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Basalts and serpentinite from the Puerto Rico Trench, 1. Petrology
Marine Geology, 16(1974): 191--203
©Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
BASALTS AND SERPENTINITE FROM THE PUERTO RICO TRENCH, 1. PETROLOGY
FUMIKO
SHIDO' , A K I H O M I Y A S H I R O '
and M A U R I C E
EWING'
'Department of Geological Sciences, State University of N e w York at Albany, Albany, N.Y. (U.S.A.) 2The Marine Biomedical Institute, The University o f Texas at Galveston, Galveston, Texas (U.S.A.)
(Accepted for publication January 10, 1974}
ABSTRACT Shido, F., Miyashiro, A. and Ewing, M., 1974. Basalts and serpentinite from the Puerto R i c o Trench, 1. Petrology. Mar. Geol., 16: 191--203. Typical abyssal t h o l e i i t e s o c c u r on both north and south wallsof the Puerto Rico Trench. Probably they were erupted in the Mid-Atlantic Ridge in Cretaceous time and afterwards were moved to the present position. In addition, doleritic basalts containing phenocrysts of pigeonite rimmed by augite were found from the north wall, and these rocks presumably represent a n e w t y p e of abyssal tholeiites characterized by CaO/Na20 ratios lower than t h o s e o f ordinary abyssal tholeiites. A new analysis of serpentinite from the north wall is also recorded here.
INTRODUCTION
A large a m o u n t of petrographic and chemical data has been published on igneous rocks from ocean floors in the past decade. Most of the rocks studied came f r o m the mid-oceanic ridge system and seamounts. We have come to know t h a t the surface layer of the mid-oceanic ridges is made up mainly of low-K, high-Na tholeiites (e.g., Engel and Engel, 1964), usually called oceanic tholeiites or abyssal tholeiites. (The latter name is preferred in this paper, because some authors include oceanic island tholeiites in the category of oceanic tholeiites, though such rocks differ in chemical characteristics from the ocean-floor tholeiites now under discussion.) We k n o w the composition ranges and the trends of crystallization differentiation of abyssal tholeiites (Miyashiro et al., 1970; Shido et al., 1971; Hekinian and Aumento, 1973; Shido and Miyashiro, 1973). The deeper parts of the ridges appear to be made up mainly of metamorphosed tholeiite and gabbro possibly associated with serpentinite or peridotite (e.g., Miyashiro et al., 1971). However, our knowledge o f hard rocks in other parts of the ocean floor is still very limited.
192 Hard rocks sampled from a few trenches have been reported. A fresh dolerite dredged from the b o t t o m of the Vitiaz Deep in the Mariana Trench was described by Yagi (1960). Consideration of his analytical data in light of our present-day knowledge of ocean-floor rocks indicates that the rock is tholeiitic dolerite probably derived from abyssal tholeiite magma. Fisher and Engel {1969) recorded some altered gabbroic and basaltic rocks together with fresh peridotite from the nearshore flank of the Tonga Trench. Serpentinites and basalts from the north wall of the Puerto Rico Trench were described by Bowin et al. (1966), Chase and Hersey (1968), Nalwalk {1969), Hart and Nalwalk (1970), and Hart (1971). Unfortunately, however, the Puerto Rico Trench basalts were so strongly altered that their original characteristics were n o t clear. In 1964, the research vessel " R o b e r t Conrad" of Lamont-Doherty Geological Observatory made two successful rock dredges in the Puerto Rico Trench, one on the north and the other on the south wall. The degree of alteration of the basaltic rocks obtained is much less than that of the Puerto Rico Trench basalts reported in the past. The original chemical features are so well preserved that we could identify them as abyssal tholeiites. Therefore, we intend to describe these basalts in this paper. A specimen of serpentinite from the north wall was obtained through the courtesy of Drs. J. B. Hersey and C. O. Bowin of Woods Hole Oceanographic Institution. It will also be described in this paper. PUERTO RICO TRENCH AND DREDGE STATIONS Ordinary trenches such as the Mariana and Tonga are formed in the zones of plate convergence along island arcs or continental margins. On the other hand, the Puerto Rico Trench is now characterized by strike-slip motion with only a small component of underthrusting (e.g., Molnar and Sykes, 1969; Malfait and Dinkelman, 1972). In the Island of Puerto Rico, intense volcanism of the island arc type occurred from middle Cretaceous to Eocene time {e.g., Mattson, 1966). Hence, we may consider that there was a subduction zone, presumably to the north of Puerto Rico, from the Middle Cretaceous to the Eocene, and t h a t thereafter the direction of plate movement has changed and strike~slip movement has become dominant. The top of the north wall of the trench is about 5500 m deep (Fig.l). The wall dips to the south at an average angle of 5 ° , but locally there are many pronounced scarps which presumably represent parallel faults running generally in east--west directions. The north wall has an especially steep and complicated fault structure in the area of 65 ° 00'--65°15'W longitude {Nalwalk, 1969, fig.3). Our dredge RC8-RD1 was made there {Table I). The trench floor, about 7500 m deep, is covered by thick sediments. Dredge RC8-RD4 was recovered from the northeastern slope of a ridge, called the Main Ridge by Conolly and Ewing (1967), which lies at the foot of the south wall and trends approximately WNW. No rock dredges have been reported, so far as we know, from this ridge or other parts of the south wall of the trench.
193
5
5
0
TrenchFloor 5
5
0
0
m
~
~. 4"/01:. X~__. ~__~____ ~;n,g/.:.,, RCB-RD4 0
m
~
IO0km Fig.1. Dredge s t a t i o n s in t h e P u e r t o Rico T r e n c h . Dredge haul C H 1 9 - D 1 0 was d e s c r i b e d b y B o w i n e t al. (1966). Dredge hauls CH34-D3 a n d D4 were d e s c r i b e d b y Chase a n d Hersey ( 1 9 6 8 ) a n d Nalwalk ( 1 9 6 9 ) . T h e dredges o f t h e AII-11 g r o u p d e s c r i b e d by Nalwalk ( 1 9 6 9 ) were m a d e nearly a t t h e s a m e p o s i t i o n s as o u r dredge R C 8 - R D 1 .
TABLE I Dredge s t a t i o n s Dredge No.
Latitude
Longitude
Corrected depth (m)
Note
RC8-RD1 RC8-RD4 CH19-D10
20 ° 0 6 . 8 ' N 19 ° 3 6 ' N 20 ° 0 0 ' N
65 ° 0 6 . 5 ' W 65 ° 0 4 ' W 66 ° 31' 5 7 " W
6591 7260 ?
n o r t h wall s o u t h wall n o r t h wall
Dredge RC8-RD1 This dredge haul, obtained from the north wail (Table I; Fig.l), includes a large number of angular basaltic fragments, of which 43 were thin-sectioned. The manganiferous coating is absent in many specimens and is t o o thin to measure in others. These basaltic rocks may be classified into three types: Type 1A: relatively poorly crystalline basalts (abyssal tholeiites). These basalts usually contain a large a m o u n t of spherulitic and dendritic aggregates of microlites in the groundmass (Table II). Glass, if present, has been altered to smectite. They are typical abyssal tholeiites with plagioclase phenocrysts. Type 1B: holocrystalline basalts (abyssal tholeiites). This t y p e has a better crystallized groundmass with no aggregates of microlites, but appears to be similar to type 1A in other respects. Most of the rocks of this type contain only plagioclase as phenocrysts. However, fragment No.2 contains both plagioclase and augite (2 V = 45o--50 ° ) as phenocrysts. The groundmass
194 TABLE II Primary mineral composition of each rock type Dredge
Type Phenocrysts
RC8-RD1 1A
1B
RC8-RD4 A
B
p|agioclase
Microphenocrysts
Groundmass
Rock name (CaO/Na20)
plagioclase
plagioclase, (augite), microlites
typical abyssal tholeiite (4.8--3.9)
plagioclase; plagioclase rarely with augite (2V = 450--50 ° )
plagioclase, typical abyssal augite tholeiite (2V= 45°--52 ° ) (4.4--4.1) magnetite
zoned clinozoned clinopyroxene pyroxene (2V = 0°--20 ° , 45°--47 ° )
plagioclase, augite, magnetite
doleritic basalt, a new type of abyssal tholeiite (2.2--1.8)
plagioclase; rarely with augite ( 2 V = 54 °)
plagioclase, augite, magnetite
typical abyssal tholeiite (4.7--4.1)
plagioclase; rarely with augite ( 2 V = 54 °)
plagioclase + augite ( 2 V - - 48 ° )
doleritic basalt and dolerite of abyssal tholeiite composition (3.9)
Note: for the distinction between phenocrysts and microphenocrysts, refer to p.48 of Miyashiro et al. (1969a).
pyroxene is augite with 2 V = 45°--50 ° (fragment No.2), 47 ° (No.4), and 52 ° (No.9). Type 1A grades into t y p e 1B with increasing crystallinity. Though olivine is not present now, the shapes of some aggregates of alteration products (mainly smectite) suggest the possibility of the former presence of olivine as microphenocrysts as well as in the groundmass. Table III shows the chemical composition of rocks of types 1A and lB. They all are ordinary abyssal tholeiites. The rocks of t y p e 1B show higher total iron and MgO contents than those of type 1A. This may be due to the accumulation of mafic minerals during the slow cooling which resulted in t y p e lB. The original magmas of the two types were probably virtually identical. Type 2: holocrystaUine doleritic basalts with zoned clinopyroxene phenocrysts. This t y p e is coarser grained than types 1A and lB. The degree of alteration is higher so that the color o f hand specimens is greyish yellow ocher.
195 TABLE III Compositions of basaltic rocks of types 1A and 1B in RC8-RD1 Specimen no. and type
8(1A)
4(1B)
FeO*/MgO CaO/Na20
1.09 4.1
1.12 4.3
1.14 4.8
1.14 4.6
1.15 4.5
1.26 3.9
1.28 4.4
SiO2 TiO 2 A120 ~ Fe~O 3 FeO MnO MgO CaO
48.71 1.40 15.95 3.65 5.57 0.18 8.09 11.40
48.79 1.40 15.39 4.36 5.18 0.19 8.16 11.63
48.97 1.31 15.38 3.51 5.63 0.20 7.68 11.94
48.86 1.33 15.14 4.06 5.57 0.21 8.10 11.83
49.07 1.23 15.64 3.61 5.52 0.19 7.65 11.77
49.51 1.27 16.05 3.20 5.24 0.16 6.47 12.10
48.49 1.44 15.32 5.02 4.91 0.19 7.35 11.96
Na~O K20
2.78 0.05 0.96 1.15 0.11 100.00
2.72 0.04 0.99 1.03 0.10 99.98
2.47 0.06 0.96 1.81 0.09 100.01
2.59 0.03 1.19 1.02 0.10 100.03
2.63 0.06 1.33 1.48 0.09 100.27
3.08 0.05 1.54 1.25 0.10 100.02
2.72 0.07 1.30 1.35 0.11 100.23
8.86 pl pl
9.10 pl p!
8.79 pl --
9.22 pl + aug --
8.77 -pl
8.12 pl --
9.43 pl --
0.36 20.90 30.70 3.21 14.30 22.69 2.18 2.49 0.21
0.18 21.92 29.60 6.69 11.15 23.11 2.18 2.53 0.23
0.36 22.26 30.70 4.91 12.29 22.02 2.18 2.34 0.21
0.30 26.06 29.82 4.75 7.28 24.04 2.18 2.41 0.2~
0.41 23.02 20.39 7.59 7.83 23.83 2.18 2.74 0.26
H~OIg. loss P20s Total FeO* Pheno. Micropheno.
3(1A)
2(1B)
I(1A)
6(1A)
9(1B)
CIPW norm
or ab an ol hy di mt il ap
0.30 23.52 30.90 8.59 9.06 20.22 2.18 2.66 0.26
0.24 23.02 29.67 8.21 9.26 22.22 2.18 2.66 0.23
Note: FeO* in this and the succeeding tables means total iron as FeO. The norms were calculated after partial reduction of F % O 3 so as to give Fe20 ~ = 1.50% in all these tables. Analyzed by F. Shido. Type 1B shows higher crystallinity than type 1A.
The phenocrysts (if present) and microphenocrysts of zoned clinopyroxenes are made up of a pigeonite core ( 2 V = 0o--20 ° ) rimmed b y augite ( 2 V = 45 ° 47 ° ). The boundary between the core and rim is sharp and there is no pyroxene of intermediate 2 V and hence of intermediate composition. The pigeonite core is always markedly lower in birefringence than the augite rim. Augite occurs also in the groundmass. Plagioclase is confined to the
196 T A B L E IV Compositions of basaltic rocks o f t y p e 2 in RC8-RD1 S p e c i m e n no. FeO*/MgO CaO/Na20
11
18
17
12
23
1.21 2.2
1.31 1.9
1.34 1.9
1.35 1.9
1.35 1.8
SiO2 TiO2 AI203 Fe203 FeO MnO MgO CaO
52.05 1.90 14.60 4.77 4.24 0.09 7.05 7.61
50.82 2.09 16.73 5.97 3.51 0.20 6.77 7.44
50.87 2.09 16.69 5.85 3.59 0.17 6.61 7.58
50.33 2,14 15.48 6.06 3.26 0,09 6,44 7.34
50.43 2.04 16.39 6.27 3.72 0.20 6.92 7.44
Na20 K20 H20Ig. loss P205 Total
3.47 0.51 1.71 1.86 0.22 100.08
3.99 0.75 0.34 1.53 0.22 100.36
3.98 0.72 0.37 1.48 0.24 100.24
3.80 0.69 2.37 1.86 0.25 100.11
4.05 0.72 0.38 1.59 0.22 100.37
FeO* Pheno. Micropheno.
8.53 -(altered)
8.88 zoned p y r zoned pyr
8.86 z o n e d pyr zoned pyr
8.71 -zoned pyr
9.36 zoned pyr
CIPW norm
Q or ab an oi hy
2.56 3.01 29.36 22.76 -21.21
di mt il ap
10.99 2.18 3.61 0.51
. 4.43 33.76 25.53 7.59 12.00
. 4.26 33.68 25.55 6.58 12.67
. 4.08 32.16 23.15 1.55 18.23
. 4.26 34.27 24.42 10.44 8.98
8.08 2.18 3.97 0.51
8.52 2.18 3.97 0.56
9.45 2.18 4.06 0.58
9.00 2.18 3.87 0.51
N o t e : 11 and 12 analyzed by F. 8hido, and the rest by Tsugio Shibata. T y p e 2 is characterized by phenocrysts and m i c r o p h e n o c r y s t s of z o n e d c l i n o p y r o x e n e s with pigeonite cores.
groundmass (Table II). Because of the high degree of alteration, it is not clear whether olivine was ever present or not. Table IV shows the chemical analyses of rocks of this type. These rocks are generally similar to abyssal tholeiites, but do not fall in the composition range of ordinary members of the abyssal tholeiites. Their K20 and Na20 contents are higher than those of ordinary abyssal tholeiites. The SiO2 and TiO2 contents also tend to be rather high. They show unusually low CaO
197
contents and CaO/Na20 ratios. Four of the 5 analyses given in Table IV show normative olivine and hypersthene after the partial reduction of iron to Fe203 = 1.50%. The high K20 contents and the normative quartz in one analysis might be due to secondary alteration. With increasing FeO*/MgO, the FeO*, TiO2, Na20 and K20 contents tend to increase, whereas the MgO content tends to decrease, though the variations are very small. Here, FeO* means total iron as FeO. The SiO2 content remains nearly constant. These variations may be regarded as representing a tholeiitic trend of fractionation.
Dredge RC8-RD4 This dredge haul, recovered from the south wall of the Trench (Table I; Fig.l), contains more than a hundred fragments of basaltic rocks, of which 50 were thin-sectioned. Most fragments have no manganiferous coating, but some are covered by a manganiferous film too thin to measure. They are classified into t w o types: Type A: abyssal tholeiites. Most of the fragments belong to this type. Some samples are poorly crystalline so as to contain a large a m o u n t of dendritic crystals and microlitic spherulites in the groundmass, and others are made up of well-crystallized mineral grains. Though there are a few nonporphyritic varieties, most of the rocks contain phenocrysts of plagioclase rarely accompanied by those of augite (2V = 53 ° ). Type B: doleritic basalts and dolerites o f abyssal tholeiite composition. Only a few fragments belong to this type.. They are coarser grained and show a subophitic texture. Table V shows that analyzed specimens of type B show a little lower A1203 and CaO contents tklan the rocks of type A, conceivably owing to the accumulation of mafic minerals during the formation of type B, just as in that of type 1B of RC8-RD1. The shapes of some aggregates of alteration products (smectite, etc.) suggest the possible former presence of olivine in some thin sections. However, olivine is not present now. ALTERATION
All of our 20 analyzed basalts show Fe203 > 2.9%. The amounts of ignition loss of the t w e n t y basalts are in the range of 0.7--1.9%. These indicate that the basalts are secondarily altered to a considerable extent. The main alteration product appears to be smectite or related minerals. Small amounts of carbonate and secondary opaque minerals are common. Zeolite was observed in several thin sections (e.g., RC8-RD1-8 and RC8-RD4-2 among analyzed rocks). The alteration would be a combination of deuteric alteration, weathering and ocean-floor metamorphism. There is no positive evidence to suggest the effect of regional (i.e., orogenic) metamorphism.
198 TABLE V Composition of basaltic rocks of RC8-RD4 Specimen no. and type FeO*/MgO CaO/Na20
5(B)
4(A)
8(A)
9(A)
11(A)
I(B)
10(A)
2(A)
1.10 3.9
1.13 4.3
1.16 4.4
1.20 4.1
1.21 4.6
1.23 3.9
1.26 4.7
1.33 4.2
SiO2 Ti02 A1203 Fe203 FeO MnO MgO CaO
48.62 1.28 14.40 4.24 5.47 0.19 8.43 11.10
49.65 1.32 15.40 2.96 6.00 0.19 7.66 12.13
49.30 1.53 15.13 3.59 5.71 0.19 7.71 12.03
48.68 1.43 15.74 3.89 5.07 0.19 7.13 12.32
49.19 1.20 15.99 3.71 5.25 0.18 7.12 12.30
50.62 1.60 14.41 5.12 4.47 0.16 7.37 10.79
49.13 1.43 14.79 4.24 5.40 0.16 7.31 12.21
49.21 1.31 15.56 3.70 5.75 0.19 6.84 12.08
Na20 K20 H20-
2.83 0.06 1.41 1.90 0.10 100.03
2.85 0.06 0.83 0.76 0.10 99.91
2.73 0.07 1.03 1.02 0.11 100.15
2.99 0.05 1.24 1.38 0.10 100.21
2.68 0.07 0.89 1.33 0.10 100.01
2.82 0.14 1.31 1.12 0.12 100.05
2.61 0.09 0.95 1.39 0.10 99.81
2.87 0.05 1.34 1.09 0.09 100.08
9.29 ---
8.66 pl pl
8.57 .pl pi
8.59 pl pl
. 0.30 25.30 29.38 9.28
0.41 22.68 31.40 4.89
Ig. loss
P205 Total
FeO* Pheno. Micropheno.
8.94 pi + aug pl
9.08 9.22 -pl -- pl + aug
9.08 pl pl
CIPW norm
Q or ab an ol
hy di mt il ap
. 0.36 23.95 26.41 8.91
9.24 22.74 2.18 2.43 0.23
.
.
0.36 24.12 29.05 6.42
8.59 24.73 2.18 2.51 0.23
. 0.41 23.10 28.83 5.54
10.2.1 24.47 2.18 2.91 0.26
.
2.71 25.26 2.18 2.72 0.23
0.81 0.83 23.86 26.25 --
0.53 22.09 28.38 3.84
0.30 24.29 29.43 5.69
9.94 18.48 11.55 23.57 2.18 2.28 0.23
21.55 2.18 3.04 0.28
25.69 2.18 2.72 0.23
8.44 24.41 2.18 2.49 0.21
Note: analyzed by F. Shido. Type A represents abyssal tholeiites, and type B doleritic basalts and dolerites of abyssal tholeiite composition. CIPW N O R M S
AND THE COURSES OF CRYSTALLIZATION
OF PYROXENES
Calculation of CIPW norms, made directly from our chemical analyses, gives s m a l l a m o u n t s o f n o r m a t i v e q u a r t z f o r m a n y a n a l y s e s , j u s t as f o u n d for the analyses of Chase a n d Hersey ( 1 9 6 8 ) a n d N a l w a l k (1969). As the h i g h F e 2 0 3 c o n t e n t s .are p r o b a b l y d u e t o s e c o n d a r y a l t e r a t i o n , t h e f e r r i c
199 iron was partly reduced to Fe203 = 1.50%, and then norms were calculated as shown in Tables III--V. In this case, the rocks show b o t h normative hypersthene and olivine, except for t w o rocks in which normative quartz occurs. Thus, at least most of our rocks are probably olivine tholeiites in the normative definition proposed by Yoder and TiUey (1962), as are most abyssal tholeiites. It is n o t clear whether the normative quartz in the two exceptions is due to chemical migration during secondary alteration. The pigeonite-bearing rocks ( R C 8 - R D l - t y p e 2) are higher in normative hypersthene and lower in normative diopside than the other basaltic rocks. Their original magmas were probably in the pigeonite-crystallization field. Their A1203 contents and hence their normative anorthite contents are not higher than those of the other rocks. Hence, the high contents of normative hypersthene cannot be ascribed to assimilation of aluminous sediments. There is no significant difference in the a m o u n t of normative olivine between different types of rocks. The only direct cause for the high contents of normative hypersthene consists in the lower CaO contents. The normative pyroxenes are recalculated into molecular percentages of Wo(CaSiO3 ), En(MgSiO3 ), and Fs(FeSiO3 ), and are shown in Fig.2. It is observed in Fig.2 that rocks in which augite is the only pyroxene show higher Wo contents than rocks in which pigeonite was the first pyroxene to crystallize o u t of magma. In other words, augite and pigeonite were the first pyroxene to crystallize o u t of magma higher and lower, respectively, in Wo component. Hence, augite and pigeonite are in a cotectic-like relation with an apparent cotectic line lying near 25% Wo for Wo + En + Fs = 100%. However, pigeonite is always rimmed by augite and occurs only in phenocrysts and microphenocrysts and n o t in the groundmass (Table II). This gives us an impression that the crystallization of pigeonite was discontinued
Di
Hd
5of
40/' IO
o
En
~o
~
~o
~
go
~
so
F's
Fig.2. Molecular compositions o f normative pyroxenes in basaltic rocks from the Puerto Rico Trench. Dots represent rocks in which augite was the first and the only pyroxene to crystallize out o f magma (RC8-RD1 type 1 and RC8-RD4 types A and B), whereas crosses represent rocks in which pigeonite was the first pyroxene to crystallize (RC8-RD1 type 2).
200 when augite mantles began to form around pigeonite crystals in magmas of RC8-RD1 t y p e 2. However, the possibility is n o t precluded that pigeonite crystallized in the groundmass also, b u t afterwards it was lost by secondary alteration. The calculated compositions of normative pyroxenes depend on the assumption Fe203 = 1.50%. I-f we take an alternative assumption that Fe203 = 0.00%, Fs molecule increases, and the apparent cotectic line shifts toward a higher Wo content by a few percent. CHEMICAL COMPARISON OF PUERTO RICO TRENCH THOLEIITES OF THE MID-ATLANTIC RIDGE
BASALTS WITH ABYSSAL
Recently we f o u n d that the abyssal tholeiites in the Mid-Atlantic Ridge crest show a marked compositional variation at the latitude of the Azores (about 39 ° N). The abyssal tholeiites from north of the Azores tend to show higher CaO and lower Na20 contents than those from south of the Azores. CaO/Na20 ratios are usually 7.5--4.0 in the former, and 4.6--3.5 in the latter (Shido and Miyashiro, 1973). Our Puerto Rico Trench abyssal tholeiites of RC8-RD1 types 1A and 1B and RC8-RD4 types A and B are in the composition range of the abyssal tholeiites from south of the Azores. The rare-earth element abundance patterns of these Puerto Rico Trench rocks were determined by Jibiki and Masuda (1974), who found that they are virtually identical to the abundance patterns of abyssal tholeiites from the Mid-Atlantic Ridge crest. On the other hand, our doleritic basalts of RC8-RD1 t y p e 2 show CaO/Na20 = 2.2--1.8, that is, lower than those of the known abyssal tholeiites of the Mid-Atlantic Ridge crest. The occurrence of phenocrysts with a pigeonite core in these rocks is a result of the low contents o f CaO and the low CaO/Na20 ratios. The close association of these rocks with, and their general chemical similarity to, typical abyssal tholeiites suggest that these rocks represent a new t y p e of abyssal tholeiites characterized b y very low CaO/Na20 ratios. Jibiki and Masuda (1974) examined one of these rocks and f o u n d that it shows a rare-earth element abundance pattern different from those of ordinary abyssal tholeiites, that is, a pattern With generally greater enrichment of light rare-earth elements as compared with the chondrite pattern. SERPENTINITE
A serpentinite from dredge haul CH19-D10 has been newly analyzed as shown in Table VI. The specimen contains irregular bastite grains set in a finer grained serpentine matrix. Talc is mixed with serpentine in some bastites. Dust of secondary magnetite is disseminated. No primary igneous minerals occur. Smectite occurs mainly as a vein mineral. The dredge site is shown in Table I and Fig.1. The analysis shows A12 03 and CaO contents lower than most serpentinites from mid-oceanic ridges (cf. Miyashiro et al.,
201 TABLE VI Compositions of serpentinites from dredge CH19-D10 in the Puerto Rico Trench 4
6
SiO: TiO~ Al:O3 Fe~O~ FeO MnO MgO CaO
38.66 0.10 0.87 8.22 1.09 0.08 35.43 0.14
39.34 <0.005 0.61 7.25 0.94 0.04 37.28 0.15
Na20 K20 H~O÷ P20~ Cr203 NiO
0.24 0.028 1.19 12.55 0.02 0.42 0.27
0.24 <0.005 2.08 11.32 0.05 0.42 0.43
Total
99.31
H20-
100.16
Note: Specimen 4 analyzed by F. Shido. The data of specimen 6 by Bowin et al. (1966), containing also 0.019% F and 0.024% Cl.
1 9 6 9 b ) . B o w i n et al. ( 1 9 6 6 , table 7, 2nd c o l u m n ) r e p o r t e d an analysis o f a s e r p e n t i n i t e f r o m t h e same dredge haul. T h e t w o results are similar. DISCUSSIONS AND CONCLUSIONS T y p i c a l abyssal tholeiites o c c u r o n b o t h n o r t h and s o u t h walls o f the P u e r t o Rico T r e n c h . We f o u n d n o rocks o f t h e c o n t i n e n t a l (island arc) t y p e . P r o b a b l y , the abyssal tholeiites were f o r m e d b y e r u p t i o n in o r n e a r the axial part o f a mid-oceanic ridge ( p r e s u m a b l y the Mid-Atlantic Ridge), and were t r a n s p o r t e d f r o m the ridge as p a r t o f a m o v i n g plate t o w a r d t h e subd u c t i o n z o n e n o r t h o f P u e r t o Rico. Just b e f o r e these r o c k s r e a c h e d the s u b d u c t i o n zone, the s u b d u c t i o n had been drastically slowed owing t o a change in t h e d i r e c t i o n o f plate m o v e m e n t , p r e s u m a b l y near t h e e n d o f E o c e n e time. Strike-slip m o v e m e n t began along the t r e n c h and the original oceanic crust was cut b y a strike-slip fault. Thus, very similar r o c k s o f the mid-oceanic ridge t y p e are n o w e x p o s e d on t h e o p p o s i t e walls o f t h e trench. If this i n t e r p r e t a t i o n is a c c e p t e d , the basaltic r o c k s n o w e x p o s e d in the P u e r t o Rico T r e n c h w o u l d have been e r u p t e d in C r e t a c e o u s t i m e in the Mid-Atlantic Ridge. B o w i n et al. ( 1 9 6 6 ) r e p o r t e d t h e o c c u r r e n c e o f L a t e C r e t a c e o u s ( C e n o m a n i a n ) fossils in s e d i m e n t a r y rocks d r e d g e d f r o m the
202
n o r t h wall. All t h e volcanic a n d s e d i m e n t a r y layers in the l o w e r wall o f the t r e n c h w o u l d have been d e p o s i t e d in C r e t a c e o u s time. Nalwalk {1969) c o n s i d e r e d t h a t the s e r p e n t i n i t e in the P u e r t o R i c o T r e n c h was i n t r u d e d into t h e C r e t a c e o u s rocks. This i n t e r p r e t a t i o n appears t o be c o n s i s t e n t with the rarity of this r o c k t y p e in dredges. Nalwalk {1969) r e p o r t e d t h e result o f K - - A r dating o f t h r e e basalts f r o m the n o r t h wall. Their ages are: 7.0 m.y., 4.8 m.y. and less t h a n 12 m , y . T h o u g h the possibility o f e r u p t i o n at such late times is n o t p r e c l u d e d , it appears m o r e likely t h a t these y o u n g ages are due t o w e a t h e r i n g o f the samples. ACKNOWLEDGEMENTS We are i n d e b t e d t o the crew a n d scientists o f the " R o b e r t C o n r a d " cruise 8, a n d particularly t o Drs. A l l y n Vine, Marcus L a n g s e t h a n d R. Gerard. Drs. J. B. Hersey a n d C. O. B o w i n k i n d l y gave us a sample o f s e r p e n t i n i t e f r o m the P u e r t o Rico Trench. Mr. Tsugio S h i b a t a p a r t i c i p a t e d in o u r chemical work. Drs. Peter Mattson, Paul J. F o x and S t e p h e n D e L o n g read t h e m a n u script with helpful criticism. T h e p r e s e n t s t u d y was m a d e with financial s u p p o r t f r o m the G. Unger Vetlesen F o u n d a t i o n a n d the N a t i o n a l Science F o u n d a t i o n { G A - 2 0 7 4 2 and A O 2 0 7 4 2 - 0 0 1 ) . This is C o n t r i b u t i o n No. 2 0 5 1 o f L a m o n t - D o h e r t y Geological O b s e r v a t o r y , and C o n t r i b u t i o n No. 33 o f E a r t h and Planetary Sciences Division, Marine B i o m e d i c a l I n s t i t u t e , University o f Texas Medical B r a n c h at Galveston. REFERENCES Bowin, C. O., Nalwalk, A. J. and Hersey, J. B., 1966. Serpentinized peridotite from the north wall of the Puerto Rico Trench. Geol. Soc. Am. Bull., 77: 257--270. Chase, R. L. and Hersey, J. B., 1968. Geology of the north slope of the Puerto Rico Trench. Deep-Sea Res., 15: 297--317. Conolly, J. R. and Ewing, M., 1967. Sedimentation in the Puerto Rico Trench. J. Sediment. Petrol., 37: 44--59. Engel, A. E. J. and Engel, C. G., 1964. Composition of basalts from the Mid-Atlantic Ridge. Science, 144: 1330--1333. Fisher, R. L. and Engel, C. G., 1969. Ultramafic and basaltic rocks dredged from the nearshore flank of the Tonga Trench. Geol. Soc. Am. Bull., 80: 1373--1378. Hart, S. R., 1971. K, Rb, Cs, Sr and Ba contents and Sr isotope ratios of ocean floor basalts. Phil. Trans. R. Soc. Lond., Ser. A, 268: 573--587. Hart, S. R. and Nalwalk, A. J., ]970. K, Rb, Cs and Sr relationships in submarine basalts from the Puerto Rico Trench. Geochim. Cosmochim. Acta, 34: 145--155. Hekinian, R. and Aumento, F., 1973. Rocks from the Gibbs fracture zone and the Minia Seamount near 53°N in the Atlantic Ocean. Mar. Geol., 14: 47--72. Jibiki, H. and Masuda, A., 1974. Basalts and serpentinite from the Puerto Rico Trench, 2. Rare-earth geochemistry. Mar. Geol., 16: 205--211. Malfait, B. T. and Dinkelman, M. G., 1972. Circum-Caribbean tectonics and igneous activity and the evolution of the Carribbean plate. Geol. Soc. Am. Bull., 83: 251--272. Mattson, P. H., 1966. Geological characteristics of Puerto Rico. In: W. H. Poole (Editor), Continental Margins and Island A r c s - Geol. Surv. Can., Paper 66--15, pp. 124--138.
203 Miyashiro, A., Shido, F. and Ewing, M., 1969a. Diversity and origin of abyssal tholeiite from the Mid-Atlantic Ridge near 24 ° and 30 ° north latitude. Contrib. Mineral. Petrol., 23: 38--52. Miyashiro, ~_, Shido, F. and Ewing, M., 1969b. Composition and origin of serpentinites from the Mid-Atlantic Ridge near 24 ° and 30 ° north latitude. Contrib. Mineral. Petrol., 23 : 117--127. Miyashiro, A., ShidO, F. and Ewing, M., 1970. Crystallization and differentiation in abyssal tholeiites and gabbros from mid-oceanic ridges. Earth Planet. Sci. Lett., 7: 361--365. Miyashiro, A., Shido, F. and Ewing, M., 1971. Metamorphism in the Mid-Atlantic Ridge near 24 ° and 30°N. Phil. Trans. R. Soc. Lond., Ser. A, 268: 589--603. Molnar, P. and Sykes, L. R., 1969. Tectonics of the Caribbean and Middle America regions from focal mechanisms and seismicity. Geol. Soc. Am. Bull., 80: 1639--1684. Nalwalk, A~ J., 1969. Geology of a portion of the north wall of the Puerto Rico Trench. Tectonophysics, 8: 403--425. Shido, F, and Miyashiro, A., 1973. Compositional difference between abyssal tholeiites from north and south of the Azores on the Mid-Atlantic Ridge. Nat. Phys. Sci., 245: 59--60. Shido, F., Miyashiro, A. and Ewing, M., 1971. Crystallization of abyssal tholeiites. Contrib. Mineral. Petrol., 31: 251--266. Yagi, K., 1960. A dolerite block dredged from the bottom of the Vitiaz Deep, Mariana Trench. Proc. Japan Acad., 36: 213--216. Yoder, Jr., H. S. and Tilley, C. E., 1962. Origin of basalt magmas: an experimental study of natural and synthetic rock systems. J. Petrol., 3: 342--532.
©Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
BASALTS AND SERPENTINITE FROM THE PUERTO RICO TRENCH, 1. PETROLOGY
FUMIKO
SHIDO' , A K I H O M I Y A S H I R O '
and M A U R I C E
EWING'
'Department of Geological Sciences, State University of N e w York at Albany, Albany, N.Y. (U.S.A.) 2The Marine Biomedical Institute, The University o f Texas at Galveston, Galveston, Texas (U.S.A.)
(Accepted for publication January 10, 1974}
ABSTRACT Shido, F., Miyashiro, A. and Ewing, M., 1974. Basalts and serpentinite from the Puerto R i c o Trench, 1. Petrology. Mar. Geol., 16: 191--203. Typical abyssal t h o l e i i t e s o c c u r on both north and south wallsof the Puerto Rico Trench. Probably they were erupted in the Mid-Atlantic Ridge in Cretaceous time and afterwards were moved to the present position. In addition, doleritic basalts containing phenocrysts of pigeonite rimmed by augite were found from the north wall, and these rocks presumably represent a n e w t y p e of abyssal tholeiites characterized by CaO/Na20 ratios lower than t h o s e o f ordinary abyssal tholeiites. A new analysis of serpentinite from the north wall is also recorded here.
INTRODUCTION
A large a m o u n t of petrographic and chemical data has been published on igneous rocks from ocean floors in the past decade. Most of the rocks studied came f r o m the mid-oceanic ridge system and seamounts. We have come to know t h a t the surface layer of the mid-oceanic ridges is made up mainly of low-K, high-Na tholeiites (e.g., Engel and Engel, 1964), usually called oceanic tholeiites or abyssal tholeiites. (The latter name is preferred in this paper, because some authors include oceanic island tholeiites in the category of oceanic tholeiites, though such rocks differ in chemical characteristics from the ocean-floor tholeiites now under discussion.) We k n o w the composition ranges and the trends of crystallization differentiation of abyssal tholeiites (Miyashiro et al., 1970; Shido et al., 1971; Hekinian and Aumento, 1973; Shido and Miyashiro, 1973). The deeper parts of the ridges appear to be made up mainly of metamorphosed tholeiite and gabbro possibly associated with serpentinite or peridotite (e.g., Miyashiro et al., 1971). However, our knowledge o f hard rocks in other parts of the ocean floor is still very limited.
192 Hard rocks sampled from a few trenches have been reported. A fresh dolerite dredged from the b o t t o m of the Vitiaz Deep in the Mariana Trench was described by Yagi (1960). Consideration of his analytical data in light of our present-day knowledge of ocean-floor rocks indicates that the rock is tholeiitic dolerite probably derived from abyssal tholeiite magma. Fisher and Engel {1969) recorded some altered gabbroic and basaltic rocks together with fresh peridotite from the nearshore flank of the Tonga Trench. Serpentinites and basalts from the north wall of the Puerto Rico Trench were described by Bowin et al. (1966), Chase and Hersey (1968), Nalwalk {1969), Hart and Nalwalk (1970), and Hart (1971). Unfortunately, however, the Puerto Rico Trench basalts were so strongly altered that their original characteristics were n o t clear. In 1964, the research vessel " R o b e r t Conrad" of Lamont-Doherty Geological Observatory made two successful rock dredges in the Puerto Rico Trench, one on the north and the other on the south wall. The degree of alteration of the basaltic rocks obtained is much less than that of the Puerto Rico Trench basalts reported in the past. The original chemical features are so well preserved that we could identify them as abyssal tholeiites. Therefore, we intend to describe these basalts in this paper. A specimen of serpentinite from the north wall was obtained through the courtesy of Drs. J. B. Hersey and C. O. Bowin of Woods Hole Oceanographic Institution. It will also be described in this paper. PUERTO RICO TRENCH AND DREDGE STATIONS Ordinary trenches such as the Mariana and Tonga are formed in the zones of plate convergence along island arcs or continental margins. On the other hand, the Puerto Rico Trench is now characterized by strike-slip motion with only a small component of underthrusting (e.g., Molnar and Sykes, 1969; Malfait and Dinkelman, 1972). In the Island of Puerto Rico, intense volcanism of the island arc type occurred from middle Cretaceous to Eocene time {e.g., Mattson, 1966). Hence, we may consider that there was a subduction zone, presumably to the north of Puerto Rico, from the Middle Cretaceous to the Eocene, and t h a t thereafter the direction of plate movement has changed and strike~slip movement has become dominant. The top of the north wall of the trench is about 5500 m deep (Fig.l). The wall dips to the south at an average angle of 5 ° , but locally there are many pronounced scarps which presumably represent parallel faults running generally in east--west directions. The north wall has an especially steep and complicated fault structure in the area of 65 ° 00'--65°15'W longitude {Nalwalk, 1969, fig.3). Our dredge RC8-RD1 was made there {Table I). The trench floor, about 7500 m deep, is covered by thick sediments. Dredge RC8-RD4 was recovered from the northeastern slope of a ridge, called the Main Ridge by Conolly and Ewing (1967), which lies at the foot of the south wall and trends approximately WNW. No rock dredges have been reported, so far as we know, from this ridge or other parts of the south wall of the trench.
193
5
5
0
TrenchFloor 5
5
0
0
m
~
~. 4"/01:. X~__. ~__~____ ~;n,g/.:.,, RCB-RD4 0
m
~
IO0km Fig.1. Dredge s t a t i o n s in t h e P u e r t o Rico T r e n c h . Dredge haul C H 1 9 - D 1 0 was d e s c r i b e d b y B o w i n e t al. (1966). Dredge hauls CH34-D3 a n d D4 were d e s c r i b e d b y Chase a n d Hersey ( 1 9 6 8 ) a n d Nalwalk ( 1 9 6 9 ) . T h e dredges o f t h e AII-11 g r o u p d e s c r i b e d by Nalwalk ( 1 9 6 9 ) were m a d e nearly a t t h e s a m e p o s i t i o n s as o u r dredge R C 8 - R D 1 .
TABLE I Dredge s t a t i o n s Dredge No.
Latitude
Longitude
Corrected depth (m)
Note
RC8-RD1 RC8-RD4 CH19-D10
20 ° 0 6 . 8 ' N 19 ° 3 6 ' N 20 ° 0 0 ' N
65 ° 0 6 . 5 ' W 65 ° 0 4 ' W 66 ° 31' 5 7 " W
6591 7260 ?
n o r t h wall s o u t h wall n o r t h wall
Dredge RC8-RD1 This dredge haul, obtained from the north wail (Table I; Fig.l), includes a large number of angular basaltic fragments, of which 43 were thin-sectioned. The manganiferous coating is absent in many specimens and is t o o thin to measure in others. These basaltic rocks may be classified into three types: Type 1A: relatively poorly crystalline basalts (abyssal tholeiites). These basalts usually contain a large a m o u n t of spherulitic and dendritic aggregates of microlites in the groundmass (Table II). Glass, if present, has been altered to smectite. They are typical abyssal tholeiites with plagioclase phenocrysts. Type 1B: holocrystalline basalts (abyssal tholeiites). This t y p e has a better crystallized groundmass with no aggregates of microlites, but appears to be similar to type 1A in other respects. Most of the rocks of this type contain only plagioclase as phenocrysts. However, fragment No.2 contains both plagioclase and augite (2 V = 45o--50 ° ) as phenocrysts. The groundmass
194 TABLE II Primary mineral composition of each rock type Dredge
Type Phenocrysts
RC8-RD1 1A
1B
RC8-RD4 A
B
p|agioclase
Microphenocrysts
Groundmass
Rock name (CaO/Na20)
plagioclase
plagioclase, (augite), microlites
typical abyssal tholeiite (4.8--3.9)
plagioclase; plagioclase rarely with augite (2V = 450--50 ° )
plagioclase, typical abyssal augite tholeiite (2V= 45°--52 ° ) (4.4--4.1) magnetite
zoned clinozoned clinopyroxene pyroxene (2V = 0°--20 ° , 45°--47 ° )
plagioclase, augite, magnetite
doleritic basalt, a new type of abyssal tholeiite (2.2--1.8)
plagioclase; rarely with augite ( 2 V = 54 °)
plagioclase, augite, magnetite
typical abyssal tholeiite (4.7--4.1)
plagioclase; rarely with augite ( 2 V = 54 °)
plagioclase + augite ( 2 V - - 48 ° )
doleritic basalt and dolerite of abyssal tholeiite composition (3.9)
Note: for the distinction between phenocrysts and microphenocrysts, refer to p.48 of Miyashiro et al. (1969a).
pyroxene is augite with 2 V = 45°--50 ° (fragment No.2), 47 ° (No.4), and 52 ° (No.9). Type 1A grades into t y p e 1B with increasing crystallinity. Though olivine is not present now, the shapes of some aggregates of alteration products (mainly smectite) suggest the possibility of the former presence of olivine as microphenocrysts as well as in the groundmass. Table III shows the chemical composition of rocks of types 1A and lB. They all are ordinary abyssal tholeiites. The rocks of t y p e 1B show higher total iron and MgO contents than those of type 1A. This may be due to the accumulation of mafic minerals during the slow cooling which resulted in t y p e lB. The original magmas of the two types were probably virtually identical. Type 2: holocrystaUine doleritic basalts with zoned clinopyroxene phenocrysts. This t y p e is coarser grained than types 1A and lB. The degree of alteration is higher so that the color o f hand specimens is greyish yellow ocher.
195 TABLE III Compositions of basaltic rocks of types 1A and 1B in RC8-RD1 Specimen no. and type
8(1A)
4(1B)
FeO*/MgO CaO/Na20
1.09 4.1
1.12 4.3
1.14 4.8
1.14 4.6
1.15 4.5
1.26 3.9
1.28 4.4
SiO2 TiO 2 A120 ~ Fe~O 3 FeO MnO MgO CaO
48.71 1.40 15.95 3.65 5.57 0.18 8.09 11.40
48.79 1.40 15.39 4.36 5.18 0.19 8.16 11.63
48.97 1.31 15.38 3.51 5.63 0.20 7.68 11.94
48.86 1.33 15.14 4.06 5.57 0.21 8.10 11.83
49.07 1.23 15.64 3.61 5.52 0.19 7.65 11.77
49.51 1.27 16.05 3.20 5.24 0.16 6.47 12.10
48.49 1.44 15.32 5.02 4.91 0.19 7.35 11.96
Na~O K20
2.78 0.05 0.96 1.15 0.11 100.00
2.72 0.04 0.99 1.03 0.10 99.98
2.47 0.06 0.96 1.81 0.09 100.01
2.59 0.03 1.19 1.02 0.10 100.03
2.63 0.06 1.33 1.48 0.09 100.27
3.08 0.05 1.54 1.25 0.10 100.02
2.72 0.07 1.30 1.35 0.11 100.23
8.86 pl pl
9.10 pl p!
8.79 pl --
9.22 pl + aug --
8.77 -pl
8.12 pl --
9.43 pl --
0.36 20.90 30.70 3.21 14.30 22.69 2.18 2.49 0.21
0.18 21.92 29.60 6.69 11.15 23.11 2.18 2.53 0.23
0.36 22.26 30.70 4.91 12.29 22.02 2.18 2.34 0.21
0.30 26.06 29.82 4.75 7.28 24.04 2.18 2.41 0.2~
0.41 23.02 20.39 7.59 7.83 23.83 2.18 2.74 0.26
H~OIg. loss P20s Total FeO* Pheno. Micropheno.
3(1A)
2(1B)
I(1A)
6(1A)
9(1B)
CIPW norm
or ab an ol hy di mt il ap
0.30 23.52 30.90 8.59 9.06 20.22 2.18 2.66 0.26
0.24 23.02 29.67 8.21 9.26 22.22 2.18 2.66 0.23
Note: FeO* in this and the succeeding tables means total iron as FeO. The norms were calculated after partial reduction of F % O 3 so as to give Fe20 ~ = 1.50% in all these tables. Analyzed by F. Shido. Type 1B shows higher crystallinity than type 1A.
The phenocrysts (if present) and microphenocrysts of zoned clinopyroxenes are made up of a pigeonite core ( 2 V = 0o--20 ° ) rimmed b y augite ( 2 V = 45 ° 47 ° ). The boundary between the core and rim is sharp and there is no pyroxene of intermediate 2 V and hence of intermediate composition. The pigeonite core is always markedly lower in birefringence than the augite rim. Augite occurs also in the groundmass. Plagioclase is confined to the
196 T A B L E IV Compositions of basaltic rocks o f t y p e 2 in RC8-RD1 S p e c i m e n no. FeO*/MgO CaO/Na20
11
18
17
12
23
1.21 2.2
1.31 1.9
1.34 1.9
1.35 1.9
1.35 1.8
SiO2 TiO2 AI203 Fe203 FeO MnO MgO CaO
52.05 1.90 14.60 4.77 4.24 0.09 7.05 7.61
50.82 2.09 16.73 5.97 3.51 0.20 6.77 7.44
50.87 2.09 16.69 5.85 3.59 0.17 6.61 7.58
50.33 2,14 15.48 6.06 3.26 0,09 6,44 7.34
50.43 2.04 16.39 6.27 3.72 0.20 6.92 7.44
Na20 K20 H20Ig. loss P205 Total
3.47 0.51 1.71 1.86 0.22 100.08
3.99 0.75 0.34 1.53 0.22 100.36
3.98 0.72 0.37 1.48 0.24 100.24
3.80 0.69 2.37 1.86 0.25 100.11
4.05 0.72 0.38 1.59 0.22 100.37
FeO* Pheno. Micropheno.
8.53 -(altered)
8.88 zoned p y r zoned pyr
8.86 z o n e d pyr zoned pyr
8.71 -zoned pyr
9.36 zoned pyr
CIPW norm
Q or ab an oi hy
2.56 3.01 29.36 22.76 -21.21
di mt il ap
10.99 2.18 3.61 0.51
. 4.43 33.76 25.53 7.59 12.00
. 4.26 33.68 25.55 6.58 12.67
. 4.08 32.16 23.15 1.55 18.23
. 4.26 34.27 24.42 10.44 8.98
8.08 2.18 3.97 0.51
8.52 2.18 3.97 0.56
9.45 2.18 4.06 0.58
9.00 2.18 3.87 0.51
N o t e : 11 and 12 analyzed by F. 8hido, and the rest by Tsugio Shibata. T y p e 2 is characterized by phenocrysts and m i c r o p h e n o c r y s t s of z o n e d c l i n o p y r o x e n e s with pigeonite cores.
groundmass (Table II). Because of the high degree of alteration, it is not clear whether olivine was ever present or not. Table IV shows the chemical analyses of rocks of this type. These rocks are generally similar to abyssal tholeiites, but do not fall in the composition range of ordinary members of the abyssal tholeiites. Their K20 and Na20 contents are higher than those of ordinary abyssal tholeiites. The SiO2 and TiO2 contents also tend to be rather high. They show unusually low CaO
197
contents and CaO/Na20 ratios. Four of the 5 analyses given in Table IV show normative olivine and hypersthene after the partial reduction of iron to Fe203 = 1.50%. The high K20 contents and the normative quartz in one analysis might be due to secondary alteration. With increasing FeO*/MgO, the FeO*, TiO2, Na20 and K20 contents tend to increase, whereas the MgO content tends to decrease, though the variations are very small. Here, FeO* means total iron as FeO. The SiO2 content remains nearly constant. These variations may be regarded as representing a tholeiitic trend of fractionation.
Dredge RC8-RD4 This dredge haul, recovered from the south wall of the Trench (Table I; Fig.l), contains more than a hundred fragments of basaltic rocks, of which 50 were thin-sectioned. Most fragments have no manganiferous coating, but some are covered by a manganiferous film too thin to measure. They are classified into t w o types: Type A: abyssal tholeiites. Most of the fragments belong to this type. Some samples are poorly crystalline so as to contain a large a m o u n t of dendritic crystals and microlitic spherulites in the groundmass, and others are made up of well-crystallized mineral grains. Though there are a few nonporphyritic varieties, most of the rocks contain phenocrysts of plagioclase rarely accompanied by those of augite (2V = 53 ° ). Type B: doleritic basalts and dolerites o f abyssal tholeiite composition. Only a few fragments belong to this type.. They are coarser grained and show a subophitic texture. Table V shows that analyzed specimens of type B show a little lower A1203 and CaO contents tklan the rocks of type A, conceivably owing to the accumulation of mafic minerals during the formation of type B, just as in that of type 1B of RC8-RD1. The shapes of some aggregates of alteration products (smectite, etc.) suggest the possible former presence of olivine in some thin sections. However, olivine is not present now. ALTERATION
All of our 20 analyzed basalts show Fe203 > 2.9%. The amounts of ignition loss of the t w e n t y basalts are in the range of 0.7--1.9%. These indicate that the basalts are secondarily altered to a considerable extent. The main alteration product appears to be smectite or related minerals. Small amounts of carbonate and secondary opaque minerals are common. Zeolite was observed in several thin sections (e.g., RC8-RD1-8 and RC8-RD4-2 among analyzed rocks). The alteration would be a combination of deuteric alteration, weathering and ocean-floor metamorphism. There is no positive evidence to suggest the effect of regional (i.e., orogenic) metamorphism.
198 TABLE V Composition of basaltic rocks of RC8-RD4 Specimen no. and type FeO*/MgO CaO/Na20
5(B)
4(A)
8(A)
9(A)
11(A)
I(B)
10(A)
2(A)
1.10 3.9
1.13 4.3
1.16 4.4
1.20 4.1
1.21 4.6
1.23 3.9
1.26 4.7
1.33 4.2
SiO2 Ti02 A1203 Fe203 FeO MnO MgO CaO
48.62 1.28 14.40 4.24 5.47 0.19 8.43 11.10
49.65 1.32 15.40 2.96 6.00 0.19 7.66 12.13
49.30 1.53 15.13 3.59 5.71 0.19 7.71 12.03
48.68 1.43 15.74 3.89 5.07 0.19 7.13 12.32
49.19 1.20 15.99 3.71 5.25 0.18 7.12 12.30
50.62 1.60 14.41 5.12 4.47 0.16 7.37 10.79
49.13 1.43 14.79 4.24 5.40 0.16 7.31 12.21
49.21 1.31 15.56 3.70 5.75 0.19 6.84 12.08
Na20 K20 H20-
2.83 0.06 1.41 1.90 0.10 100.03
2.85 0.06 0.83 0.76 0.10 99.91
2.73 0.07 1.03 1.02 0.11 100.15
2.99 0.05 1.24 1.38 0.10 100.21
2.68 0.07 0.89 1.33 0.10 100.01
2.82 0.14 1.31 1.12 0.12 100.05
2.61 0.09 0.95 1.39 0.10 99.81
2.87 0.05 1.34 1.09 0.09 100.08
9.29 ---
8.66 pl pl
8.57 .pl pi
8.59 pl pl
. 0.30 25.30 29.38 9.28
0.41 22.68 31.40 4.89
Ig. loss
P205 Total
FeO* Pheno. Micropheno.
8.94 pi + aug pl
9.08 9.22 -pl -- pl + aug
9.08 pl pl
CIPW norm
Q or ab an ol
hy di mt il ap
. 0.36 23.95 26.41 8.91
9.24 22.74 2.18 2.43 0.23
.
.
0.36 24.12 29.05 6.42
8.59 24.73 2.18 2.51 0.23
. 0.41 23.10 28.83 5.54
10.2.1 24.47 2.18 2.91 0.26
.
2.71 25.26 2.18 2.72 0.23
0.81 0.83 23.86 26.25 --
0.53 22.09 28.38 3.84
0.30 24.29 29.43 5.69
9.94 18.48 11.55 23.57 2.18 2.28 0.23
21.55 2.18 3.04 0.28
25.69 2.18 2.72 0.23
8.44 24.41 2.18 2.49 0.21
Note: analyzed by F. Shido. Type A represents abyssal tholeiites, and type B doleritic basalts and dolerites of abyssal tholeiite composition. CIPW N O R M S
AND THE COURSES OF CRYSTALLIZATION
OF PYROXENES
Calculation of CIPW norms, made directly from our chemical analyses, gives s m a l l a m o u n t s o f n o r m a t i v e q u a r t z f o r m a n y a n a l y s e s , j u s t as f o u n d for the analyses of Chase a n d Hersey ( 1 9 6 8 ) a n d N a l w a l k (1969). As the h i g h F e 2 0 3 c o n t e n t s .are p r o b a b l y d u e t o s e c o n d a r y a l t e r a t i o n , t h e f e r r i c
199 iron was partly reduced to Fe203 = 1.50%, and then norms were calculated as shown in Tables III--V. In this case, the rocks show b o t h normative hypersthene and olivine, except for t w o rocks in which normative quartz occurs. Thus, at least most of our rocks are probably olivine tholeiites in the normative definition proposed by Yoder and TiUey (1962), as are most abyssal tholeiites. It is n o t clear whether the normative quartz in the two exceptions is due to chemical migration during secondary alteration. The pigeonite-bearing rocks ( R C 8 - R D l - t y p e 2) are higher in normative hypersthene and lower in normative diopside than the other basaltic rocks. Their original magmas were probably in the pigeonite-crystallization field. Their A1203 contents and hence their normative anorthite contents are not higher than those of the other rocks. Hence, the high contents of normative hypersthene cannot be ascribed to assimilation of aluminous sediments. There is no significant difference in the a m o u n t of normative olivine between different types of rocks. The only direct cause for the high contents of normative hypersthene consists in the lower CaO contents. The normative pyroxenes are recalculated into molecular percentages of Wo(CaSiO3 ), En(MgSiO3 ), and Fs(FeSiO3 ), and are shown in Fig.2. It is observed in Fig.2 that rocks in which augite is the only pyroxene show higher Wo contents than rocks in which pigeonite was the first pyroxene to crystallize o u t of magma. In other words, augite and pigeonite were the first pyroxene to crystallize o u t of magma higher and lower, respectively, in Wo component. Hence, augite and pigeonite are in a cotectic-like relation with an apparent cotectic line lying near 25% Wo for Wo + En + Fs = 100%. However, pigeonite is always rimmed by augite and occurs only in phenocrysts and microphenocrysts and n o t in the groundmass (Table II). This gives us an impression that the crystallization of pigeonite was discontinued
Di
Hd
5of
40/' IO
o
En
~o
~
~o
~
go
~
so
F's
Fig.2. Molecular compositions o f normative pyroxenes in basaltic rocks from the Puerto Rico Trench. Dots represent rocks in which augite was the first and the only pyroxene to crystallize out o f magma (RC8-RD1 type 1 and RC8-RD4 types A and B), whereas crosses represent rocks in which pigeonite was the first pyroxene to crystallize (RC8-RD1 type 2).
200 when augite mantles began to form around pigeonite crystals in magmas of RC8-RD1 t y p e 2. However, the possibility is n o t precluded that pigeonite crystallized in the groundmass also, b u t afterwards it was lost by secondary alteration. The calculated compositions of normative pyroxenes depend on the assumption Fe203 = 1.50%. I-f we take an alternative assumption that Fe203 = 0.00%, Fs molecule increases, and the apparent cotectic line shifts toward a higher Wo content by a few percent. CHEMICAL COMPARISON OF PUERTO RICO TRENCH THOLEIITES OF THE MID-ATLANTIC RIDGE
BASALTS WITH ABYSSAL
Recently we f o u n d that the abyssal tholeiites in the Mid-Atlantic Ridge crest show a marked compositional variation at the latitude of the Azores (about 39 ° N). The abyssal tholeiites from north of the Azores tend to show higher CaO and lower Na20 contents than those from south of the Azores. CaO/Na20 ratios are usually 7.5--4.0 in the former, and 4.6--3.5 in the latter (Shido and Miyashiro, 1973). Our Puerto Rico Trench abyssal tholeiites of RC8-RD1 types 1A and 1B and RC8-RD4 types A and B are in the composition range of the abyssal tholeiites from south of the Azores. The rare-earth element abundance patterns of these Puerto Rico Trench rocks were determined by Jibiki and Masuda (1974), who found that they are virtually identical to the abundance patterns of abyssal tholeiites from the Mid-Atlantic Ridge crest. On the other hand, our doleritic basalts of RC8-RD1 t y p e 2 show CaO/Na20 = 2.2--1.8, that is, lower than those of the known abyssal tholeiites of the Mid-Atlantic Ridge crest. The occurrence of phenocrysts with a pigeonite core in these rocks is a result of the low contents o f CaO and the low CaO/Na20 ratios. The close association of these rocks with, and their general chemical similarity to, typical abyssal tholeiites suggest that these rocks represent a new t y p e of abyssal tholeiites characterized b y very low CaO/Na20 ratios. Jibiki and Masuda (1974) examined one of these rocks and f o u n d that it shows a rare-earth element abundance pattern different from those of ordinary abyssal tholeiites, that is, a pattern With generally greater enrichment of light rare-earth elements as compared with the chondrite pattern. SERPENTINITE
A serpentinite from dredge haul CH19-D10 has been newly analyzed as shown in Table VI. The specimen contains irregular bastite grains set in a finer grained serpentine matrix. Talc is mixed with serpentine in some bastites. Dust of secondary magnetite is disseminated. No primary igneous minerals occur. Smectite occurs mainly as a vein mineral. The dredge site is shown in Table I and Fig.1. The analysis shows A12 03 and CaO contents lower than most serpentinites from mid-oceanic ridges (cf. Miyashiro et al.,
201 TABLE VI Compositions of serpentinites from dredge CH19-D10 in the Puerto Rico Trench 4
6
SiO: TiO~ Al:O3 Fe~O~ FeO MnO MgO CaO
38.66 0.10 0.87 8.22 1.09 0.08 35.43 0.14
39.34 <0.005 0.61 7.25 0.94 0.04 37.28 0.15
Na20 K20 H~O÷ P20~ Cr203 NiO
0.24 0.028 1.19 12.55 0.02 0.42 0.27
0.24 <0.005 2.08 11.32 0.05 0.42 0.43
Total
99.31
H20-
100.16
Note: Specimen 4 analyzed by F. Shido. The data of specimen 6 by Bowin et al. (1966), containing also 0.019% F and 0.024% Cl.
1 9 6 9 b ) . B o w i n et al. ( 1 9 6 6 , table 7, 2nd c o l u m n ) r e p o r t e d an analysis o f a s e r p e n t i n i t e f r o m t h e same dredge haul. T h e t w o results are similar. DISCUSSIONS AND CONCLUSIONS T y p i c a l abyssal tholeiites o c c u r o n b o t h n o r t h and s o u t h walls o f the P u e r t o Rico T r e n c h . We f o u n d n o rocks o f t h e c o n t i n e n t a l (island arc) t y p e . P r o b a b l y , the abyssal tholeiites were f o r m e d b y e r u p t i o n in o r n e a r the axial part o f a mid-oceanic ridge ( p r e s u m a b l y the Mid-Atlantic Ridge), and were t r a n s p o r t e d f r o m the ridge as p a r t o f a m o v i n g plate t o w a r d t h e subd u c t i o n z o n e n o r t h o f P u e r t o Rico. Just b e f o r e these r o c k s r e a c h e d the s u b d u c t i o n zone, the s u b d u c t i o n had been drastically slowed owing t o a change in t h e d i r e c t i o n o f plate m o v e m e n t , p r e s u m a b l y near t h e e n d o f E o c e n e time. Strike-slip m o v e m e n t began along the t r e n c h and the original oceanic crust was cut b y a strike-slip fault. Thus, very similar r o c k s o f the mid-oceanic ridge t y p e are n o w e x p o s e d on t h e o p p o s i t e walls o f t h e trench. If this i n t e r p r e t a t i o n is a c c e p t e d , the basaltic r o c k s n o w e x p o s e d in the P u e r t o Rico T r e n c h w o u l d have been e r u p t e d in C r e t a c e o u s t i m e in the Mid-Atlantic Ridge. B o w i n et al. ( 1 9 6 6 ) r e p o r t e d t h e o c c u r r e n c e o f L a t e C r e t a c e o u s ( C e n o m a n i a n ) fossils in s e d i m e n t a r y rocks d r e d g e d f r o m the
202
n o r t h wall. All t h e volcanic a n d s e d i m e n t a r y layers in the l o w e r wall o f the t r e n c h w o u l d have been d e p o s i t e d in C r e t a c e o u s time. Nalwalk {1969) c o n s i d e r e d t h a t the s e r p e n t i n i t e in the P u e r t o R i c o T r e n c h was i n t r u d e d into t h e C r e t a c e o u s rocks. This i n t e r p r e t a t i o n appears t o be c o n s i s t e n t with the rarity of this r o c k t y p e in dredges. Nalwalk {1969) r e p o r t e d t h e result o f K - - A r dating o f t h r e e basalts f r o m the n o r t h wall. Their ages are: 7.0 m.y., 4.8 m.y. and less t h a n 12 m , y . T h o u g h the possibility o f e r u p t i o n at such late times is n o t p r e c l u d e d , it appears m o r e likely t h a t these y o u n g ages are due t o w e a t h e r i n g o f the samples. ACKNOWLEDGEMENTS We are i n d e b t e d t o the crew a n d scientists o f the " R o b e r t C o n r a d " cruise 8, a n d particularly t o Drs. A l l y n Vine, Marcus L a n g s e t h a n d R. Gerard. Drs. J. B. Hersey a n d C. O. B o w i n k i n d l y gave us a sample o f s e r p e n t i n i t e f r o m the P u e r t o Rico Trench. Mr. Tsugio S h i b a t a p a r t i c i p a t e d in o u r chemical work. Drs. Peter Mattson, Paul J. F o x and S t e p h e n D e L o n g read t h e m a n u script with helpful criticism. T h e p r e s e n t s t u d y was m a d e with financial s u p p o r t f r o m the G. Unger Vetlesen F o u n d a t i o n a n d the N a t i o n a l Science F o u n d a t i o n { G A - 2 0 7 4 2 and A O 2 0 7 4 2 - 0 0 1 ) . This is C o n t r i b u t i o n No. 2 0 5 1 o f L a m o n t - D o h e r t y Geological O b s e r v a t o r y , and C o n t r i b u t i o n No. 33 o f E a r t h and Planetary Sciences Division, Marine B i o m e d i c a l I n s t i t u t e , University o f Texas Medical B r a n c h at Galveston. REFERENCES Bowin, C. O., Nalwalk, A. J. and Hersey, J. B., 1966. Serpentinized peridotite from the north wall of the Puerto Rico Trench. Geol. Soc. Am. Bull., 77: 257--270. Chase, R. L. and Hersey, J. B., 1968. Geology of the north slope of the Puerto Rico Trench. Deep-Sea Res., 15: 297--317. Conolly, J. R. and Ewing, M., 1967. Sedimentation in the Puerto Rico Trench. J. Sediment. Petrol., 37: 44--59. Engel, A. E. J. and Engel, C. G., 1964. Composition of basalts from the Mid-Atlantic Ridge. Science, 144: 1330--1333. Fisher, R. L. and Engel, C. G., 1969. Ultramafic and basaltic rocks dredged from the nearshore flank of the Tonga Trench. Geol. Soc. Am. Bull., 80: 1373--1378. Hart, S. R., 1971. K, Rb, Cs, Sr and Ba contents and Sr isotope ratios of ocean floor basalts. Phil. Trans. R. Soc. Lond., Ser. A, 268: 573--587. Hart, S. R. and Nalwalk, A. J., ]970. K, Rb, Cs and Sr relationships in submarine basalts from the Puerto Rico Trench. Geochim. Cosmochim. Acta, 34: 145--155. Hekinian, R. and Aumento, F., 1973. Rocks from the Gibbs fracture zone and the Minia Seamount near 53°N in the Atlantic Ocean. Mar. Geol., 14: 47--72. Jibiki, H. and Masuda, A., 1974. Basalts and serpentinite from the Puerto Rico Trench, 2. Rare-earth geochemistry. Mar. Geol., 16: 205--211. Malfait, B. T. and Dinkelman, M. G., 1972. Circum-Caribbean tectonics and igneous activity and the evolution of the Carribbean plate. Geol. Soc. Am. Bull., 83: 251--272. Mattson, P. H., 1966. Geological characteristics of Puerto Rico. In: W. H. Poole (Editor), Continental Margins and Island A r c s - Geol. Surv. Can., Paper 66--15, pp. 124--138.
203 Miyashiro, A., Shido, F. and Ewing, M., 1969a. Diversity and origin of abyssal tholeiite from the Mid-Atlantic Ridge near 24 ° and 30 ° north latitude. Contrib. Mineral. Petrol., 23: 38--52. Miyashiro, ~_, Shido, F. and Ewing, M., 1969b. Composition and origin of serpentinites from the Mid-Atlantic Ridge near 24 ° and 30 ° north latitude. Contrib. Mineral. Petrol., 23 : 117--127. Miyashiro, A., ShidO, F. and Ewing, M., 1970. Crystallization and differentiation in abyssal tholeiites and gabbros from mid-oceanic ridges. Earth Planet. Sci. Lett., 7: 361--365. Miyashiro, A., Shido, F. and Ewing, M., 1971. Metamorphism in the Mid-Atlantic Ridge near 24 ° and 30°N. Phil. Trans. R. Soc. Lond., Ser. A, 268: 589--603. Molnar, P. and Sykes, L. R., 1969. Tectonics of the Caribbean and Middle America regions from focal mechanisms and seismicity. Geol. Soc. Am. Bull., 80: 1639--1684. Nalwalk, A~ J., 1969. Geology of a portion of the north wall of the Puerto Rico Trench. Tectonophysics, 8: 403--425. Shido, F, and Miyashiro, A., 1973. Compositional difference between abyssal tholeiites from north and south of the Azores on the Mid-Atlantic Ridge. Nat. Phys. Sci., 245: 59--60. Shido, F., Miyashiro, A. and Ewing, M., 1971. Crystallization of abyssal tholeiites. Contrib. Mineral. Petrol., 31: 251--266. Yagi, K., 1960. A dolerite block dredged from the bottom of the Vitiaz Deep, Mariana Trench. Proc. Japan Acad., 36: 213--216. Yoder, Jr., H. S. and Tilley, C. E., 1962. Origin of basalt magmas: an experimental study of natural and synthetic rock systems. J. Petrol., 3: 342--532.