 km southwest of Anjouan (Fig. 1). It Comore, distributed over a distance of 275 km is roughly elliptical in shape, 30 km by 20 km along a west-northwest-trending submarine pla- with its long axis trending parallel to the teau (Fig. 1). principal axis of the archipelago. It can be Lacroix (1922) was the first to point out the divided into two distinct topographic regions, volcanic nature of the islands, and he provided the relatively low-lying ( <400 m) eastern pla- Geological Society of America Bulletin, v. 83, p. 389-406, 14 figs., February 1972 389 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/83/2/389/3443106/i0016-7606-83-2-389.pdf by guest on 02 October 2021 390 D. F. STRONG Figure 1. Map of the Comores Archipelago. Note northwest directions, caused by eruption along fissures the diminishing importance of coral reefs (indicated by common to all the islands. Submarine contours in stippling) and progressively smoother topography of the meters. Surface contours at intervals of 500 m. Insert islands from east to west, reflecting their decreasing age. shows the location of the archipelago in the Mozambi- Note also the linear topography in roughly north and que channel. teau, and the more mountainous western area posedly formed during Miocene time. The (Fig. 2). The latter is dominated by a jagged "phase intermediare," of the eastern half, was median ridge which rises to a maximum height supposedly formed during middle to late of 790 m. The southern slopes of this ridge are Quaternary time. In this report, however, the distinctly more rugged than the northern ones, rugged topography of the west and the deep indented by amphitheater-headed valleys due soil cover in the east are taken to indicate that to more intense precipitation and erosion relative ages are in the reverse order of that caused by the prevailing southeasterly winds. suggested by De Saint Ours (that is, with the A group of small elongate islets lie off the south older part being in the east). This is also in line coast and, along with a number of peninsulas on with the age progression described above for this coast, radiate from the higher western the archipelago as a whole, although detailed peaks of mainland Moheli. The island is en- mapping and radiometric dates are necessary tirely surrounded by fringing reefs, but in to provide a definite statement of the ages of contrast to Anjouan and Mayotte, there is no each region. Instead of retaining the ter- indication of any barrier reef development. minology of De Saint Ours, the present writer prefers to use as the approximate English Geology equivalents the terms "Older" phase and De Saint Ours (1960) thought the western "Middle" phase to describe the eastern (in- mountainous half of Moheli represents the cluding southern islets) and western parts of oldest phase of volcanic activity on the island, Moheli, respectively (Fig. 2). which he called the "phase inferieur," sup- A still younger phase of volcanic activity, Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/83/2/389/3443106/i0016-7606-83-2-389.pdf by guest on 02 October 2021 PETROLOGY OF MOHELI, WESTERN INDIAN OCEAN 391 termed "Younger" phase here, is represented Younger lavas to be distinguished as a separate by localized areas of very fresh lava flows phase of activity in the present study. found primarily in the northern and western part of Moheli (Fig. 2). These flows are seen as Geological Development prominent areas of convex topography on the The present elongate form of Moheli is un- topographic map, in contrast to the concave doubtedly the result of repeated eruptions erosional slopes of the Middle lavas and low- along west-northwest fissures, a pattern clearly lying terrain of the Older lavas (Fig. 2). Al- established on the other three Comores Islands. though these flows are generally covered with However, the submarine contours are almost vegetation, they are easily recognized in coastal circular (Fig. 1), suggesting the former presence exposures (Fig. 3). of a large central volcano equal in size to that Numerous well-preserved cones of cinder and of Anjouan. This is also suggested by the small tuff are found on Moheli, generally aligned islets and peninsulas off the south coast, the parallel to the long axis of the island. Although radiating pattern of which is explicable in they appear to represent the most recent vol- terms of their representing flows filling former canic activity on the island, they do not appear river valleys in the ancient volcano and sub- significantly separated in time from the sequently preserved as resistant ridges. Figure 2. Simplified geological map and cross lines around coast indicate fringing reefs; numbers sections of Moheli. Shaded areas = Older phase lava; refer to sample locations; contour interval at 200 m. clear areas = Middle lava; irregular lined areas = Numbers refer to sample locations. Younger kva; radiating lines = pyroclastic cones; broken Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/83/2/389/3443106/i0016-7606-83-2-389.pdf by guest on 02 October 2021 392 D. F. STRONG Figure 3. Unconformity between fresh younger coast of Moheli, east of Bangoma. Hammer handle is 2 phase lava and weathered Middle phase lava. North ft long. With the above features in mind, a proposed cent in the present study (compare Coombs, sequence of events in the development of 1963). Thus it is difficult to draw any real Moheli is shown in Figure 4. distinction between these rocks on the basis of It is stressed, however, that detailed field normative nepheline, and this approach has relations on the island are still scanty, and that been avoided in the present study. The ratio of the main purpose of the present paper is to alkalis to silica is a convenient measure of their present what is known of the petrography and undersaturation and has been used in this geochemistry of samples considered to be fairly study to distinguish between alkali basalt, representative of the island. basanite, and melanephelinite (Fig. 5). These distinctions roughly correspond to 5 percent PETROGRAPHY and 14 percent normative nepheline. With an increase in alkalis and silica, and in Introduction the relative proportions of salic minerals, the basanite grades into nepheline hawaiite, and The extrusive rocks of Moheli consist of the melanephelinite grades through nephelinite alkali basalt gradational through basanite into into the trachyte and phonolite. The main melanephelinite. Although these gradations in- intrusive rock types found are alkali gabbro, volve increase in both normative and modal theralite, and ijolite, corresponding to the nepheline, a modal classification is not strictly alkali basalt, basanite, and nephelinite, re- adhered to in the present study. Furthermore, spectively. the amount of normative nepheline is partly The general features of these rock types are determined by the oxidation state of iron in the described below, and descriptions of the an- rocks, and this has been normalized to 2.00 per- alyzed specimens are given in Table 1.