44 Earth and Planetary Science Letters, 106 (1991) 4 4 - 6 3 Elsevier Science Publishers B.V., A m s t e r d a m
[CHI
Sr-Nd-Pb isotopic evolution of Gran Canaria: evidence for shallow enriched mantle beneath the Canary Islands
Kaj H o e r n l e a,1, G e o r g e T i l t o n a a n d H a n s - U l r i c h S c h m i n c k e b Department of Geological Sciences, University of California, Santa Barbara, CA 93106, USA
b GEOMAR, Wischhofstrasse 1-3, D-2300 Kiel-14, German),"
Received July 24, 1990; revised version accepted June 13, 1991
A B S T R A C T
We report the Sr, N d and Pb isotopic compositions (1) of 66 lava flows and dikes spanning the circa 15 Myr subaerial volcanic history of G r a n Canaria and (2) of five Miocene through Cretaceous sediment samples from D S D P site 397, located 100 km south of G r a n Canaria. The isotope ratios of the G r a n Canaria samples vary for SVSr/S6Sr: 0.70302-0.70346, for 143Nd/]a4Nd: 0.51275-0.51298, and for 2°6pb/2°4pb: 18.76-20.01. The Miocene and the P l i o c e n e - R e c e n t volcanics form distinct trends on isotope correlation diagrams. The most SiO2-undersaturated volcanics from each group have the least radiogenic Sr and most radiogenic Pb, whereas evolved volcanics from each group have the most radiogenic Sr and least radiogenic Pb. In the Pliocene-Recent group, the most undersaturated basalts also have the most radiogenic Nd, and the evolved volcanics have the least radiogenic Nd. The most SiO2-saturated basalts have intermediate compositions within each age group. Although the two age groups have overlapping Sr and N d isotope ratios, the Pliocene-Recent volcanics have less radiogenic Pb than the Miocene volcanics.
At least four c o m p o n e n t s are required to explain the isotope systematics of G r a n Canaria by mixing. There is no evidence for crustal contamination in any of the volcanics. The most undersaturated Miocene volcanics fall within the field for the two youngest and westernmost Canary Islands in all isotope correlation diagrams and thus appear to have the most plume-like (high 23SU/2°4pb) HIMU-like composition. During the Pliocene-Recent epochs, the p l u m e was located to the west of G r a n Canaria. The isotopic composition of the most undersaturated Pliocene-Recent volcanics m a y reflect entrainment of asthenospheric material (with a depleted mantle (DM)-like composition), as p l u m e material was transported through the upper asthenosphere to the base of the lithosphere beneath G r a n Canaria. The shift in isotopic composition with increasing SiO2-saturation in the basalts and degree of differentiation for all volcanics is interpreted to reflect assimilation of enriched mantle (EM1 and EM2) (cf. [1]) in the lithosphere beneath G r a n Canaria. This enriched mantle m a y have been derived from the continental lithospheric mantle beneath the West African Craton by thermal erosion or delamination during rifting of Pangaea. This study suggests that the enriched mantle c o m p o n e n t s (EM1 and EM2) m a y be stored in the shallow mantle, whereas the H I M U c o m p o n e n t m a y have a deeper origin.
1. Introduction
In recent years, there has been an explosion in the amount of Sr-Nd-Pb isotope data from ocean islands. At least four components are required to explain the range in the ocean island Sr-Nd-Pb basalt (OIB) data [1]. These are depleted mantle (DM), high-~ ( 2 3 8 u / / Z ° 4 p b ) mantle (HIMU), and two enriched mantle components (EM1 and EM2).
The location of these components in the mantle
Present address: D e p a r t m e n t of Earth Sciences; University of California; Santa Cruz, CA 95064, USA.
remains a topic of c o n j e c t u r e - - i.e., whether they reside in the lithosphere, asthenosphere a n d / o r mantle plumes. Knowledge of the location and distribution of these components, however, is cru- cial in understanding their origin. Although man- tle xenoliths in OIB could potentially provide im- portant information about the composition of the oceanic lithosphere, the xenoliths are almost al- ways found in the highly SiO2-undersaturated basalts erupted during the latest evolutionary stages of ocean island volcanism. Therefore, even if the xenoliths have not re-equilibrated with the host basalts, the original isotopic composition of
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Sr-Nd-Pb ISOTOPIC E V O L U T I O N O F G R A N C A N A R I A 45
the xenoliths may have been significantly altered by interaction with the magmas that form the island [2].
In order to define and to constrain the location of the different isotopic endmembers observed in the Canary Islands, we have undertaken a detailed isotopic study of well-documented volcanic rocks from Gran Canaria. In addition to age constraints, major and trace element data are available on all of the samples [3,4,5 and references therein]. To- gether with the 36 ignimbrites and lava flows from the upper Hogarzales, Mogan and lower Fataga Formations (ca. 14.1-12.5 Ma) studied by Cou-
sens et al. [6], the 66 samples from lava flows and dikes in this study provide one of the most de- tailed data sets on a single volcano. The variation in isotope ratios with age, degree of differentia- tion, SiO2-saturation in the basalts, and magma production rates constrain the composition of the lithosphere, asthenosphere and the plume beneath the Canary Islands.
2. General geology
The 600 km long Canary Island chain sits on the continental rise (Fig. 1), adjacent to the lower
N
T
K E Y
~ Plio-Recent Tejeda Fmn.
Fetaga Fmn.
Mogan Fmn.
Shield Basalts
0 10
• , |
KILOMETRES
Fig. 1. Geologic m a p of G r a n Canaria with an inset m a p of the Canary Islands [4].
46 K . H O E R N L E E T A L .
Proterozoic to Archean (1.8-3.4 Ga) West African Craton [7]. G r a n Canaria, the third most easterly island, is located 150 km from the West African continental margin. Seismic refraction studies in- dicate that up to 10 km of sediments m a y underlie the easternmost islands. Igneous oceanic crust un- derlies at least G r a n Canaria and the western islands [7,8]. Based on the magnetic anomalies to the north, the age of the oceanic crust beneath the Canary Islands p r o b a b l y ranges from 160 to 180 Ma [9]. While each Island, except G o m e r a , has had volcanic activity within the last 5000 yr [9], the age of the oldest subaerially exposed volcanics on each island decreases from east (30-80 Ma) to west (2 4 Ma) [10,11], consistent with a hotspot origin for the Canary Islands and with estimates for plate motion of 2 - 8 m m / y r [12-14]. The presence of multiple cycles of volcanism on the older islands in the chain and the overall low productivity of the Canary hotspot suggests an origin involving an intermittent plume, or blobs, rather than a continuous plume [4,5].
The - 1 5 Myr subaerial volcanic history of G r a n Canaria can be divided into two major cycles of volcanism (Fig. 2): the Miocene or shield cycle ( < 15 9 M a ) - - s i m i l a r to the Hawaiian preshield,
0-
2 -
8<'-
>-
§8
3 1 0 -
12:
14:
SUBAERIAL VOLCANIC HISTORY ~
m CVOLOAN,OS D EVOLVEOVOLe' ,OS
NEPHELINITES- ALKALI BASALTS 1 .Q
hi, .~E...- M ELILITITES - BASANITES 4
~ i i ~ ALKAU BAsALTs" TRACHYTES, BASANITEs - PHONOLITES ~ _ ~
BASANITES - THOLEIITES - -
~-NEPHELINITES- BASANITES 1 m
~ T NEPHELINITES,
RACHYPHONOLITES 4 0111
ALKALI BASALTS - PERALKALINE RHYOLITES Z
~ PICRITES - THOLEIITES Ill
1000 2000 3000 4000 5000
ERUPTION RATE (km3/Ma)
Fig. 2. Eruption rate versus age for the three cycles--Miocene (shield), Pliocene (first rejuvenated) and Quaternary (second rejuvenated?)--of subaerial volcanism on Gran Canaria [4]•
Stippled pattern denotes predominantly marie (Mg# > 62) volcanism; whereas areas with no pattern denote periods of predominantly evolved (Mg# < 62) volcanism. The Miocene cycle makes up -80% of the subaerial volume, the Pliocene
cycle - 18% and the Quaternary cycle - 2%.
shield and postshield s t a g e s - - a n d the Pliocene or first rejuvenated cycle (5.5-1.7 M a ) - - s i m i l a r to the Hawaiian rejuvenated stage [4,5,9]. Quaternary (1.3-0 Ma) volcanism on G r a n Canaria m a y rep- resent the initiation of a second rejuvenated cycle.
Cycles can be further subdivided into as m a n y as four stages (see Fig. 2) [4,5]. A submarine Miocene stage 1, c o n t a i n i n g highly u n d e r s a t u r a t e d volcanics, is inferred from the neighboring island of Fuerteventura (Fig. 1), where the earliest volcanics, which are intercalated with continental rise sediments, are subaerially exposed [11]. The oldest subaerial Miocene volcanics on G r a n Canaria (Guigui Formation, > ca. 14.3 Ma) be- long to the end of stage 2 and consist of picrites and tholeiites (Figs. 2 and 3). Miocene stage 3 volcanics (Hogarzales and Mogan Formations, ca.
14.3-13.5 Ma) become progressively more dif- ferentiated upsection and range from tholeiites and alkali basalts to trachytes and peralkaline rhyolites. Miocene stage 4 volcanics and intrusives (Fataga and Tejeda Formations, ca. 1 3 - 9 Ma) are
the m o s t S i O 2 undersaturated, consisting of
trachyphonolites, nepheline syenites and rare nephelinites. Eruption rate and m a g m a production rates (i.e., eruption rate corrected for volume loss due to crystal fractionation) were at a m a x i m u m during stage 2 and decreased throughout stage 3 (Fig. 2) [4]. There was a mild resurgence in both rates at the beginning of stage 4, followed by a continual decrease until volcanism ceased. During the Miocene, volcanism was highly centralized, with most m a g m a s p r o b a b l y passing through a c o m m o n plumbing network enroute to the surface.
The Pliocene cycle began after a > 3 Myr hiatus in volcanism (Fig. 2). Pliocene stage 1 volcanics (El Tablero Formation, ca. 5.3-4.8 Ma) consist of highly SiO2-undersaturated basalts, in- cluding nephelinites, basanites and rare tephrites.
During Pliocene stage 2 (lower Roque Nublo Group, ca. 4.7-4.0 Ma), only basalts were erupted, which became more SiOz-saturated, from basanites to alkali basalts to tholeiites, with decreasing age.
Pliocene stage 3 (upper Roque N u b l o Formation, ca. 3.9-3.4 Ma) contains complete suites of alkali basalts through trachytes and basanites through phonolites, with the latter suite becoming more prevalent higher in the section. Eruption and m a g m a production rates increased systematically with decreasing age during stages 1 and 2 and
S r - N d - P b I S O T O P I C E V O L U T I O N O F G R A N C A N A R I A 4 7
20- MIOCENE VOLCANICS
15-
h ~
+10-
z 5 -
UV
0 i i i i i i i
20- PLIOCENE - RECENT VOLCANICS
Z 5
0 U
35 4'0 45 5'0 5'5 6'0 6'5 7'0 75 SiO 2
Fig. 3. Plot of SiO 2 versus the alkalis for the Miocene and Pliocene-Recent volcanics on Gran Canaria (only samples for which isotope data is available from this study and Cousens et al. [6] are included). The Miocene volcanics have been divided into three groups based on age and chemical compositions: (1) undersaturated volcanics (UV), which consist of nephelinites and trachyphonolites = stage 4; (2) saturated primitive volcanics (SPV), which have SiO 2 > 43 and magnesium number (Mg#) > 62, and include picrites and tholeiites = stage 2; and (3) saturated evolved volcanics (SEV), which have Mg# _< 62 and range from tholeiites to peralkaline rhyolites = stage 3.
The Pliocene-Recent volcanics have also been divided into three groups: (1) undersaturated primitive volcanics (UPV), which have SiO 2 < 43 and MgCX > 62, and range from basanites to melilitites = primarily stages 1 and 4; (2) saturated primitive volcanics (SPV), which have SiO 2 > 43 and Mg# > 62, and range from basanites to tholeiites = primarily stage 2; and (3) evolved volcanics (EV), which have Mg# < 62 and range from alkali basalts through trachytes and basanites through phono- lites = primarily stage 3. The Miocene and Pliocene-Recent UV and UPV groups are collectively referred to as the most undersaturated groups, and the SEV and EV as the evolved groups. XRF data is from Cousens et al. [6], Hoernle and
Schmincke [4] and references therein.
t h e n d e c r e a s e d d u r i n g stage 3 (Fig. 2) [15]. F o l l o w - ing a p o s s i b l e b r i e f h i a t u s in v o l c a n i s m , t h e r e was a r e s u r g e n c e in v o l c a n i s m , d u r i n g w h i c h o n l y h i g h l y u n d e r s a t u r a t e d b a s a l t s were e r u p t e d . T h e s e P l i o c e n e stage 4 b a s a l t s ( L o s L l a n o s a n d L o s P e c h o s F o r m a t i o n s , ca. 3 . 2 - 1 . 7 M a ) r a n g e in c o m - p o s i t i o n f r o m b a s a n i t e to melilitite. D u r i n g s t a g e 4, b o t h S i O 2 - s a t u r a t i o n a n d e r u p t i o n r a t e in-
c r e a s e d i n i t i a l l y a n d t h e n d e c r e a s e d . T h e o l d e s t Q u a t e r n a r y b a s a l t s t h a t h a v e b e e n d a t e d are n e p h e l i n i t e s , w h e r e a s the m o s t r e c e n t Q u a t e r n a r y e r u p t i v e s a r e p r e d o m i n a n t l y b a s a n i t e s , w i t h r a r e t e p h r i p h o n o l i t e s a n d p h o n o l i t e s . I n c o n t r a s t to P l i o c e n e s t a g e 1 a n d 4 a n d Q u a t e r n a r y stage 1 volcanics, w h i c h were e r u p t e d f r o m vents s c a t t e r e d a c r o s s the w h o l e i s l a n d , m o s t of the P l i o c e n e stage 2 a n d 3 v o l c a n i c s o r i g i n a t e d f r o m a c e n t r a l e r u p - t i o n c o m p l e x , l o c a t e d w i t h i n the M i o c e n e c a l d e r a (Fig. 1).
3. Analytical methods
S a m p l e p r e p a r a t i o n a n d p r o c e d u r e s for the iso- t o p i c a n a l y s e s p r e s e n t e d in T a b l e 1 a r e the s a m e as those o u t l i n e d in H o e r n l e a n d T i l t o n [16]. B a s e d o n r e p l i c a t e a n a l y s e s o f s a m p l e m a t e r i a l , the two s i g m a r e p r o d u c i b i h t y is e s t i m a t e d to b e b e t t e r t h a n + 0 . 0 0 0 0 2 5 for Sr, + 0 . 0 0 0 0 2 0 for N d , a n d + 0 . 0 5 % p e r a m u for P b i s o t o p e ratios. C o m p a r i - son o f the i s o t o p i c c o m p o s i t i o n o f l e a c h e d a n d u n l e a c h e d s a m p l e m a t e r i a l i l l u s t r a t e s t h a t a c i d w a s h i n g d o e s n o t s i g n i f i c a n t l y affect the N d a n d P b i s o t o p e r a t i o s b u t m a y slightly l o w e r the 87Sr//86Sr r a t i o . A t a b l e c o n t a i n i n g u n l e a c h e d iso- t o p e d a t a a n d r e p l i c a t e a n a l y s e s for G r a n C a n a r i a s a m p l e s is a v a i l a b l e f r o m the first a u t h o r u p o n request. A m o r e d e t a i l e d d i s c u s s i o n o f the effects o f a c i d w a s h i n g , a n d o f the r e p r o d u c i b i l i t y of i s o t o p i c a n d t r a c e e l e m e n t d a t a is p r e s e n t e d in H o e r n l e a n d T i l t o n [16]. I s o t o p e r a t i o s were m e a - s u r e d o n a m u l t i p l e c o l l e c t o r F i n n i g a n M A T 261 m a s s s p e c t r o m e t e r o p e r a t i n g in s t a t i c m o d e . F o r c o n s i s t e n c y , the Pb, Sr a n d N d a n a l y s e s were n o r m a l i z e d to the s a m e v a l u e s as t h o s e u s e d in the e a r l i e r s t u d y o n the M o g a n a n d l o w e r F a t a g a i g n i m b r i t e s f r o m G r a n C a n a r i a also p e r f o r m e d at U C S B [6].
4. Results and observations
T h e M i o c e n e a n d t h e P l i o c e n e - R e c e n t v o l c a n i c s f o r m d i s t i n c t , e l o n g a t e fields o n i s o t o p e c o r r e l a t i o n d i a g r a m s (Figs. 4 - 7 ) . I n the S r - N d , P b - S r , D e l t a 2 0 8 / 2 0 4 P b - S r a n d P b - N d di- a g r a m s , the M i o c e n e a n d P l i o c e n e - R e c e n t fields f o r m d i f f e r e n t t r e n d s . F o r e x a m p l e , the M i o c e n e v o l c a n i c s f o r m t r e n d s w i t h zero s l o p e in the S r - N d a n d P b - N d d i a g r a m s , w h e r e a s the P l i o c e n e - R e -
TABLE 1 Isotope data from Gran Canaria volcanics in order of increasing age and sediments from DSDP site 397 87 86 143Nd / Sample Rock type Rock Age SiO 2 M8# Rb Sr Sr/ Srin Sm Nd number group (Ma) (wt%) (ppm) (ppm) (ppm) (ppm) 144Ndin"
Epsilon Pb U Th 2°6Pb/ 2°7pb/ 2°8pb/ Nd (ppm) (ppm) (ppm) 2°4Pbin" 2°4Pbin" 2°4pbin" Quaternary Q1499 basanite UPV 0.0 42.7 71.0 29.91 880.2 0.703172(16) 10.68 56.26 0.512909(5) 5.29 Q1433 basanite SPV 0.0 43.6 67.4 29.38 934.1 0.703158(21) 11.11 56.05 0.512930(8) 5.70 Q1414 basanite UPV 0.2 42.8 65.7 37.93 1165 0.703164(14) 12.17 65.59 0.512907(7) 5.24 Q1714A basanite EV 0.5 42.2 52.8 * 11 * 2832 0.703136(16) 0.512866(8) 4.45 Q1438 tephrite EV 0.9 43.7 61.0 * 38 * 1538 0.703250(14) * 15.73 * 91.17 0.512872(9) 4.56 QC21 nephelinite UPV 1.3 40.1 71.8 * 35 * 1439 0.703118(17) * 14.45 * 84.3 0.512953(6) 6.14 Pliocene LLC22 melilitite UPV 1.7 38.3 76.1 * 14 * 1333 0.703104(12) * 15.5 * 88 0.512957(6) 6.22 LL1415 melilitite UPV 1.8 39.3 72.5 32.70 1351 0.703117(16) 18.78 104.53 0.512930(6) 5.69 LLC9 melilitite UPV 1.9 38.5 69.4 * 12 * 1787 0.703126(16) * 18.8 * 106 0.512931(5) 5.71 LL1410 hawaiite SPV 1.9 46.1 64.4 41.89 1203 0.703186(11) 12.59 66.75 0.512849(12) 4.11 LLl142 nephelinite UPV 2.4 39.7 68.9 "41 * 1490 0.703189(17) * 16.6 * 84.4 0.512898(3) 5.07 LLll41 nephelinite UPV 2.6 40.0 72.0 26.26 1604 0.703183(12) 17.73 100.01 0.512894(10)5.00 LLC16 nephelinite UPV 2.9 40.7 72.3 * 30 * 1204 0.703066(13) * 13.6 * 73 0.512967(6) 6.42 LL1424 nephelinite UPV 3.1 39.9 70.7 24.47 1478 0.703097(11) 17.04 93.01 0.512899(15)5.09 RNB122 phonolite EV 3.4 56.0 28.7 * 143 * 2394 0.703290(12) 4.38 32.26 0.512779(7) 2.75 RNC44 basanite UPV 3.4 41.6 70.2 * 33 * 1061 0.703090(15) 0.512981(6) 6.69 RNC1 phonolite EV 3.4 56.9 10.6 231.1 139.7 0.703493(23) 1.83 15.92 0.512846(12) 4.07 RNC1 anortho- clase 124.7 773.7 0.703295(15) RNC49 basanite UPV 3.5 42.0 70.0 * 24 * 1358 0.703122(12) * 11.4 * 59 0.512927(6) 5.64 RNl179 hawaiite EV 3.5 46.7 50.7 * 73 * 1665 0.703173(16) 16.26 92.31 0.512827(3) 3.68 RNl143 phono- tephrite EV 3.5 49.9 47.8 101.1 2148 0.703272(15) 16.38 98.16 0.512846(12) 4.05 RNC25 phonolite EV 3.5 59.6 12.2 187.5 56.92 0.703606(16) 2.57 28.28 0.512789(7) 2.94 RNC25 anortho- clase 71.28 610.2 0.703199(11) RNB105 basanite SPV 3.5 44.7 64.1 * 48 * 911 0.703156(14) * 10.5 * 53.4 0.512879(6) 4.71 RNl130 basanite UPV 3.6 42.7 67.5 19.39 940.6 0.703079(12) 12.47 RNB44 phono- tephrite EV 3.6 50.7 47.1 42.01 1835 0.703324(17) 16.13 RNB36 mugearite EV 3.6 51.1 47.9 46.12 1739 0.703273(15) 14.79 RN74 trachyte EV 3.6 59.5 31.0 110.1 2189 0.703293(15) 6.05 RNB31 benmoreite EV 3.7 56.2 43.9 62.06 1753 0.703232(13) 12.08 RN86 alkali basalt SPV 3.7 46.0 62.8 * 31 * 825 0.703114(13) 11.40 RN63.3 tholeiite EV 3.9 49.6 61.7 15.63 529.0 0.703191(11) 7.48
60.67 0.512901(7) 5.13 97.53 0.512761(7) 2.39 87.98 0.512755(6) 2.29 50.91 0.512761(9) 2.40 76.46 0.512777(5) 2.70 54.63 0.512879(7) 4.70 31.07 0.512883(9) 4.78 3.04 1.30 4.40 19.542 15.592 39.382 3.28 1.25 4.82 19.473 15.583 39.333 3.32 1.49 5.66 19.507 15.587 39.385 19.479 15.579 39.334 5.48 2.60 10.41 19.490 15.566 39.338 * 9.5 19.485 15.584 39.247 4.97 2.42 10.14 19.567 15.605 39.518 4.58 2.52 10.13 19.329 15.568 39.126 2.24 10.26 19.531 15.589 39.390 5.63 * 1.9 7.06 19.159 15.558 39.061 3.75 1.87 7.32 19.364 15.577 39.225 3.34 2.49 9.72 19.502 15.583 39.382 3.65 1.76 7.21 19.383 15.567 39.153 2.71 2.10 7.68 19.478 15.582 39.307 14.74 3.61 21.41 19.187 15.536 39.197 19.271 15.576 39.116 28.93 13.06 40.32 19.174 15.526 39.175 4.78 1.35- 5.39 19.361 15.562 39.207 5.01 2.22 9.54 19.076 15.519 38.978 9.20 4.58 17.23 19.165 15.541 39.185 17.85 8.03 28.58 19.216 15.530 39.184 2.42 1.04 4.82 19.177 15.529 39.002 2.70 * 1.06 4.36 19.243 15.551 39.035 7.07 3.03 12.71 19.165 15.534 39.241 6.98 2.96 11.30 18.979 15.511 38.984 12.28 3.97 17.59 19.133 15.542 39.269 8.99 2.57 16.06 19.115 15.530 39.103 1.95 0.84 3.53 19.174 15.537 39.006 1.48 0.42 1.86 18.994 15.588 38.992 ,v o m z m
RN63.2 tholeiite SPV 3.9 48.9 63.1 * 13 * 550 0.703160(15) RN63.1 tholeiite EV 3.9 49.4 51.6 7.44 579.3 0.703188(14) 8.22 RNB14 tholeiite SPV 4.0 48.8 65.1 11.00 536.7 0.703204(16) 7.75 RN60 tholeiite SPV 4.0 48.4 63.3 9.55 564.8 0.703147(6) 8.53 RN951 basanite SPV 4.1 44.2 65.2 35 * 1262 0.703191(15) 11.05 RN1258 basanite SPV 4.1 44.8 65.2 * 34 * 1061 0.703154(17) 11.88 RN53 alkali basalt SPV 4.1 45.7 65.5 22.83 871.8 0.703186(24) 10.96 RNB125 basanite SPV 4.2 43.4 64.1 * 46 * 1076 0.703126(10) * 13 RN54 basanite EV 4.2 45.0 60.8 * 31 * 1011 0.703094(12) 11.74 RNB60 basanite SPV 4.3 44.2 68.0 * 42 * 1106 0.703218(14) RN1249 alkali basalt SPV 4.4 45.0 65.5 * 23 * 1328 0.703165(15) 13.45
0.512886(4) 4.84 35.63 0.512890(7) 4.92 33.00 0.512879(7) 4.71 36.97 0.512879(6) 4.71 69.28 0.512857(8) 4.28 63.74 0.512865(5) 4.43 53.63 0.512872(6) 4.56 * 68 0.512847(10) 4.08 60.11 0.512916(7) 5.42 0.512831(7) 3.76 73.23 0.512828(6) 3.70 RN104 basanite EV 4.4 44.0 61.4 * 25 * 1144 0.703226(13) * 12.82 68.11 0.512818(5) 3.51 RN99.2 basanite SPV 4.5 44.4 66.7 * 30 * 1002 0.703112(16) * 12.41 60.71 0.512913(6) 5.37 RN94 alkali basalt SPV 4,5 45.1 69.5 * 32 * 868 0.703082(13) * 9.23 46.02 0.512959(6) 6.27 RN95 basanite SPV 4.6 44.6 69.9 *43 * 1147 0.703095(14) * 11.35 57.65 0.512934(6) 5.78 RN677 basanite SPV 4.7 43.1 64.4 * 24 * 1234 0.703101(15) 12.03 70.19 0.512898(5) 5.07 ETB45 nephelinite UPV 5.0 40.0 71.0 27.25 1064 0.703089(18) 12.87 66.77 0.512888(6) 4.88 ET88 nephelinite UPV 5.0 40.3 70.5 6.66 1348 0.703106(16) 12.05 59.18 0.512907(6) 5.25 ETB47 nephelinite UPV 5.0 41.0 68.2 25.90 939.9 0.703078(16) 11.31 57.80 0.512898(8) 5.07 ET108 tephrite EV 5.2 45.9 52.4 47.48 1579 0.703235(14) 16.89 93.43 0.512771(11) 2.60 ET106 basanite UPV 5.3 41.0 66.3 * 35 * 857 0.703092(16) 11.59 54.46 0.512920(4) 5.49 ET878 basanite UPV 5.3 41.0 65.7 28.73 921.6 0.703121(12) 11.34 56.29 0.512900(10) 5.11 Miocene T1376 trachyte UV 11.4 61.3 17.3 135.0 3.67 0.703075(15) 28.04 174.1 0.512887(7) 4.85 T1374 trachyte UV 11.4 62.6 16.0 119.1 4.44 0.703075(39) 15.08 101.7 0.512887(5) 4.86 TC43 phonolite UV 11.6 62.2 18.8 118.0 7.19 0.703020(25) 20.32 127.2 0.512893(6) 4.70 TC43 felsic phases 11.6 163.6 2.47 0.703020(11) F253872 nephelinite UV 11.8 40.5 53.4 19.51 * 2872 0.703079(6) 16.35 H1389 tholeiite SEV 14.3 51.7 48.6 28.80 695.4 0.703350(17) 12.58 H1390 tholeiite SEV 14.3 49.3 49.2 21.79 636.7 0.703341(14) 11.01 H1384 mugearite SEV 14.3 52.9 44.2 36.57 774.8 0.703337(19) 15.20 H1383 tholeiite H1382 tholeiite G1392 tholeiite G1378 tholeiite G1379 tholeiite G1377 tholeiite G1265 tholeiite G1262 tholeiite
SEV 14.3 49.9 44.4 39.07 598.2 0.703319(17) 13.90 SEV 14.3 48.9 45.5 23.73 588.3 0.703319(9) 10.74 SEV 14.4 49.8 58.5 15.76 539.3 0.703462(16) 9.97 SPV 14.4 46.7 65.9 19.93 513.2 0.703193(20) 9.12 SPV 14.4 46.4 69.2 21.62 533.1 0.703196(18) 9.24 SPV 14.5 46.6 74.3 7.63 323.4 0.703280(19) 6.30 SPV 14.5 46.9 70.4 22.88 358.1 0.703267(17) 7.88 SPV 14.5 45.4 78.5 14.80 237.9 0.703260(16) 5.83 86.45 0.512901(3) 5.13 58.26 0.512906(9) 5.23 50.78 0.512899(9) 5.09 73.73 0.512895(12) 5.02 66.67 0.512911(7) 5.33 49.67 0.512913(7) 5.36 45.62 0.512890(8) 4.91 43.64 0.512906(9) 5.23 44.81 0.512895(7) 5.02 28.47 0.512908(7) 5.27 37.14 0.512898(14) 5.07 26.25 0.512915(5) 5.41 19.019 15.578 39.004 1.73 0.52 2.05 19.007 15.559 38.974 1.39 0.45 1.96 19.008 15.569 38.944 1.75 0.45 2.11 19.036 15.571 38.966 3.33 1.55 7.30 19.219 15.570 39.228 3.51 1.57 6.64 19.124 15.541 39.074 2.87 1.05 3.88 19.008 15.545 38.955 3.27 1.14 5.33 19.181 15.540 39.070 2.36 0.74 3.79 19.190 15.553 39.078 4.20 1.63 7.29 1.27 6.42 19.030 15.521 39.024 2.49 0.97 4.07 19.149 15.532 38.994 2.57 0.93 3.79 19.361 15.549 39.101 3.51 1.51 5.81 19.494 15.570 39.276 3.34 1.35 6.54 19.209 15.573 39.129 3.09 1.03 5.71 19.475 15.572 39.226 2.66 1.13 5.16 19.499 15.573 39.216 2.22 1.20 5.12 19.443 15.571 39.185 5.31 1.55 8.10 18.760 15.501 38.866 2.49 0.92 3.71 19.359 15.575 39.170 1.96 0.77 3.84 19.312 15.593 39.154 13.60 5.25 14.18 19.888 15.585 39.603 13.21 5.53 20.29 19.921 15.631 39.725 12.02 4.32 17.76 20.014 15.627 39.783 2.08 0.95 3.58 19.885 15.609 39.618 2.50 1.22 4.28 19.516 15.601 39.252 2.41 1.06 3.17 19.536 15.602 39.281 3.47 1.63 4.39 19.577 15.591 39.283 3.50 1.58 4.58 19.562 15.596 39.292 2.50 1.10 3.45 19.512 15.592 39.218 2.37 0.77 2.83 19.366 15.606 39.108 2.14 0.78 2.74 19.636 15.627 39.409 2.19 0.71 2.92 19.647 15.611 39.376 1.50 0.44 1.90 19.572 15.599 39.257 2.09 0.65 2.36 19.549 15.621 39.307 1.30 0.48 1.67 19.527 15.619 39.288
.Ix x,O
TABLE 1 (continued) Sample Rock type Rock Age SiO 2 Mg# Rb Sr 87Sr/86Srin. Sm Nd 143Nd/ Epsilon Pb U Th 2°rpb/ 2°Tpb// 2°spb/ number group (Ma) (wt%) (ppm) (ppm) (ppm) (ppm) 144Ndin. Nd (ppm) (ppm) (ppm) 2°4pbin" 2°4pbin" 2°4Pbin" DSDP 397-60-4 sediments Tmu 165.8 397.1 0.710291(12) 5.42 28.74 0.511958(7) -13.26 25.02 2.54 10.81 19.008 15.744 39.187 397-101-1 sediments Tmm 39.23 1240 0.723619(13) 2.60 14.10 0.512065(7) -11.18 6.01 2.00 3.25 18.836 15.696 39.057 397-30-1 sediments Tml 69.38 135.7 0.709362(23) 3.68 19.86 0.511646(12) -19.35 11.34 1.83 6.02 18.936 15.724 38.973 397-40-2 sediments Kh 45.99 1127 0.709363(16) 2.76 14.67 0.512038(6) -11.70 6.08 1.65 3.45 18.605 15.685 38.947 397-49-1 sediments Kh 44.16 1203 0.709288(25) 3.03 16.49 0.512062(8) -11.24 5.71 1.35 3.41 18.640 15.680 38.925 Capital letters in front of sample numbers stand for the following formations: Q = Quaternary, LL = Los Llanos, RN = Roque Nublo, ET = El Tablero, T = Tejeda, F = Fataga, H = Hogarzales, and G = Guigui. The sources of most ages are reported in [4]. Ages for samples T1374, T1376 and TC43 were determined from the Sr isotope data, using the average initial 87Sr/S6Sr ratio (0.703075) of Fataga and Tejeda samples [this study and 6] for samples T1374 and T1376. Mg# (= Mg/(Mg+Fe +2) with Fe+3/(Fe+3 + Fe +2) = 0.2} and SiO 2 are from X-ray fluorescence data, recalculated on an anhydrous basis, reported in Schmincke [9,42] and Hoernle and Schmincke [4]. Trace element concentrations were determined by isotope dilution, except those preceded by an asterisk which were determined by XRF (Rb, St) or INAA (Sm, Nd, U, Th) [4]. The precision of all concentrations determined by isotope dilution is better than 1%. For determination of isotope ratios, a separate split of the same powder as used for the trace element analyses was acid washed for 45 min with a mixture of 50°C 6 N HC1 and 7 N HNO3, except for samples T1374, T1376, TC43 and the DSDP sediment samples. Felsic mineral separates were washed for 15 min with 2 N HCI. All isotope ratios are age corrected, except for the DSDP samples and the samples for which there is insufficient parent-daughter data. Errors refer to the least significant digits and are 2 sigma mean within-run precision. The 87Sr/86Sr ratio was normalized within-run to 8%r/8SSr = 0.1194, and then adjusted to a 87Sr/86Sr value of 0.710250 for NBS 987. The 143Nd/144Nd ratio was normalized within-run to 146Nd/la4Nd = 0.721900, and then adjusted to a 143Nd/144Nd value of 0.511850 for the La Jolla standard. Pb isotope analyses were corrected to NBS981 [43] for fractionation. An average of twelve measurements of NBS981 yielded: 2°rpb/2°4pb = 16.904(7), 2°7pb/2°4pb = 15.447(8) and 2°spb/E°4pb = 36.560(21). Blanks for Sr, Nd and Pb were < 0.3 nanograms and thus are negligible, eNd was calculated using the initial 143Nd/144Nd ratio except for the sediment samples, eNd = ([(143Nd//144Nd)sample//(143Nd/144Nd)bulk earth]-- 1 } X 104; Bulk Earth = 0.512638; 147Sm/144Nd = 0.1966.
Sr-Nd-Pb ISOTOPIC E V O L U T I O N O F G R A N C A N A R I A 51
0.5131
0.5130 70 Z
0.512(.
"O z
0.5128
o.5127
0.7030 0.7032 0.7034 0.7036
87Sr/86Sr
Fig. 4. S r - N d isotope correlation diagram for Gran Canaria volcanics. The fields for the Miocene (solid symbols) and Pliocene-Re- cent (open symbols) volcanics form different trends. The Miocene volcanics trend from a HIMU-like to an EM2-1ike composition.
The Pliocene-Recent volcanics trend from an intermediate ( H I M U + D M + E M ) composition towards EM1. The most under-
saturated volcanics (UV, UPV; diamonds) from the two groups overlap with the field for the western Canary Islands of La Palma and Hierro [Hoernle et al., manuscript in preparation], which primarily reflects interaction between the Canary Plume (HIMU-like) and the asthenosphere (DM + EM). The saturated primitive volcanics (SPV; squares) and the evolved volcanics (SEV, EV; triangles) extend towards more enriched compositions, with the evolved volcanics having the most enriched compositions. The trend towards more enriched compositions probably results from lithospheric contamination, with the lower lithosphere beneath Gran Canaria having an EM2-1ike and the upper lithosphere an EMl-like composition. Also shown is the field for St. Paul's Rocks, a piece of mantle peridotite subaerially exposed along a mid-Atlantic-ridge transform fault [19]. The figure also contains Miocene data from
Cousens et al. [6]. The mantle components are from Zindler and Hart [1] and the field for Atlantic MORB is from Ito et al. [23].
cent volcanics form trends with a negative slope in the S r - N d diagram and a positive slope in the P b - N d diagram (Figs. 4 and 7). Although the
40.0 (a) N H R L J
MIOOE.N E ~ ,,~
a- Q 39.6 PLIOCENE
RECENT ~
% 392 N e~ 38.8
DM
38.4 i i / " i i i i i er/r
(b)
15 7 . - " A t l a n t i c ', ... .
" ( S e d i m e n t s . ) N H R L ~ , ~
~.
]
EM2oa 156 t
| / "x~...-~___ .~ ( ~ , = " ~ WESTERN 3 ~ E M I ~ P V ~ b o C E N E CANARIES / - - D M / T M EV ~ " - . . .
/ i I I i i i i i I
18.4 18.8 19.2 19.6 20.0 20.4
206pb / 2O4pb
Miocene samples cover a time period > 3 Myr, 143Nd/144Nd is surprisingly constant at 0.512900 _+0.000018 (2 sigma, N = 15, this study) and 0.512913 -t- 0.000030 (2 sigma, N = 30, [6]), within the analytical precision for each study. The aver- age ta3Nd/144Nd ratio for all the analyzed Miocene samples (0.512910 -+ 0.000030, 2 sigma, N = 45) is also indistinguishable from the average value for the western islands of La Palma and Hierro (0.512921 -+0.000041, 2 sigma, N = 1 7
Fig. 5. P b - P b isotope correlation diagrams for Gran Canaria volcanics. The Pliocene-Recent group has less radiogenic Pb isotopes than the Miocene group. Both groups form trends which have nearly horizontal to positive slopes. Within a group, the most undersaturated volcanics have the most radio- genic Pb and the evolved volcanics the least radiogenic. On the 2°7pb/2°4pb diagram both groups fall on the northern hemi- sphere reference line (NHRL) [17]. Neither group trends to- wards the field for Atlantic sediments, which contains data from this study and Sun [18]. On the 2°8pb/a°4pb diagram, some of the Miocene evolved volcanics and the Pliocene-Re- cent volcanics fall above the N H R L . For additional informa-
tion and references see caption to Fig. 4.
52 K. HOERNLE ET AL.
(a) 0"7036 t
0.7034]
~ -
0.7032-
0.7030
0.7028
DM ,Atlantic, MORB /
)
07036 t
(b) !i
0.7034]
0,7032" MIC
o~ U) li- e)
0.7030
... U ~._
HIMU Western IJm
Canaries ~
(Plume)
u./uzo , ,
18.5 19.0 19.5 2().0 0"7028-30 -2b -lb 6 lb ;~0 3b 4'0 5'0
206pb / 204 Pb DELTA 208/204 Pb
Fig. 6. Pb-Sr isotope correlation diagrams for Gran Canada volcanics: (a) 2°6pb/2°4pb versus 87Sr/S6Sr, and (b) Delta 208/204 Pb versus 87Sr/S6Sr. See caption to Fig. 4 for additional information and references.
t
Hoernle et al., in prep.]). As illustrated in Fig. 6, 7Sr/86Sr correlates negatively with the Pb isotope ratios and positively with Delta 208/204 Pb (the deviation from the Northern Hemisphere Refer- ence Line ( N H R L ) [17]). The range in Pb isotopes for Gran Canaria (2°6pb/2°4pb=18.76-20.01) covers the entire range reported for the other C a n a r y Islands [18]. The P l i o c e n e - R e c e n t volcanics have less radiogenic Pb isotopes than the Miocene volcanics (Fig. 5). In the 2°6pb/2°4pb versus 2°7pb/2°4pb diagram, both fields have slightly positive slopes and intersect the N H R L . Neither field trends towards Cretaceous-Recent Atlantic sediments. In the 2°6pb/2°4pb versus2°8pb/2°apb diagram, the trends for both groups also have positive slopes. The Miocene volcanics with the most radiogenic 2°6pb fall on the N H R L in this diagram; some of the Miocene samples with the least radiogenic
2°6pb
and the Pliocene- Recent samples, however, fall above the N H R L and thus have Delta 208/204 Pb > 0 (Fig. 6b).Based on their major element compositions and stage of eruption, b o t h the Miocene and Pliocene-Recent volcanics can be subdivided into three groups (Fig. 3), which also have different isotopic compositions. The Pliocene-Recent un- dersaturated primitive volcanics (UPV) have mag- nesium numbers ( M g # ) > 66 and S i O 2 < 43
0.5131
0.5130
~ . 0.5129
0.5128
• D M
Atlantic MORB( A s p h e n ~ ~
(Plume)
.->, HIMU
0.5127 +
E M I , ~ EM2 error
i i ~ i i i
18.5 19.0 19.5 20.0
206pb / 204pb
Fig. 7. Pb-Nd isotope correlation diagram for Gran Canaria volcanics. See caption to Fig. 4 for additional information and references.
S r - N d - P b I S O T O P I C E V O L U T I O N O F G R A N C A N A R I A 5 3
weight percent, and range from basanites to melilitites; the Miocene undersaturated volcanics (UV) are nephelinites and trachyphonolites. The UPV were erupted during stages 1 and 4 of the Pliocene and Quaternary Cycles, and the UV dur- ing Miocene stage 4 (Fig. 2). As mentioned previ- ously, Miocene stage 1 volcanics are not exposed on G r a n Canaria. The saturated primitive volcanics (SPV) have M g # > 62 and SiO 2 > 43 weight percent and range from basanites to tholei- ites and picrites. These volcanics were primarily erupted during stage 2 of each cycle. The Miocene saturated evolved volcanics (SEV) and the Plio- c e n e - R e c e n t evolved volcanics (EV) have M g # <
62 and were erupted primarily during stage 3 of each cycle.
In all isotope correlation diagrams (Figs. 4-7), the most SiO2-undersaturated volcanics from each cycle (UV, UPV) have the most restricted com- positional range and fall at the end of the field for each cycle that overlaps the field for the western Canary Islands. For each cycle, these volcanics have the least radiogenic 87Sr/86Sr, the most ra- diogenic 206 Pb/204 Pb and 208pb/204pb, and lowest Delta 208/204 Pb values. The UPV also have the most radiogenic ta3Nd/a44Nd ratios of the Plio- c e n e - R e c e n t volcanics. The Miocene SPV fall in the middle of the Miocene field, and the Pliocene-Recent SPV are concentrated in the middle of the Pliocene-Recent field but also over- lap the UPV. The evolved (SEV, EV) groups cover most of the field for a cycle but are concentrated on the opposite ends of the fields from the most undersaturated volcanics. The evolved groups have the most radiogenic Sr and the least radiogenic Pb isotopic compositions, and the highest Delta 208/204 Pb values. The EV also have the least radiogenic Nd isotopic compositions of the Plio- c e n e - R e c e n t volcanics. In Figs. 4, 6 and 7, the Miocene volcanics trend towards the field for St.
Paul's Rocks [19], with the SEV overlapping the field. St. Paul's Rocks are a piece of mantle peri- dotite that is subaerially exposed along the St.
Paul Fracture Zone on the Mid Atlantic Ridge.
Sr, Nd and Pb isotopes show systematic and similar variations with M g # for the Pliocene volcanics (Fig. 8). With decreasing M g # , 87Sr/86Sr increases and the 143Nd/144Nd and Pb isotope ratios decrease until M g # is - 5 0 ; below this value all ratios remain roughly constant. M g # =
0.7034
¢~ 0.7033-
~ 0.7032-
0.7031
0.7030 0.5130-
"1o Z
~ 0 . 5 1 2 9 - Z
0.5128-
P L I O C E N E V O L C A N I C S
OL+ CPX = Mantle FracUonation
OL+ CPX + PLAG = Crustal Fractionation
A ,E.I A
i i
I
e r r o r
t j e r r o r
AI A ~,
I t
0 . 5 1 2 7 ' ' I . . . .
.~ 19.5 I e r r o r T
13-
c~ 19.3
18.9 i
I
18.7 Aj
80 50 40 3'o 1'0
Magnesium Number
Fig. 8. Plots of magnesium number (Mg#) versus St, Nd and Pb isotope ratios for the Pliocene volcanics illustrate assimila- tion during crystal fractionation. With decreasing Mg~, the 87Sr/S6Sr ratio increases and the 143Nd/144Nd and Z°6pb/2°4pb ratios decrease and then remain constant. The change in slope of the trends at Mg# = 50 corresponds with the crystallization of plagioclase. The presence of plagioclase on the liquidus may mark the transition from mantle to crustal
levels of cooling and fractionation [4].
50 also divides rocks which only have olivine and clinopyroxene on the liquidus ( M g # > 50) from rocks which also have plagioclase on the liquidus ( M g # < 50). Although not as abundant as in the hawaiite to trachyte suite, plagioclase is present as a phenocryst phase in the tephrite to phonolite suite and is required in the mass balance calcula- tions to link rocks in this suite by fractional crys- tallization [15]. There is no simple correlation be- tween M g # and isotopic composition for the Miocene volcanics.
The mafic and evolved volcanics from the nearby Cape Verde Islands exhibit similar isotopic
