The Neogene ash-flow caldera of A'ithayn: an eruptive centre upon the trap series in central North Yemen

Tect~ophy~ic~~

223

198 (1991) 223-238

Elsevier Science Publishers B.V., Amsterdam

The Neugene ash-flow caldera of A’ithayn: an eruptive centre upon the Trap Series in central North Yemen K. Heyckendorf

and D. Jung

ABSTRACT Heyckendorf, K. and Jung, D., 19%. The Neogene ash-flow caldera of A’ithayn: an eruptive centre upon the Trap Series in central North Yemen. In: J. Makris. P. Mohr and R. Rihm (Editors), Red Sea: Birth and Early History of a New Oceanic Basin. Tectonophysics, 198: 223-238. The main type of eruptive vents on the Yemen Plateau are dykes; ~nogenic and ~rn~site dykes are present. br addition, a mo~hol~c~ly well preserved ash-flow caldera with a diameter af 10 km is identified as the eruptive centre of extensive surro~~ng volcanics. It is situated in the centre of the Yemen Arab Republic, 40 km south of Sana’a and NW of Ma’bar. The caldera and its products are younger than the underlying Trap Series, and Perhaps of Pliocene age. in ~ornpa~~n to the 5-6.5 Ma old calderas in South Yemen the structure shows a much batter state of preservation. Striking differences exist between the silicic rocks erupted from the caldera and those in the underlying Trap Series. We emphasise that a high proportion of silicic rocks is a feature of the central Yemen Trap Series. In contrast to the Ethiopian plateaus, a dominant role for basalts is not evident at the structural levels examined.

Introduction

According to Gass (1975), u~o~~g of the Afro-Arabian dome started in late Eocene (ca. 40 Ma): continental rifting and crustal tinning opened pathways for ascending magmas, which led to the generation of the Ethiopian and Yemenite volcanogenic plateaus. In the Upper Oligocene/Lower Miocene, continental rifting resulted in the breakaway of Arabia from Africa, accompanied by the formation of the Red Sea basin (Chiesa et al., 1983). The initial, ‘“punctiform” ~ani~tion of the Red Sea started about 5 Ma ago (Bonatti, 1985). Tertiary volcanic racks of Yemen Arab Republic (Y.A.R.)

The Terti Trap Series of Yemen covers approximately 40,000 km2 (Fuchs, 1985; Capddi et al., 1988), about l/4 of Yemen Arab Republic. Lamare (1925, 1930), Roman (e.g. 1925), Comucci ~195~/9~/SO3.~

1991 - Elsevier Scietrce ~bI~sh~

(1930,1933), Rathjens and Wissmann (1934,1942), Lipparini (1954), Shukri and Basta (1955a, b), and Karrenberg f1956, 1959) contributed early works on the Tertiary magmatism in North Yemen. Reviews of regional ma~atic development of the Afro-Arabian dome have been made by Gass (197Oa, b, 1975) Brown and Jackson (1979). and Almond (1986). According to Chiesa et al. (1983) and Capaldi et al. (1988), the oldest dated Tertiary igneous rocks in Yemen are about 30 Ma old (i.e. Upper Oligocene). Lipparini (1954) mentioned widespread intra-Trapper lacustrine i~t~r~lations of Middle-Lower Eocene (‘“medio-inferiore”‘) age, but Geukens tl9~) assigned all Yemen intraTrappean sediments to the Oligocene/ Miocene. Recent mapping and dating of Yemen Arab Republic leads to new, evincing evidence indicating at least an Eocene age for the oldest volcanic rocks of the Trap Series (Kruck et al., 1992; Men&s et ai., 1990) Trap Series volcanics (Geukens, 1966) rest

B.V. All rights reserved

224

mainly

k HtY(‘KENIX)Kl

on

(Cretaceous)

the

continental

Tawilah

Sandstone

or locally on the Paleocene

Series (Kruck,

Medj-Zir

1983, 1984). In the east, they over-

.1

,,’ h

lap onto

Precambrian

1960. 1966: Grolier 1983; Chiesa

I)

,I

Vc,

rocks (Geukenx.

and Overstreet,

1978: Kruck.

et al.. 1983). Capaldi

et al. (1988).

Y

\

crystalline

&VI)

1

-\ \

n

in km

of A’ithayn. Tectonic setting. Distribution of Caldera lgnimbrite series (according to Kruck. 1983). A/ = A’ithayn; Al = Al Jiya; D = Daf; M = Ma’bar; Tj- Tafadil; TW = Tawalib: Y = Yakar. A-B = profile line of Fig. 5

Fig. 1. Caldera

THE NEOGENE

ASH-FLOW

YEM

CALDERA

225

QF A’ITHAYN

22

Tawali

b

-

7. C4lDqRA

IGNIMBRITE

-with

vltrophyrr

barol

0 trc

-

6. basalt

-5.

0

basalt

tbl

-wrth basal red brrccia

-I.

m

30

-

basalt

3.

COnglOmQratQ and coyllomrmtic

sandstone

20 M---S---

10 -2.

5 t

ignimbritt

e-----m-

- dominated

complax

0

trT

t. basalt, aphanitic

Fig. 2. Succession of the topmost part of the Yemen Trap Series and basal part of the Caldera Ignimbrite series (= ttC). Profile at the northern slope of summit 2817, SW of Tawalib. hT ~4Tawalib I~b~te, fbT = Tawalib Basalts.

226

and

Chiesa

northern

et al. (1989)

and

continental

distinguish

a southern flood

region

basalt

parent

magmatic

magmas,

province”

between

Wadi Zabid-Dhamar that volcanic

ceased

cene (26 Ma) in the northern of Yemen southern

parts

magmatic Oligocene (Capaldi

(Sa’dah

Trap

by contrastnature

of

contamina-

them approximates

line. Isotopic

activity

(i.e.

development,

and grades of crustal

tion. The border

a

of the “Yemen

Series). These two regions are marked ing temporal

between

the

data indicate Oligo-

and central

regions

and Sana’a areas), while in the a second

phase

of

activity developed from the Upper to the Lower Miocene (ca. 23-19 Ma) et al., 1988; Chiesa et al., 1989).

Basaltic dyke swarms and stock-like granophyre bodies intruded into the escarpment and Red Sea coastal plain zone (Tihama) (ca. 21 Ma) (Capaldi

rocks of unknown

tion and deposition; different

grades

ignimbrites bodies;

(b) plateau of

matrix

in combination

(d) cinerites;

in the Lower Miocene

et al., 1987c, 1988). Coeval

intercalations; glomeratic

(g)

mode of erupignimbrites,

welding;

with

(c)

rheo-

with basal vitrophyric

(e) basalt

ally wedge out rapidly

in the Upper

of the country

silicic volcanic

bodies.

which usu-

laterally;

(f) agglomeratic

conglomerates

and

sandstones;

and (h) paleosols

conat vari-

ous levels. According

to Chiesa

rocks of the Yemen alkali basalts,

hawaiites,

rhyolites.

The rhyolites

(Capaldi scarce.

et

al.,

et al. (1983)

Plateau

1988).

the eruptive

are characterized

mugearites,

trachytes

are of comenditic Intermediate

as and

nature

rocks

are

Chiesa et al. (1989) called the Yemen Trap Series a “continental flood basalt province”. This is a misleading characterization for central Yemen. According

to Capaldi

et al. (1988)

basaltic

with the younger Trap activity, voluminous alkaline to peralkaline granite plutons were formed

rocks “dominate” between Sana’a

in two main phases: late Upper Oligocene (ca. 26 Ma), and Lower Miocene (ca. 21 Ma) (Capaldi et

ordinate intercalations of silicic lavas and ignimbrites. But in the regional Trap pile, a high

al., 1987b; cf. Civetta

et al.,

proportion of acidic rocks is demonstrably present: basaltic to acidic ratio is ca. 1 : 1. For the

(1988) mention post-Trap Miocene age scattered over

Cainozoic Ethiopian volcanic province, Mohr (1968, 1971) cites a ratio of 6 : 1 for basaltic to

1989). Capaldi et al. volcanics of Up,per

et al., 1978: Coleman

the Trap sequences in the area and Dhamar, with only sub-

Y.A.R. (ca. 11-9 Ma, Capaldi et al., 1987~). In the area of Dhamar, another phase of activity is

acidic rocks, an unambiguous dominance basalts. To emphasize the high proportion

known,

strongly fractionated felsic volcanic rocks in the Yemen Trap Series, Grolier and Overstreet (1978) dropped the old name, and introduced the neutral

about

5 Ma old (Pliocene),

which perhaps

led up to the Pliocene/Recent volcanism of the Dhamar-Rada’ field. Kruck (1983, 1984) mapped volcanic rocks of questionable Pliocene age in

term “ Yemen

of of

Volcanics”.

central Yemen, which seem to occur as a unit with the regional Trap Series. These rocks are termed “Caldera Ignimbrites” in this paper (see below).

In large part the described profile belongs unambiguously to the Trap Series. Regional Trap activity ended in the Upper Oligocene in the

General lithology and stratigraphy of the Tertiary volcanics in central North Yemen

northern region according to Chiesa et al. (1989). The sequences which we call Caldera Ignimbrites are synonymous with “tr3” of Kruck (1983), and

A combined,

near-complete

profile

of the Trap

Series, of about 1000 m thickness, was sampled in central Yemen. The chemical composition of the volcanic rocks extends from extreme basic to acidic, with a range of nearly 40% SiO*. The uppermost members are profiled in Fig. 2. The Trap pile in central Yemen is built up of a sequence

of: (a) rhyolites-highly

differentiated

are perhaps of Pliocene age (Fig. 3). The type section comprises the following

units

(from top to base). 8. Caldera Ignimbrites < trC > 7. Tawalib Basalts < tbT > 6. Conglomerates and conglomeratic sandstones, outcropping best in the area of the village of Tawalib, S of Sana’a < tcT > 5. Tawalib Ignimbrite < trT >

cmLIxaa4

SOW dark budtic

and tuffaceoua

Ignisbrita lorutioa

tamlib chaotic

Series

(try)

Sana'a S*ries

CtblW

_~__________~___~~~~_I___________

Attan Pyrocfutie

Plateau Igniabritc

________________~___~~~~~~~~~~~~~





Conglomaratos

Taralib


ttrC>

ctbT>

Ignimbritar:

Tawalib Basaltr

Caldcra

mswm

cCw&rrl north Y-0, Leucocratic

pyroclastics

m?Cx 119t31

Igoimbrite yellowish.

with and 9rwuish

intercalated pink

tbl

flow

toff

flows

pyrcclutics

and gyroclartics

flows,

Chaotic Formath

stocbl

0%‘.

em_---

Leueocrrtic pyroclastics iacfudio~ layers of green taff _*--.. ___________-____________ tbl Basaltic pyroclutics, and flows

trl

____________________-~~------ ..__---

_______ ____________________-------tr2 Laucocratic pyroclastics ___________________________I______ tb2 Basaltic pyroclutic8, aaiafg ash and tuff

tb3 Basaltic

tr3 teueocratic

UUCK (198.6) (Sheet Al Kudagdoh 1:250.000~

of Kruck is comparable with the

mod pyrocla$ticr. intorealated lascocratic tuff containing basalt bombs

Basaltic

___________~*_~_________________I_______

trl

tb3 overlying trl, undivided _~________~__~__~~~~_________________I__

trl

and flora

and flow

(Sheet Sana'a' 1:250.000>

tb3 Kolaaocratic pyroclastics (basalt stocktf

trf

(Paleocene)

I

Fig. 3. Comparative stratigraphies, of central Yemen Tertiary volcanics: Sana’a Basalts to Caldera Ignimbrites. tb3/trl Kruck, BGR Hannover, pers. commun., 1991).

Tawilah Sandstone (CretaceouuJ I Wedj-Zir Series

TEyl predmiaaatly basaltic, but includes grem falsic coogiomxatr, porpbyritic tracbytr, and PiaL tufts (otrrliat the tarilab Wroup” * Tarilah Saodstoaa~

TKy2 predominantly falsic (older thaa TKp3)

TCyS predoninmtly fslsie aad tutfaceous (oldrr than TKylf

TKr4 pradiminallt1y flllBiC and tuffacrcw, with 8ow ba8a1tic flowa tm&rlie8 TKy%

?_~--*______--_______---~______________”~~

ritk

(1978)

felric tufts flows

olmm%sT cY4mi?n>

TK* genrra11y 18ucoerrtic

!

22x

4. Chaotic Formation < tCh > (cf. Fuchs, 1985), mainly built up of different rhyolites and various basaltic flows; 4.1. Rhyolitic to more intermediate rocks < trCh > ; 4.2. Basalts, with both aphyric and phyric types < tbCh > 3. Plateau Ignimbrite < trP > , a prominent acidic marker horizon, comprising two units (Kruck: trl partim), that covers a wide area 2. Attan Pyroclastic Series < trA > , dominated by green lithologies, with vitrophyre, of intermediate to acidic composition, and intercalations of conglomerates and finer-grained sediments (Kruck: trl partim); Attan village is situated 4.5 km SW of Sana’a 1. Sana’a Series < tbSa > (tbl of Kruck, 1983, 1984); dominated by basalts; our samples from top members are intermediate rocks (SiO, ca. 58%) From the central areas of the Trap Series, a maximum thickness of 200Q m is reported by Capaldi et al. (1988; cf. Grolier and Overstreet, 1978). An EXXON Oil Company well near Ma’bar ended at a total depth of 1600 m, without reaching the base of the Series. To the east, the total thickness rapidly decreases. Monogenic basaltic, subordinate acidic and composite dykes occur (Jung and Heyckendorf, 1988; cf. Chiesa et al., 1989). Some dykes have been traced for 10 to 20 km in the field and on satellite images. The composite, multiphase dykes contain both acidic and basaltic rock-types, with close petrographic and chemical relationships to corresponding eruptive rocks. Stock-like bodies, built up by volcanic breccia in a trachytic to more acidic host, are a second type of eruptive channel for the Trap Series. They are surrounded and crossed by numerous dykes. Examples are exposed in the village of Tafadil, NE Ma’bar (Fig. l), and at Jabal Fidah (Road entrance to Wadi Zahr, NW of Sana’a). A Neogene caldera eruptive centre Morphology and strucrure

A type of eruptive structure presently known from only one locality in Y.A.R., forms the circular caldera of A’ithayn, NW of Ma’bar (Fig. 1).

Fig. 4. Structure of the A’ithayn caldera (includes data of Kruck, 1983).

It is dominated by sequences of silicic ignimbrites. The outer rim has a diameter of 9-10 km, the floor 2.5-3 km. The 1: 250,000 geological map of Kruck (1983) cautiously calls this striking structure, which is clearly visible on satellite images, a “circular feature”. Grolier and Overstreet (1978) interpreted the outer ringfault as “large volcanic crater rimcrest”. Outside the caldera rim, volcanic layers dip radially away from the structure. Inside the circular rim, individual megablocks are tilted towards the centre (Fig. 4). Along the rim, and inside the caldera, the rocks have often been intensely affected by presumed fumarolic and hydrothermal activity. At the caldera western rim, a subparallel dyke set crops out, dipping eastwards towards the centre. This set may belong to a cone sheet system. The caldera is substantially eroded. The erosion products have been tran~rted and deposited into the central depression, resulting in a partial covering of the original morphology. The circular central plain has a median altitude of 2400 m a.s.1. (Topographic Map 1: 50,000 of the Y.A.R, Sheet ( 1444 Al > A’ithayn). The su~oun~ng summits in the north and south show respective maximum altitudes of 3004 and 2920 m, and in the east and west 2600 m a.s.1.

THE NEOGENE

ASH-FLOW

CALDERA

229

OF A’ITHAYN

Kruck,

A’ithayn caldera is situated inside the central Yemen graben system, which generally strikes in a regional NNW-SSE direction, parallel to the coastlines of the Red Sea. This terrain has also been affected by major NE-SW faults (Kruck, 1983). A relatively shallow magma chamber was positioned at the intersection of these two systems (Fig. 1). The A’ithayn caldera is positioned on an intragraben horst. At the eastern and western flank of the Qa al Haql depression, which borders the caldera to the west (Figs. 1 and 4; Capaldi et al., 1988, fig. 5) fractures with NNE-SSW strike are visible. The eastern slope of Qa al Haql reveals a vertical throw of at least 1000 m. The Qa al Jahran valley plain is located east of the structure, with an average altitude of 2340 m; on two sides it is cut by prominent NNW-SSE and NNE-SSE faults. At the eastern flank, in the village of Tafadil, the remnants of a stock-like vent crop out (Figs. 1 and 4).

1983) with underlying rocks (,‘ tb3” = “melanocratic pyroclastics and flows; basalt stocks”, Kruck, 1983) crops out (Fig. 3). Stock-like rocks, and horizontal and slightly dipping layered volcanic rocks occur inside the A’ithayn caldera. These comprise trachytes, acidic ignimbrites, cinerites, and agglomerates. They could be either caldera infill or relics of a collapsed caldera roof. Exotic xenoliths in the ignimbrites include angular fragments of fine layered pelites, possible indicators of a temporary lake inside the caldera. The Caldera Ignimbrites cover a NE-SW elongated area of about 30 x 15 km2. Together with a median thickness of at least 100 m, a preserved volume of more than 45 km3 is established (Fig. 1). Grolier and Overstreet (1978) assigned these rocks to their unit “Tkys” (“generally leucocratic felsic tuffs with some basaltic flows, associated with the formation and collapse of a circular volcanic structure.. . “) (Fig. 2), belonging to the young “Yemen Volcanics” (Trap Series of Geukens, 1966).

Caldera-associated

Lithology of Caldera Ignimbrites

flows”;

Tectonic setting The

volcanic rocks

Caldera products outside the structure comprise the characteristic, acidic Caldera Ignimbrites, deposited on top of the Trap volcanics in this area. NE of Ma’bar, N of Yakar (Fig. l), the slightly discontinuous contact of the Caldera Ignimbrites (“tr3” = “ leucocratic pyroclastics and

A

Al Jiya

kithayn

al

Daf 1

1

1

10s

The base of the Caldera Ignimbrites is well exposed at the northern slope of summit 2817 SW of Tawalib, and NE of Wa’alan (Locality YEM 22, Figs. 1 and 2). On top of the underlying Tawalib basal& a thin layer (0.5 m) of an acid ash tuff is followed by vitrophyre of varying thickness.

Haql-1

LQa ca.

Fig. 5. Schematic W-E profile

through

25

km

B

Tafadil I

al JahranJ~“~ *

the central Yemen graben, N of Ma’bar. Altitude in m a.s.1. Profile line: gee Fig. 1.

230

Above the vitrophyre, the typical facies of Caldera Ignimbrites is developed: porphyritic rocks that are clearly ignimbritic, and light grey in colour. They weather strikingly white to yellowish, or else with thick black weathering crusts. Both columnar jointing and parallel bedding structures occur. Locally, intraformational breccia are intercalated with the ignimbritic rocks; they contain clasts with diameters exceeding 10 cm.

In the basal ash tuff, dominant phenocrysts are subhedral felspars with diameters up to 0.8 mm, which occupy 20% of the total volume. Sanidine predominates over scarce plagioclase. Only a few fragments of tschermakitic amphibole and salitic augite occur, with sizes comparable to that of the felspars. Scattered magnetite is accessory. About 20 vol.% of angular fragments of brownish pumice have diameters up to 0.8 mm, and contain round, sometimes collapsed vesicles. The matrix is built up of pumice shards which are partly altered; acicular zeolites form an outer rim, and the cores consist of chlorite. Pores are filled with secondary acicular zeolites. Fragments of angular quartz, considered to be derived from the underlying Cretaceous and Paleocene sediments, with diameters again up to 0.8 mm have a modal value up to 20 vol.%; they do not show any unambiguous features of volcanic generation such as magmatic resorption. Singular extremely fine-grained basaltic xenoliths have a maximum diameter of 0.15 mm.

schlierenic. Perlite cracks are visible: perlite, as hydrated obsidian, represents the first step of glass alteration. Hydration is usually accompanied by ion-exchange with groundwater, involving in particular K, Na, Si and Ca (Fisher and Schmincke, 1984). Separated orbicules (spheroids) with diameters up to 3 cm occur abundantly at some levels. and indicate high-temperature devitrification (Hughes, 1982). Devitrified trachytic to rhyolitic lithoclasts with diameters of 1-2 cm develop rims as a result of thermal contact. In some samples from the base, these clasts are highly concentrated. Rocks of the higher vitrophyre levels contain only a few xenoliths. Other features reveal differences: phenocrysts, concentrated up to 5 vol.%, consist mainly of euhedral oligoclase and andesine, with sizes up to 1 x 0.5 mm. Sanidines are an order smaller, only 0.1 X 0.02 mm. Opaque ore patches show diameters exceeding 0.1-0.15 mm. Tschermakitic amphiboles are rare. Roundish, flattened and altered glass shards reach maximum sizes of 3.5 mm, but the majority is smaller. These compacted shards are arranged parallel to each other. Their outer rim of quasi-isotropic zeolite encloses acicular natrolite. The core is built up of a mosaic of heulandite, but some cores remain hollow. These observations point to a high mobility of chemical components during the advanced stage of alteration of silicic glass (cf. Fisher and Schmincke, 1984). Complete dissolution of shards is followed by precipitation of zeolites into these secondary cavities (Fisher and Schmincke, 1984; Jezek and Noble, 1978). The reddish matrix is partly submicroscopically devitrified.

Vitrophyre

Cover of the vitrophyre

Rocks of the basal vitrophyre differ from those from higher levels. Only a few subhedral to cataelastic and corroded oligoclase crystals occur, with diameters up to a maximum of 1 mm. Most euhedral plagioclases are smaller than 0.1 mm, as are the sanidines. A few clearly corroded augites occur. Magnetite with a maximum diameter of 0.1 mm, and ilmenite up to 0.2 mm, are scattered through the matrix. The structure of the pale reddish matrix, which occupies 80 vol.%, is

Phenocrysts occupy about 10% of the total volume. Some oligoclases reach maximum sizes of 1 x 0.5 mm, but the majority are around 0.05 X 0.01 mm. Some plagioclases are zoned. Grass-green chlorite is pseudomorphous after prismatic mafic minerals. Small amounts of magnetite and apatite are scattered through the matrix which is submicroscopically devitrified. The structure is schlierenic/pseudofluidal, with felspars arranged parallel to one another. Lithophysae are filled with

Mineralogy of Cakra

Ignimbrites

Basal ash tuff

THE

NEOGENE

ASH-FLOW

CALDERA

231

OF A’ITHAYN

TABLE 1 Yemen Trap Series-XRF-analyses Stratigr. unit: trA SampIe: Y 131 LocaIity:YEM14

Y 133 YEM14

of sihcic volcanic rocks of defied

trP Y 126 YEMll

YEM22

73.69 11.70 2.70 0.30 0.07 0.08 0.19 3.97 5.44 0.49 0.05 0.69 0.13 0.01 99.51

73.85 11.23 2.51 0.22 0.03 0.13 0.17 4.01 4.92 0.42 0.06 0.78 0.12 0.01 98.52

67.39 13.66 4.79 0.16 0.14 0.18 1.18 4.37 3.19 0.81 0.20 1.65 0.64 0.01 99.05

71.21 13.69 1.77 0.26 0.03 0.13 0.38 3.23 6.87 0.64 0.06 0.64 0.84 0.01 99.11

71.65 13.78 1.53 0.19 0.02 0.08 0.36 3.40 6.54 0.63 0.06 0.69 0.97 0.01 99.16

25.50 0.97

3.03 9.00 0.02 9.41 0.73 49.53 1.06

2.81 11.68 0.04 8.93 0.82 52.53 1.06

4.97 29.94 0.03 8.16 1.1s 6.91 0.83

2.06 6.81 0.05 10.10 0.47 26.58 0.93

1.74 8.05 0.04 9.94 0.52 27.61 0.92

95.26

1.07 93.04

0.98 93.38

85.23

94.17

94.72

1.89 5.64 0.06 9.01 0.65

1.04 92.37

93.96

93.5s

0.33 91.91

92.95

6.38 11.17 ‘7.81 3.06 5.95

YEM22

2.52 19.83 0.04 8.29 0.94 25.91 0.95

I.77 89.06

4.76 12.38 9.49 3.65 8.45

YEM22

2.98 5.37 0.03 9.94 0.70 22.09 1.06

0.05 12.25 0.39 35.00 1.00

2.94 12.80 8.32 3.89 8.S9

YEM14

0.09 0.05 0.10 0.16 2.19 7.19 0.60 0.03 1.48 0.52 0.01 98.97 2.97 31.89 0.03 9.97 0.28 62.31 0.90

Ti/Zr Zr/Y Zr,‘Nb Zr/Ce Zr/Nd

Y177A

YEM9

73.39 12.10 1.58 0.28 0.01 0.13 0.34 3.55 5.46 0.62 0.12 1.14 0.31 0.01 99.03

3.19

1646 212 n.d. n.d. 12 110 128 n.d. 24 58 151 648 83 109

Y177B

YEM14

72.69 11.83 2.38 0.12 0.04 0.13 0.32 4.02 4.27 0.59 0.09 0.97 0.67 0.01 98.12

3.23 20.40 0.04 9.28 0.22 23.80 0.99

446 190 11 n.d. n.d. 13s 34 n.d. 15 56 102 693 73 82

trT Y 176

71.57 12.29 2.47 0.46 0.07 0.11 0.45 4.10 5.84 0.48 0.02 1.14 0.23 0.06 99.29

5.01 17.44 0.03 11.83 0.25 31.13 1.04

Trace elements (ppm): Ba 616 336 Ce 12 Cr n.d. Ni Pb 18 Kb 145 26 Sr 25 Tit V 27 Y 102 Zn 145 Zr 1306 Nb 157 152 Nd

Y 145

69.76 13.33 2.87

64.57 15.12 3.19 0.00 0.74 0.19 0.35 3.43 8.82 0.69 0.09 0.74 0.45 0.01 98.39

Mg’ Na,O-f-I&O Na ,O/K,O Alk./CaO AI. Acmite (CIPW) D.I.

Y 114

Y 113 YEM9

12.54 11.03 3.06 0.15 0.06 0.17 0.39 1.66 1.62 0.55 0.12 1.24 0.59 0.05 99.23

FesO: Fe,O,/FeO

Y 143

Y 112 YEM9

Major elements (wt.%): 65.66 SiOz 13.60 A12o3 4.71 Fe203 0.27 Fe0 0.07 MnO 0.15 MgQ 0.38 CaO 2.39 Na,O 9.44 KzQ 0.64 TiOz 0.15 PZQS 0.76 HsO+ 0.76 H,O0.01 80, 98.99 Sum

595 14s n.d. n.d. 10 121 61 11 19 53 117 672 79 79 5.35 12.68 8.51 4.63 8.51

347 187 n.d. n.& 26 137 40 16 n.d. 71 176 957 108 9s 3.01 13.48 8.86 5.12 10.07

Y 144 YEM14

horizons

775 168 n.d. n.d. n.d. 90 103 n.d. 22 53 118 583 73 94 6.07 11.00 7.99 3.47 6.20

633 174 n.d. n.d. n.d. 67 84 n.d. 15 48 87 589 73 79 6.31 12.27 8.07 3.39 7.46

206 234 n.d. n.d. n.d. 128 24 17 n.d. 64 160 965 107 10s 3.04 15.08 9.02 4.12 9.19

241 173 n.d. n.d. n.d. 130 34 17 15 66 127 925 104 77 2.72 14.02 8.89 5.3s 12.01

855 147 n.d. n.d. n.d. 68 229 n.d. 19 65 176 683 62 78 7.11 10.51 11.02 4.65 8.76

757 154 n.d. n.d. 11 88 10s n.d, n.d. 52 136 755 62 68 5.08 14.52 12.18 4.90 11.10

7% 176 n.d. n.d. 16 9s 107 12 15 59 I70 778 68 82 4.88 13.19 11.44 4.42 9.49

Fe0 anaIyxed by modified vanadate met&d of Peters (1968). FesO$’ = total iron as FezO,. Mg’ = Mg/(Mg + Fe*++ Fe3+). A.I. = agpaitic index (Shand, 19Sl), D.I. = differentiation index (Thornton and Tuttle, 1960). Stratigraphic units: see text. Localities: YEM9 = J. Buraya, ESE AI ‘Ubr, SE Sana’a; YEMll = Quarry SW of the oil depot of Sana’a; YEMl4 = between Haddah and Sana, SW Sana’a; YEM22 = SW Tawahb, NE Wa’aIan, S Sana’a.

232

K. HEYCKENDORF

AND

D. .IUN(i

twinned heulandites. Rounded basalt lithoclasts, with intersertal to sub-ophitic structures, are not affected by any contact reactions. TABLE

2

Central

Yemen Caldera

Ignimbrites

(trC)-chemical

analyses

Sample:

Y1821

Y 179

Y180

Y174

Y181

Locality:

YEM22

YEM22

YEM22

YEM21

YEM22

Major elements (wt.%): SiO*

68.73

70.24

70.98

72.55

76.21

A1203

13.18

13.80

14.01

13.35

10.17

Fe203

1.13

2.81

2.48

1.68

1.73

Fe0

1.02

0.36

0.55

0.23

0.39

MnO

0.13

0.10

0.06

0.08

0.04

MgO CaO

0.56

0.37

0.35

0.16

0.30

1.40

1.64

1.47

0.80

1.16

Na,O

3.40

4.74

4.78

4.38

3.35

K2O

4.57

3.29

3.55

3.75

2.73

TiO,

0.52

0.75

0.73

0.46

0.53

p29

0.09

0.17

0.16

0.06

0.12

H,O+

3.79

0.26

0.74

0.90

1.63

H,O-

0.53

0.46

0.20

0.42

1.02

so3

0.01

0.01

0.01

0.06

0.02

99.07

98.99

100.07

98.88

99.40 2.16

Sum Fe,O;L Fe,O,/FeO

2.25

3.21

3.09

1.93

1.11

7.81

4.51

7.30

4.44

Mg’ Na,O+K,O

0.18

0.06

0.09

0.07

0.11

7.97

8.03

8.33

8.13

6.08

Na20/K20

0.74

1.44

1.35

1.17

1.23

Alk./CaO

5.69

4.90

5.67

10.16

5.24

A.I.

0.80 _

0.82

0.84 _

0.84

0.83

87.20

83.22

87.33

91.66

90.33

Acmite (CIPW) D.I. Trace elements

(ppm):

Ba

690

667

689

737

Ce

128

99

116

107

Cr

n.d.

n.d.

n.d.

n.d.

nd.

Ni P

nd.

n.d.

n.d.

n.d.

n.d.

21

14

19

21

15

Rb

104

91

103

108

62

Sr

267

383

377

200

296

Th

13

12

15

16

n.d.

V

n.d.

n.d.

14

n.d.

nd.

Y

43

42

43

46

Zn

102

103

107

91

76

Zr

334

333

330

368

234

495 83

Rocks of higher levels

Phenocrysts again form about 10% of the total volume. Zoned, magmatically corroded, and cataclasised plagioclase (andesine) is again dominant and of varying sizes. Only a few salitic augite crystals occur, reaching diameters of 0.3 mm. Opaque ore grains show maximum diameters of 0.1 to 0.5 mm. The matrix is totally devitrified, and consists of microlites and schlieren oriented subparallel to bedding. On the walls of horizontal movement cracks, alkali felspar, tridymite, limonite, and a small amount of zircon have recrystallized. The presence of tridymite, together with the high grade of welding, points to the originally high temperature of these ash-flow materials. The precipitation of secondary zircon indicates that Zr must have been in solution in the fluid phase; yet Zr is regarded as being “immobile” in magmatic rocks (Floyd and Winchester, 1975, 1978; Winchester and Floyd, 1977). The horizontal cracks generate typical macroscopic separation planes in the Caldera Ignimbrites. They may be shear planes, produced when the ash clouds collapsed and laminar flow movements continued. Similar features are known from rheo-ignimbrites, such as those from Monte Amiata (Toscana, Italy) (i.e. Rittmann, 1958,1963, 1981). The matrix often shows spherulitic devitrification. Crescent-like contraction lithophysae around the orbicules are coated or filled with alkali felspar and/or mosaic quartz. Basaltic xenoliths are

34

Nb

38

36

39

40

28

Notes to Table 2:

Nd

48

47

57

55

34

Y1821=

basal

analyzed

by

Ti/Zr

9.33

13.50

13.26

7.49

13.59

Zr/Y

7.77

7.93

7.67

8.00

6.88

Zr/Nb

8.79

9.25

8.46

9.20

8.36

Zr/Ce

2.61

3.36

2.85

3.44

2.82

Zr/Nd

6.96

7.09

5.79

6.69

6.88

vitrophyre, modified

with

F%Os* = total iron as FqOs. AI. = agpaitic

index

dex (Thornton

and

Abu Rayan, NE Wa’alan,

(Shand, Tuttle,

WSW Wa’ahm, S Sana’a.

high

vanadate

contents

method

of

Mg’ = Mg/(Mg+

of HaO. Peters

Fe2+ + Fe3+ ).

1951), D.I. = differentiation 1960).

Localities:

S Sana’a;

Fe0

(1968).

YE&ill

in-

= Hish

YEM22 = SW Tawalib,

THE

NEOGENE

ASH-FLOW

CALDERA

233

OF A’ITHAYN

a single flow unit or one cooling unit of the Caldera Ignimbrite sequence. The ash flow(s) must have formed at relatively high temperatures. The unwelded, ash-tuff-like deposit is followed by vitrophyres. The higher levels reveal a progressive increase in welding, and patterns of growing intensity of temperature and fluid phase reactions in the rocks (see Hughes, 1982; Fisher and S&mincke, 1984; Schmincke, 1988). Mineralogy demonstrates that the geochemical results (see below) should be treated with caution, since the ash-flow deposits have been affected by both deuteric processes and diagenetic alteration.

Subalkaline

-I I

20

*,l,‘i’,‘l*t 55

60

65

70

75

Geachemisw

60

SiO2 Mt.-%) Fig. 6. SiO,/total alkalies diagram of silicic horizons of Trap Series and Caldera Ignimbrites samples. Trap Series domain cross-hatched; dots indicate Caldera Ignimbrites. Dividing line according to Macdonald (1968).

0.5 to 1.0 mm in diameter, as are individual, rounded quartz grains. At the eastern caldera rim ignimbritic autobreccia were found. They consist mainly of angular fragments of densely welded ignimbrites, with eutaxitic matrices. The above observations present the typical sequence for (of base and higher levels of, perhaps)

t

\

031 -

3 Sr

Rb

Chemical analyses reveal significant differences between Caldera rebates and those of the older acidic horizons in the Trap Series (Tables 1 and 2). On the Total Alkali-Silica (TAS) diagram (Fig. 6) the Caldera Ignimbrites plot as subalkaline rocks of rhyolitic composition (Irvine and Baragar, 1971; Macdonald, 1974; Le Bas et al., 1986: cf. Heyckendorf and Jung, in prep.). The Attan Pyroclastics, Plateau Ignimbrite, and Tawalib Ignimbrite show a general tendency towards more alkaline features (Table 1). In particular, silica and alkalis must be regarded as late/postma~ati~~y

;

Y t Ba

Nb

Ce

Zr

TiOp

Y

Fig. 7. Selected by~o~~atop~le element abundances for Central Yemen Trap series (shaded) and Caldera Ignimbrites (striped), Dots show mean values (n = 10) of Yemen Tertiary alkali granite of Jabal Hufash, sampied at the Sana’a-Al-Hudaydah road (Coleman et al., 1989; Heyckendorf and Jung, in prep.). Circles: Jabal Hufash. Values normalized to granite (S&roll, 1975).

234

K.HEYCKENDORFANDI).JI!N(;

mobile in ash-flow deposits (Fisher and Schmincke, 1984). Secondary enrichment or depletion is common in these rocks. A review and cross-check of the results of TAS classification is therefore necessary. The best method, in view of the problems described above, is to check against “immobile” element contents (Floyd and Winchester, 1975, 1978; Winchester and Floyd, 1977). Ranges of selected hygromagmatophile elements (Fig. 7) reveal a clear and systematic difference between Trap Series and Caldera Ignimbrites rocks, especially for the immobile elements. Nb, Ce, Zr (Fig. 8) and Y are relatively enriched in the acidic rocks of the Trap Series. In principle, these elements can develop diadochic relations (G. Bayer et al., J. Felsche and A.G. Herrmann in Wedepohl, 1969-1978). Figure 9 demonstrates a good correlation between Zr and Y. Mean values of these elements for the Tertiary alkali granites of Jabal Hufash (A-type granite, Coleman et al., 1989) fall within the ranges in acidic rocks of the Trap Series. Among the Taupo rhyolites and pantellerites from Major Island, New Zealand (Hughes, 1982) the peralkaline pantellerites have higher abundances of REE and Nb, and one order higher contents of Zr. Intermediate to acidic, agpaitic 102/1306 t

B 20

&O

80 Y

Fig. 8. Si02/Zr

IOa

(ppm)

diagram for silicic rock samples from the Trap

Series and Caldera Ignimbrites.

Cross-batched

field defines the

Trap Series domain (Table 1). Dots: Caldera Ignimbrites.

50

1

55

f

60

502 Fig. 9. Y/Zr

I

I

65

I

I

70

I

80

(wt.-%1

diagram for silicic rock samples

Series and CaIdera Ignimbrites.

1

I

75

Cross-hatched

from the Trap field defines the

Trap Series domain (Table 1). Dots: Caldera Ignimbrites.

rocks generally tend to enrich Zr (Scharbert, 1984) as well as REE, Nb and Y (Schroll, 1976). In general, incompatible elements can be highly concentrated through melting or crystallization processes (Cox et al., 1979). Enrichment of REE in acidic alkaline rocks, compared with basaltic rocks of the same magma association, provides an indicator of processes of crystal differentiation (S&roll, 1976). Some Trap Series rocks analysed in this work have relatively high alkali element contents (Fig. 6, Table 1). Normative acmite values (Macdonald, 1974), and agpaicity indices exceeding 1 (Shand, 1951) indicate peralkalinity. In the case of the Caldera Ignimbrites, however, these features are not developed: the agpaicity index ranges 0.80 to 0.84, and normative acmite is absent (Tables 1, 2). This clear difference between Caldera Ignimbrites and the central Yemen Trap Series (Tables 1, 2) confirms the results of Chiesa et al. (1989) based on a wider sampling of Trap Series localities. Whereas the acidic rocks of the Trap Series and the Tertiary alkali granites of Jabal Hufash present unambiguously alkaline to peralkaline tendencies, this is not the case for the Caldera

THE

NEOGENE

ASH-FLOW

CALDERA

Ignimbrites. Although Nb, Ce, Zr and Y are enriched, relatively to talc-alkaline granite (Fig. 7), this is not as strong as in the Trap Series. Detailed geochemical results of our investigations of the central Yemen Trap Series will be presented elsewhere (Heyckendorf and Jung, in prep.). Discussion

235

OF A’ITHAYN

and conclusions

According to Mahood (1984) a characteristic feature of older, deeply eroded calderas is exposure of a “shield sequence.. . that dips radially away from the center of the shield”. He quotes the South Yemen calderas as examples (Gass and Mallick, 1968; Cox et al., 1969). In the case of the A’ithayn structure, this pattern is also clearly developed, together with blocks systematically tilted in towards the centre of the caldera. The effects of fumarolic/hydrothermal activity are especially visible at the caldera rim. On satellite images the caldera rim rocks are marked by characteristic colour differences, also a feature of the regional Quaternary volcanic edifices. Gass and Mallick (1968) described the post-collapse infill of the upper Miocene caldera of Jebel Khariz (South Yemen) as comprising agglomerates, horizontally layered eruptive rocks, and stocks. The North Yemen caldera infill shows close parallels with the equivalent Jebel Khariz rocks. The rocks derived from the A’ithayn caldera are chemically different from the underlying, older silicic rocks of the Trap Series. The Caldera series consists mainly of acidic ignimbrites with rheoignimbritic features. High-temperature deposition is indicated by, for example, the presence of tridymite. Thus the caldera is classified as an “ash flow caldera” according to the scheme of Wood (1984), equivalent to the “Valles type” of Williams and McBimey (1968). Usually, the collapse of ash-flow calderas follows eruptions of extremely large volumes (100-1000 km3) of dacitic to rhyolitic ash flows (Wood, 1984). At A’ithayn the preserved volume is relatively small (> 45 km3). The associated magma chamber could not have been very shallow, since, unlike in the case of many other calderas, roof collapse is not totally complete. This is represented by the ring of intra-

caldera blocks, which are merely tilted towards the centre of the structure. Ash-flow calderas are characterized by median diameters of about 25 km. At only 10 km, the A’ithayn caldera is a relatively small structure (Wood, 1984, fig. 3). According to Wood, however, a “subclass” exists, bound to the East African Rift, with median diameters of about 11 km. Calderas are very often located with graben systems (Walker, 1984), and this is the case with the A’ithayn caldera. Besides the A’ithayn caldera, Grolier and Overstreet (1978) show further “large volcanic crater rimcrests” related to the Trap Series on their geological map of Yemen. According to Chiesa et al. (1989), the Trap Series is mainly the product of fissure eruptions: “central edifices and lavadomes contributed little or nothing” to build up the of volcanic vents sequence. “Individualization” developed subsequently. By contrast, Coleman et al. (1989) point to “welded tuffs and concentric faults around some of the Tertiary granitic intrusions”, which could indicate caldera collapses as early as mid-Tertiary times. Rocks associated with the younger Tertiary calderas in South Yemen (Aden, Little Aden, Jebel Khariz) have been dated at 6.5 to 5.0 Ma (Cox et al., 1969). The erosion of these structures is significantly greater than that of the A’ithayn caldera. Yet the state of preservation is dependent on lithology; the densely welded ignimbrites around the A’ithayn caldera have a relatively strong resistance to erosion. Nevertheless, the good preservation of this edifice shows that the caldera must be younger than 26 Ma when, according to Capaldi et al. (1988) and Chiesa et al. (1989) Trap activity in the “northern region” ended. Capaldi et al. (1988) reported two younger phases of Tertiary magmatic activity in Yemen, around 10 Ma and around 5 Ma (Upper Miocene and Lower Pliocene, cf. Kruck, 1983). In light of these observations and data, an ascribed age of Neogene for the A’ithayn caldera seems justified. Acknowledgments

This publication is our initial contribution to an understanding of the Cainozoic volcanism of

236

central Yemen (Y.A.R.). The project was sponsored by the German Science Foundation (DFG) (Project JU 60,/27-l). Fieldwork was carried out during the project “Structure of the southern Red Sea”, organized by the Institute of Geophysics, Hamburg University, and sponsored by the Federal German Ministry of Research and Technology (BMFT), and which we are grateful to have been invited to join. Our Yemenite colleagues supported our efforts in the field, and provided valuable help in overcoming administrative problems. On behalf of many others we would like to thank Said AI Dubai, Abdul Wameed Ahmed Saleh, and, last but not least, Deputy Minister Eng. Ali Jabr Alawi, Ministry of Oil and Mineral Resources (MOMR) of the Yemen Arab Republic. References Almond, D.C., 1986. The relation of Mesozoic-Cainozoic volcanism to tectonics in the Afro-Arabian Dome. J. Volcanol. Geotherm. Res., 28: 225-246. Al-Shanti, A. (Editor}, 1979. Evolution and Miner~~tion of the Fabian-Nubian Shield. Proc. Symp. Inst. Appl. Geoi. Jeddah Bull., 3 (1). Pergamon, Oxford, 187 pp. Bailey, D.K.. Barberi, F. and MacDonald, R. (Editors), 1974. Oversatured Peralkaline Rocks. Bull. Volcanoi., Spec. Issue, 38 (3). Bonatti, E., 1985. Punctiform initiation of seafloor spreading in the Red Sea during tr~sition from a continental to an oceanic rift. Nature, 316 (6023): 33-37. Brown, G.F. and Jackson, R.O., 1979. An overview of the geology of western Arabia. In: A. Al-Shanti (Editor), Evolution and Mineralization of the Arabian-Nubian Shield. Proc. Symp. Inst. Appl. Geoi. Jeddah Bull., 3 (1): 3-10. Capaldi, G., Chiesa, S., Conticelli, S. and Manetti, P., 1987a. Jabal An Nar: an upper Miocene volcanic centre near Al Mukha (Yemen Arab Republic). J. Volcanoi. Geotherm. Res., 31: 345-351. Capaldi, G., Chiesa, S., Manetti, P., Orsi, G. and Poli, G., 1987b. Tertiary anorogenic granites of the western border of the Yemen Plateau, Lithos, 20: 433-444. Capaldi, G., Manetti, P., Piccardo, G.B. and Poli, G., 1987~. Nature and geodynamic significance of the Miocene dyke swarm in the North Yemen (YAR). Neues Jahrb. Mineral., Abh., 156 (2): 207-229. Capaldi, G., Chiesa, S., Civetta, L., La Volpe, L., Manetti, P., Orsi, G. and Piccardo, G.B., 1988. Magmatic and tectonic activities in North Yemen during Tertiary and Quatemary times. Mem. Sot. Geol. Ital. (for 1986), 31: 375-393.

Chiesa, S., La Volpe, L., Lirer, L. and Orsi, G., 1983. Geologycal and structural outline of Yemen Plateau: Yemen Arab Republic. Neues Jahrb. Geol. Pallontol., Monatsh., 1983 (11): 641-656. Chiesa, S., Civetta, L., De Fino, M., La Volpe, L. and Orsi, G., 1989. The Yemen Trap Series: genesis and evolution of a continental flood basalt province. J. Volcanol. Geotherm. Res., 36: 3377350. Civetta, L., La Volpe, L. and Lirer, L., 1978. K-Ar ages of the Yemen Plateau. J. Voicanol. Geotherm. Res., 4: 307-314. Coleman, R.G., De Bari. S. and Peterman, Z., 1989. A-type granite and the Red Sea opening. Afro-Arabian Rift System, Symposium, Karlsruhe 1989 (manuscr.). Comucci, I’., 1930. Sopra Alcune Rocce Vulcaniche della Regione di Sanaa (Arabia). Mem. Sot. Toscana Sci. Nat. Piss. 40: 70-78. Comucci, P., 1933. Rocce deli0 Iemen Raccolte dalla Missione de SE. Gasparini. Period. Mineral., 4: 89-131. Cox, K.G., Gass, LG. and Mallick, D.I.J., 1969. The evolution of the volcanoes of Aden and Little Aden, South Arabia. Q.J. Geol. Sot. London (for 1968), 124: 283-308. Cox, K.G.. Gass, LG. and Mallick, D.I.J., 1970. The peralkaline volcanic suite of Aden and Little Aden, South Arabia. J. Petrol., 11 (3): 433-461. Cox, K.G., Bell, J.D. and Pankhurst, R.J., 1979. The Interpretation of Igneous Rocks. Allen and Unwin, London, 450 PP.

Fisher, R.V. and Schmincke, H.-U., 1984. Pyroclastic Rocks. Springer, Berlin, 472 pp. Floyd, P.A. and Winchester, J.A., 1975. Magma type and tectonic setting discrimination using immobile elements. Earth Planet. Sci. Lett., 27: 211-218. Floyd, P.A. and Winchester, J.A., 1978. Identification and disc~~nation of altered and metamorphosed volcanic rooks using immobile elements. Chem. Geol., 21: 291-306. Fuchs, D., 1985. Die tern&en Vulkanite der Arabischen Republik Jemen. Eine geologische und geochemische Studie zur petrologischen und genetischen Stellung dieser Vulkanprovinz am sijdtitlichen Ende der Grossstruktur des Roten Meeres. Ph.D. thesis, Univ. Saarbriicken, 198 pp. (unpubl.). Gass, LG., 1970a. The evolution of volcanism in the junction area of the Red Sea, Gulf of Aden and Ethiopian rifts. Philos. Trans. R. Sot. London, A 267: 369-381. Gass, I.G., 1970b. Tectonic and magmatic evolution of the Afro-Arabian dome. In: T.N. Clifford and I.G. Gass (Editors), African Magmatism and Tectonics. Oliver and Boyd, Edinburgh, pp. 285-300. Gass, LG., 1975. Magmatic and tectonic processes in the development of the Afro-Arabian Dome. In: A. Pilger and A. Rosier (Editors), Afar Depression of Ethiopia. Proc. Symp. Afar Region and Refated Rift Problems, Bergzabern, April 1-6, 1974. Schweizerbart, Stuttgart, 1: 10-17. Gass, LG. and Mallick, D.I.J., 1968. Jebel Khariz: an upper Miocene strato-volcano of comenditic affinity on the south Arabian coast. Bull. Volcanol., 32: 33-88.

THE

NEOGENE

ASH-FLOW

CALDERA

OF A’ITHAYN

Geukens, F., 1960. Contribution a Ia &oIogie du Yemen. Inst. Geol. Louvain M&m., 21: 122-179. Geukens, F., 1966. Geology of the Arabian Peninsula, Yemen. Translated by S.D. Bowers. Geol. Surv. Prof. Pap., 560-B, I-V: l-23. Grolier, M.J. and Overstreet, W.C., 1978. Geologic map of the Yemen Arab Republic (San’a) 1 : 500,000. Misc. Geol. Invest., Map I-1143-B.U.S. Geoi. Surv., Washington DC. Heyckendorf, K. and Jung, D., 1989. Tertiary Trap volcanism of the Yemen Arab Repubhc-first results of 1988 field campaign. Ann. Geophys., 1989 Spec. Iss., XIV General Assembly European Geophys. Sot.. Barcelona 13-17 March, 1989, 91. Heyckendorf, K. and Jung, D., 1992. Contributions to the understanding of geochemistry and petrology of the Tertiary volcanic rocks in central Yemen (Yemen Arab Republic). In prep. Hughes, C.J., 1982. Igneous Petrology. DeveIopments in Petrology, 7. Elsevier. Amsterdam, 551 pp. Irvine, T.N. and Baragar, W.R.A., 1971. A guide to the chemical classification of the common volcanic rocks. Can. J. Earth Sci., 8: 523-548. Jezek, P.A. and Noble, DC., 1978. Natural hydration and ion exchange of obsidian: an electron ~croprobe study. Am. Mineral., 63: 266-273. Jung, D. and Heyckendorf, K., 1988. Geologisch-petrograph&he Gelandearbeiten tiber tertilre und quart&e Vufkanite der A.R. Jemen. Im Rabmen des BMFT-Projektes “Geophysikalische Untersuchungen im shdlichen Roten Meer” (SO 53) des Institutes fiir Geophysik der Universitlt Hamburg. Ber. Mineralogisch-Petrographisches Institut der Universitat Hamburg, pp. l-63 (unpubl.). Karrenberg, H., 1956. Junger Magma&mm und Vulkanismus in ~dwestarabien (Jemen). 20th Int. Geol. Congr., Mexico City 1956, Resumenes, Sect. l(1): 171-185. Karrenberg, H., 1959. Junger Ma~t~smus in SW Arabien (Jemen). Tech. Mitt. Krupp, 17 (1): 33-36. Kruck, W., 1980. Geological map of the Yemen Arab Republic. Sheet Al Hazm 1: 250,000. Federal Inst. Geosci. Nat. Resources, Hannover. Kruck, W., 1983. Geological map of the Yemen Arab Republic. Sheet San’a’ 1:250,000. Federal Inst. Geosci. Nat, Resources, Hannover. Kruck, W., 1984. Geological map of the Yemen Arab Republic. Sheet Al Hudaydah 1: 250,000. In cooperation with A. Anissi and M. Saif. Federal Inst. Geosci. Nat. Resources, Hannover. Kruck, W., Schlffer, U. and Thiele, S., 1992. Geologic mapping of the Yemen Arab Republic. In prep. Lamare, P., 1925. Le volcanisme dam Ie Yemen. Bull. Volcanol., 3/4: 109-112. Lamare, P., 1930. Les manifestations volcaniques post-c&a&es de la Mer Rouge and des pays limitrophes. Sot. G&I. Fr. Mem., n. ser. 6, 14: 221-223. Le Bas, M.J., Le Maitre, R.W., Streckeisen, A. and Zanettin,

231 B., 1986. A chemical classification of volcanic rocks based on the total Alkali-Silica diagram. J. Petrol., 27 (3): 74% 750. Lipparini, T., 1954. Contributi alla conoscenza geologia de1 Yemen {SW Arabia). Boll. Serv. Geol. Ital., 76: 95-118. Macdonald, G.A., 1968. Composition and origin of Hawaiian lavas. Geol. Sot. Am. Mem., 116: 477-522. MacDonald, R., 1974. Nomenclature and petrochemistry of the peralkaline oversatured extrusive rocks. In: D.K. Bailey, F. Barberi and R. MacDonald (Editors), Oversatured Peralkaline Rocks. Bull. Volcanol., 38 (3): 498-516. Mahood, G.A., 1984. Pyroclastic rocks and calderas associated with strongly peralkaline magmatism. J. Geophys. Res., 89 (BIO): 8540-8552. Menzies, M., Bosence, D., El-Nakhal, H.A., Khirbash, S., Al-Kadasi, M.A. and Al Subbary, A., 1990. Lithospheric extension and the opening of the Red Sea: sediment-basalt relationships in Yemen. Terra Nova, 2 (4): 340-350. Mohr, P.A.. 1968. The Cainozoic volcanic succession in Ethiopia. Bull. Volcanol., 32: 5-14. Mohr, P.A., 1971. Ethiopian rift and ptateaus: some vokanic petrochemical differences. J. Geophys. Res., 76 (8): 19671984. Peters, A., 1968. Ein neues Verfahren zur Bestimmung van Eisen(II)oxid in Mineralen und Gesteinen. Neues Jahrb. Mineral., Monatsh., 1968 (3/4): 119-135. Rathjens, C. and Wissmann, H. von, 1934. Stidarabien-Reise, 3, Landeskundliche Ergebnisse. Friederichsen, DeGruyter and Co., pp. l-229. Rathjens, C. and Wissmann, H. von, 1942. ~b~htungen in Yemen. In: H. von Wissmann, C. Rathjens and F. Kossmat (Editors), Beitrage zur Tektonik Arabiens. Geol. Rundsch., 33: 248-279. ~ttmann, A., 1958. Cenni sulle colate di i~mb~ti. BOB. Acad. Gioenia Sci. Nat. Catania, Ser. IV, 4: 524-533. Rittmann, A., 1963. Erkiirungsversuch zum M~hanismus der Ignimb~tausb~che. Geol. Rundsch. (for 1962), 52: 853861. Rittmann, A., 1981. Vulkane und ihre Tltigkeit. 3. viiliig umgearb. A&., Enke, Stuttgart, 399 pp. Roman, D., 1925. Studii petrograficie in Yemen (Regiunea Hodeida-Sanaa). Inst. Geol. al Romaniei Anuarul, 10: 301-341. Scharbert, H.G., 1984. Einftthrung in die Petrologie und Geochemie der Magmatite, Band I. Allgemeine Probleme der magmatisch~ Petrologie und Geochemie. Unter Mitarbeit von R. Fischer. Deutike, Wien, 312 pp. Schmincke, H.-U., 1988. Pyroklastische Gesteine. In: H. Ftichtbauer (Editor), Sedimente und ~imentgest~ne. Schweizerbart, Stuttgart, pp. 731-778. Schroll, E., 1975. Analytische Geochemie. Band I: Methodik. Enke, Stuttgart. Schroil, E., 1976. Anaiytische Geochemie, Band II. Gnmdlagen und Anwendungen. Enke, Stuttgart. Shand, H.W., 1951. Eruptive rocks. 4th ed., Wiley, New York, N.Y., 488 pp.

238

Shukri,

N.M. and

Basta,

kaline rocks of Yemen. Shukri,

N.M. and

Basta,

E.Z., 1955a.

Petrography

Inst. Egypte E.Z., 1955b.

A classification

pyroclastic

rocks of Yemen (Egyptian

Expedition

to S.W. Arabia).

Smith,

R.L., 1960. Zones

W.E.

Elston

(Editors),

Scientific

Inst. Egypte Bull., 36: 165-175.

Ash-Flow

Geol.

Sot.

C.P. and Tuttle,

rocks. 1. Differentiation Walker,

G.P.L..

Williams. physical

and

Winchester,

Am.

H. and

O.F.,

1960. Chemistry

of igneous

index. Am. J. Sci., 258: 664-684.

1984. Downsag

calderas,

ring faults,

growth.

J. Geophys.

Res.. 81~

caldera

1969-1978.

McBirney.

crimination

and

Eugene, Floyd,

of different usmg

A.R..

of Geochem-

1968. Geologic

of Calderaa.

of Oregon, J.A.

Handbook

Berlin.

Features

tion products

Spec. Pap., 180: 5-27. Thornton,

caldera

K.H. (Editor).

istry. Springer,

University

In: C.E. Chapin Tuffs.

Wedepohl.

ash

in welded

154-159.

magmatism.

sizes, and incremental X407- 8416.

of the

University

and zonal variations

flows. Geol. Surv. Prof. Pap., 354-F: Smith. R.L., 1979. Ash-flow

of the al-

Bull., 36: 129-163.

Center

immobile

and Gee-

Volcanology.

87 pp.

P.A..

magma

for

1977.

Geochemical

dis-

series and their differentiaelements.

Chem.

Geol..

20:

325-343. Wood.

C.A., 1984. Calderas:

phys. Res., 89: X391-8406.

a planetary

perspective.

J. Geo-