Magnetic fabric of peridotite with intersecting petrofabric surfaces, Tinaquillo, Venezuela

Physics of the Earth and Planetary Interiors, 51 (1988) 301—312 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands

301

Magnetic fabric of peridotite with intersecting petrofabric surfaces, Tinaquillo, Venezuela W.D. MacDonald Department of Geological Sciences, Stale University of New York, Binghamton, NY 13901 (USA.)

Brooks B. Eliwood Department of Geology, The University of Texas at Arlington, Arlington, TX 76019 (USA.) (Received April 9, 1986: revision accepted January 30, 1987)

MacDonald, W.D. and Ellwood, B.B., 1988. Magnetic fabric of pendotite with intersecting petrofabric surfaces, Tinaquillo, Venezuela. Phys. Earth Planet. Inter., 51: 301—312. Two intersecting structural surfaces of different ages and origins are characteristic of the Tinaquillo (Venezuela) peridotite. An older penetrative mylonitic foliation is cut by younger serpentine veinlets. The amsotropy of magnetic susceptibility (AMS) is influenced by both petrofabric components. The dominant fabric surface from the AMS perspective is defined by serpentine veinlets, probably because of the abundant secondary magnetite associated with them. The minimum AMS axis (K 3) is typically orthogonal to the plane of the dominant veinlet Set, and the maximum (K1) and intermediate (K2) axes lie in the veinlet plane. The mylonitic foliation surface also influences the magnetic fabric, so that the K1 axis lies parallel to the line of intersection between the mylomtic foliation and the veinlet plane. A macroscopically prominent mineral lineation, defined by stretched enstatite laths in the foliation surface, shows no AMS correlation. Remanent magnetization directions are multicomponent and show much scatter, probably due to the deformation and complex magnetization history of the rock.

1. Introduction The magnetic fabric of a rock is defined by the orientation and relative magnitudes of its maximum (K1), intermediate (K2), and minimum (K3) magnetic susceptibility axes. The K1 and K2 directions are commonly oriented parallel to a mineralogically controlled foliation plane, such as gneissic banding, slaty cleavage, schistosity, bedding, and so on (Graham, 1954; Granar, 1958; Stone, 1963; Graham, 1966; Hamilton and Rees, 1971; Hrouda and Janak, 1976). In rocks where a mineralogically defined lineation is also present, the K1 axis is commonly parallel to that direction. This study describes the magnetic fabric associated with an intersection petrofabric in a peridotite from northern Venezuela, amplifying our 0031-9201/88/$03.50

© 1988 Elsevier Science Publishers B.V.

earlier report (MacDonald and Eliwood, 1985). The older, macroscopically dominant surface in this petrofabric is a penetrative mylonitic foliation. The later, and generally less obvious, fabric surface consists of discrete magnetite—serpentine veinlets in oriented sets. The magnetic foliation plane, containing the K1 and K2 axes, does not coincide with the macroscopically dominant penetrative mylonitic foliation, but instead corncides with the more subtly expressed magnetite— serpentine veinlets. Although the peridotite shows a pronounced lineation, emphasized by large, stretched enstatite laths, these linear features do not significantly influence the magnetic fabric. The magnetic lineation K1 axis parallels the line of intersection between mylonitic foliation and serpentine veinlets.

302

~ phyllite ~AR ~

—c.

-)

~

•

Tinaquillo ~

~iL. peridotite

~v

~

~

72

70

~ 89

y

—V.-!.)

TINAQUILLO r,T!~*~

/

0 I

5km 1

I B. 10

t

~

yneiss

I

~

,~lr

~

1~

—,

..~..-

Fig. 1. Study area (rectangle) is located near town of Tinaquillo, in western Caribbean Mountains of northern Venezuela (inset). Sampling sites lie along the east central exposed limit of Tinaquillo peridotite. Site 88 is in amphibolite host-rock. Base map after MacKenzie (1960). Qal is Quaternary alluvium.

1.1. Geologic setting and petrology The Tinaquillo peridotite is located at the west end of the Caribbean Mountains of northern Venezuela, near their junction with the Venezuelan Andes (Fig. 1). The peridotite crops out 5 km to the west of the town of Tinaquillo, and measures about 15 km long by 7 km wide. Its extent to the east is unknown because of Quaternary alluvial cover. The geology of the Tinaquillo area is known mainly from the work of MacKenzie (1960), from which much of the following description is taken. The age of the peridotite is not yet established. The metamorphic host-rock for the Tinaquillo peridotite is mainly a hornblende—quartz—oligoclase gneiss. It may possibly be as old as Late Paleozoic (Ostos-Rosales, 1985). Because the peridotite foliation (Mattson, 1985; Ostos-Rosales,

1985) is regionally concordant with Cretaceous foliation trends in the Caribbean Mountains, a deformation or redeformation in the Cretaceous is suggested. A zone 2—5 km wide adjacent to the southwest contact with the peridotite consists of garnet—pyroxene—hornblende—plagioclase gneiss, and has been interpreted as a thermal metamorphic aureole (Mackenzie, 1960). The peridotite is typically a well-foliated enstatic dunite (harzburgite), with pyroxene layers, lenses, nodules and laminae, and with lenses of gabbroic composition. The rock is characterized by a penetrative mylonitic foliation (Fig. 2) with a well-developed flow lineation evident in elongate enstatitic laths. The contact metamorphic aureole and the pervasive mylonitic fabric suggest that the peridotite reached its present location as a hot, plastic diapir. The rock is generally only slightly serpentinized. To the north, the peridotite and its

Fig. 2. Serpentinization along joints has produced serpentine veinlets with secondary magnetite in Tinaquillo peridotite. The dominant veinlet set (site 86) is N—S, vertical and is orthogonal to the mylonitic peridotite foliation, which dips 30°S.(A) Outcrop appearance, showing locally well-developed planar-parallel serpentine veinlets; elsewhere, typical veinlets are centimeter length, thinner, and less well developed. (B) Oriented horizontal thin section shows N—S vertical veinlet set, with secondary magnetite, cross-cutting the south-dipping E—W foliation. Arrow indicates north. Section length 3 mm. (C) Block sketch of idealized arrangement of AMS axes relative to the intersecting veinlet and mylonitic foliation surfaces.

303

IIi~

a.

:-~

I

VEINLET

K~

FOLIATION~

~

K

3

* INTERSECTION—V

K1

~

/

EN S TATITE LINEATION

2

304

country rock are thrust over phyllite of probable Late Cretaceous age.

detergent and subsequently handled using surgical gloves to prevent darkening of the surfaces by adsorption of oils.

2. Methods 3. Results Oriented samples from six sites in the peridotite and one site in the host amphibolite were used for the study of petrofabric—magnetic fabric relationships. Approximately six 2.4 cm diameter cores were drilled at each site. The sampling zone in the peridotite stretches 2 km in a N—S direction, perpendicular to the regional foliation, along the central east limit of the exposures (Fig. 1). The samples were taken in Quebrada Pedernales and its tributaries. A seventh site was sampled in the amphibolite 7 km away (site 88), approximately along strike, near the southeast limits of Tinaquillo. In the laboratory, cores were cut to 2.2 cm length for measurement of anisotropy of magnetic susceptibility (AMS) and of natural remanent moment (NRM). NRM measurements were made using a Moispin Minispin magnetometer. Pilot samples from each site were a.f. demagnetized in the sequence 5, 10, 15, 20, 30, 40, 60, 80, and 100 mT. All other samples were demagnetized at 20, 30, 40, 60, 80, and 100 mT. Vector orthogonal projections were made for each sample. AMS measurements were made using a low field (<10 mT) torsion fiber magnetometer calibrated using a seamless copper ring (Noltimier, 1964). Background on the AMS procedures is given in Ellwood (1984). Isothermal remanent moment (IRM) measurements of pilot samples from each site were also performed, in fields up to 0.6 T. Fourteen oriented thin sections were cut for petrographic study. These were cut in diverse 3rentations: horizontal, vertical east—west, vertical north—south, and in planes perpendicular to macroscopically visible foliation and veinlets. The orientations of 6—20 serpentine veinlets from 4 cores for each site were measured using the standard oriented specimen cores. Cores were placed in a custom adapter which replaced the hemispheres in a four-axis universal stage. To facilitate marking and measuring the traces of the petrofabric elements, cores were first soaked in strong

3.1. Petrofabric The peridotite shows a pervasive mylonitic foliation. As noted by MacKenzie (1960), abundant enstatite laths, flattened parallel to [100] and elongated parallel to Z, accentuate the foliation and also define a prominent subhorizontal lineation. In the sampling region, the foliation dips 20—30° SSW, and the lineation trends subhorizontally WNW—ESE. Extension fractures in the enstatite laths are commonly filled with finegrained olivine. Veinlet serpentinization of many generations post-dates the lineation development. 3.1.1. Serpentine veinlets Serpentine in the Tinaquillo peridotite is not abundant, making up less than a few percent of a typical sample. It occurs in diverse habits, as both massive antigorite and fibrous chrysotile. The most important serpentine from the point of view of this study is in serpentine veinlets. Early replacements by serpentine follow the foliation irregularly, forming ‘ground-mass’ serpentine. Serpentine veinlets are of many generations and at least three generations can be seen from cross-cutting relationships in a single thin section. Not all serpentine veinlets are accompanied by secondary magnetite. Instead, some veinlets show resorption of earlier magnetite at veinlet intersections. Veinlets contain both massive and fibrous serpentine, both with and without magnetite, and probably represent hydrous reactions along fracture sets in the peridotite. The veinlets are variable in shape and arrangement; intertwining, intersecting and truncating one another. Some typical patterns of veinlet poles and foliation poles are shown in the right-hand column of hemispheric projections in Fig. 3. Two different patterns emerge for the pendotite. At the four most northerly sites (86, 90, 91 and 92), a dominant NNW veinlet trend is found

305

with vertical to steep easterly dips. Schematic representations of these results are given in Fig. 4. A

N

N 90

different pattern of veinlet orientation is found at the two southern sites (87 and 89). Here a weakly developed E—W veinlet trend with near vertical dip was found for site 89 (Fig. 5). A more random veinlet arrangement was determined for site 87. However, both sites have similar magnetic fabrics. N

N //~s~\

89

-(S

7.7

N

F~

N

~~92

N

N Fig. 4. Combined AMS fabric—petrofabric modal plots for 91

£~

petidotite sites with dominant N—S serpentine Vemiet (V) great-circle intersects mylomtic foliation veinlets. (F) great-circle near K 1 axis (U). Symbols as in Fig. 3.

F

•

. ..

—

.

.

•

~

•I

A single site (88) in the host-rock amphibolite

B N

N

U 4A A

J

•F

J

:

Fig. 3. Plots of typical AMS axes (left) paired with petrofabric elements (right) for the Tinaquillo peridotite and its amphibolite host. AMS axes (left): U = K1 (maximum); A K2 (intermediate); S = k3 (minimum). Petrofabnc axes (right): S = veinlet pole; F = foliation pole, L = mineral lineation pole. The peridotite and amphibolite have different types of foliations and veinlets (see text). Equal-area lower hemisphere projection. (A) Peridotite site 89; dominant serpentine veinlet set trends E—W, vertical. (B) Peridotite site 91; dominant serpentine veinlet set trends N—S, vertical. (C) Amphibolite site 88; K1 axis parallels the regional E—W structural trend (see also lineations, Fig. 1).

was The rock displays prominent foliation. sampled. No macroscopic lineation a was observed at site 88. However, the magnetic fabric has a pronounced lineation which suggests that a mineral lineation may be present (Fig. 5). This rock contains abundant opaques and appears to have two generations of hornblende. Scarce quartz veinlets

cut the foliation at a low angle. 3.2. Magnetic fabric The AMS fabric patterns are of three types. These are represented in Fig. 3 (left colunm). For all peridotite sites, the K1, K2 and K3 axes are relatively well grouped, although at a few sites some groups show mixed populations of K2 and K3 axes. The K2 axes are subvertical to steeply inclined at most sites. The mean data are given in Tables I and II and are plotted in Figs. 4 and 5. For the northern group of sites, the magnetic foliation plane, containing K1 and K2, trends NNW with a steep easterly dip. K3 trends easterly

306 N

N

87

Ge Q N

88

B

Fig. 5. Comparison of combined AMS fabric-petrofabric plots for peridotite (A) and amphibolite (B). The peridotite sites have E—W vertical veinlets (A, right) or possibly only microscopic veinlets (A, left). The amphibolite has scarce quartz veinlets which intersect metamorphic foliation at a low angle. The amphibolite K1 axis (U) is approximately parallel to the regional E—W structural rend and lies in the plane of the metamorphic foliation. The intermediate AMS K2 axis (A), rather than the more typical K3 axis (S), is normal to this metamorphic foliation.

For the amphibolite (site 88), the magnetic foliation plane is not well defined. K2 and K3 show marked streaking in a great circle about K1, while not overlapping (Fig. 3C, left). K1 defines a good cluster, indicating a lineation parallel to the

macroscopic by Ostos-Rosales regional lineation fig. 3, p.trend 2571). established No macroscopic AMS lineation and (1985), was remanent measurable moment atIn site (RM) 88. usual data areThe summarized in Tables I—Ill. the V (1966), where 2 V= (K asinlineation 2fabric interpretation ofK3)/(K1 ellipticity is characterized ratios K3) such by Vas 45°. This relationship is not valid for these peridotite sites because of the —

—

dual intersecting fabrics. For example, prolate ellipticity signatures V 37.2° and V 32.3°, suggestive of strong lineations, are found for pendotite sites 91 and 92, while for the other two sites in the same northern group (86 and 90), oblate ellipticity fabrics (V 54.4° and V 48.1°) are observed (Table III; see also Figs. 8—10). The ellipticity ratio for the amphibolite, V 35.2°, is =

=

=

=

=

consistent with a lineation fabric, and is parallel to the regional lineation trend. 3.3. Magnetic mineralogy

with shallow inclination, parallel to the serpentine veinlet pole (Fig. 4). The magnetic lineation, i.e. the major axis (K1) of the AMS ellipsoid, is moderately inclined to the southeast, approximately parallel to the intersection between the mylonitic foliation and the veinlet plane (see Fig. 2C). For the southern peridotite sites 87 and 89, the magnetic foliation plane trends WNW with near vertical dip (Figs. 3A left, and 5A). Here K3 trends west of south with shallow inclination, close to the veinlet poles for this locality (Fig. 5A, right). Here also, the major susceptibility axis K1 lies close to the intersection of the mylonitic foliation with the veinlet plane at site 89. The geomettic parallelism is not as clear at site 87 where a more random veinlet pattern was obtained.

The dominant magnetic mineral in the pendotite is magnetite, of many generations. An early generation is associated with the mylonitic foliation. Several later generations can be distinguished in serpentine veinlets by their cross-cutting relationships. IRM experiments show saturation at fields near 0.2 T (Fig. 6) at room temperature, indicating the dominance of magnetic minerals of low to moderate coercivity. Only the amphibolite (site 88) shows increasing IRM intensities above 0.3 T, indicating the additional presence of high coercivity minerals, such as hematitie or ilmenite, but with magnetite still dominating. RM intensities, IRM intensities, total susceptibilities (Tables I—Ill) and thin section study show that opaque oxides are one to two orders of magnitude more abundant in the amphibolite than in

307 TABLE I Tinaquillo Peridotite AMS data Sample No.

K

KI/K

2

K2/K3

K1

K2

K3 D

I

VP-86-2 VP-86-3 VP-86-4 VP-86-5 VP-86-6 MEAN

5.06 4.36 14.32 5.02 5.27 6.81

1.06 1.01 1.08 1.14 1.07 1.07

1.18 1.20 1.57 1.04 1.07 1.28

151.3 346.7 154.4 342.5 10.4 345.5

42.7 —7.8 19.4 —36.9 —9.3 —23.9

183.5 5.4 25.9 28.9 305.1 9.1

—42.5 81.8 60.5 42.5 68.6 61.9

257.5 257.0 72.3 93.3 277.2 87.8

16.8 2.6 —21.3 —25.2 —19.1 —10.4

VP-87-1 VP-87-2 VP-87-3 VP-87-4 VP-87-5 VP-87-6 VP-87-7 MEAN

3.71 4.22 3.45 12.92 18.61 3.83 5.05 7.40

1.10 1.08 1.11 1.06 1.04 1.11 1.08 1.06

1.26 1.08 1.28 1.06 1.09 1.17 1.13 1.11

272.2 278.9 268.7 99.2 102.5 89.0 93.4 94.7

—0.8 2.9 3.4 —2.9 —23.7 2.6 13.5 —2.1

273.1 35.1 124.3 76.8 70.4 187.7 284.1 111.1

89.2 83.4 85.9 86.9 62.6 65.5 76.2 87.2

2.2 8.6 358.9 9.2 6.7 357.8 4.0 4.0

0.0 —5.9 2.4 —1.2 —12.9 24.3 —2.5 0.3

1244.54 92.90 88.28 534.53 528.10 139.49 487.84

1.01 1.04 1.02 1.01 1.01 1.02 1.01

1.00 1.02 1.02 1.00 1.01 1.02 1.01

292.0 325.2 297.4 289.3 292.1 285.9 116.3

7.6 6.1 13.4 9.0 9.4 8.8 —9.3

188.6 67.5 149.2 37.5 80.9 67.7 92.0

60.2 63.1 74.3 63.2 79.1 78.9 78.4

26.2 232.2 29.3 15.0 21.2 14.8 25.5

28.6 26.1 8.0 —25.0 —5.5 —6.8 —4.8

VP-89-1 VP-89-2 VP-89-3 VP-89-4 VP-89-5 VP-89-6 MEAN

1.66 1.73 1.94 2.31 1.90 1.79 1.89

1.13 1.03 1.02 1.09 1.10 1.04 1.07

1.04 1.10 1.14 1.12 1.04 1.11 1.09

97.9 102.9 98.3 98.1 103.9 96.2 99.6

11.5 —5.9 2.8 6.3 2.1 —9.1 1.3

192.7 39.4 346.4 289.7 193.8 28.3 15.8

22.3 76.9 82.5 83.6 —4.5 67.0 78.4

342.5 191.7 188.7 8.2 218.6 182.7 7.5

64.6 11.6 7.0 —1.3 85.1 21.0 —10.9

VP-90-1 VP-90-2 VP-90-3 VP-90-4 VP-90-6 MEAN

34.11 13.12 12.28 7.32 9.94 15.35

1.07 1.01 1.09 1.12 1.06 1.06

1.02 1.13 1.16 1.07 1.07 1.07

176.5 344.5 343.3 356.5 154.9 347.3

18.7 —28.5 —9.5 —13.7 —10.9 —12.4

321.1 280.9 288.3 348.7 120.5 302.5

67.4 39.4 73.7 76.2 76.8 74.4

262.3 49.9 71.1 266.0 63.5 72.0

—12.1 37.5 13.1 —1.8 —7.3 11.0

VP-91-2 VP-91-3 VP-91-4 VP-91-5 VP.91-6 VP-91-7 MEAN

6.66 8.84 14.07 2.00 5.17 1.79 6.42

1.17 1.04 1.06 1.05 1.20 1.04 1.09

1.08 1.04 1.17 1.02 1.05 1.01 1.09

165.6 152.6 163.1 138.7 129.4 127.9 325.6

32.3 49.8 32.1 42.5 17.2 25.8 34.4

162.2 148.0 142.7 329.2 41.2 359.3 339.4

—57.7 —40.1 —56.2 47.0 —5.8 52.2 50.3

254.6 239.8 247.1 53.6 329.3 51.2 62.8

—1.6 —2.1 —9.4 —5.2 71.8 —25.5 —4.2

VP-92-1 VP-92-2 VP-92-3 VP-92-4 MEAN

23.09 5.04 3.08 2.63 8.46

1.12 1.07 1.04 1.07 1.10

1.03 1.04 1.03 1.01 1.03

159.7 333.1 135.5 320.4 327.1

26.5 —4.2 —15.7 —17.0 —8.5

349.9 53.6 43.4 358.6 19.2

63.1 65.7 —7.4 68.8 63.6

251.7 64.9 109.0 234.2 63.5

4.1 —23.9 72.6 12.4 —14.2

VP-88-1 VP-88-2 VP-88-3 VP-88-4 VP-88-5 VP-88-6 MEAN

*

*

D

I

D

I

Site in amphibolite contact zone; AMS magnitudes and directions, for specimens and site means. K1, K2, K3 = maximum, intermediate and minimum susceptibility, 1< iO~,in SI units. D = declination in degrees, east of north; I = inclination in degrees, positive downward.

308 TABLE II Tinaquillo Pendotite Lithology

Peridotite Peridotite Amphibolite Peridotite Peridotite Peridotite Peridotite

AMS data

Site mean

Site

K

1

86 87 88 89 90 91 92

K2

K3

K1

D

I

K2

D

I

K3

D

I

7.7 8.0 442.2 2.0 16.4 7.0 9.1

345.5 094.7 116.3 099.6 347.3 325.6 327.1

—23.9 —02.1 —09.3 01.3 —12.4 —34.6 —08.5

7.2 7.5 436.7 1.9 15.4 6.4 8.3

009.1 111.1 092.0 015.8 302.5 339.4 019.2

61.9 87.2 78.4 78.4 74.4 50.3 63.6

5.6 6.7 434.5 1.7 14.3 5.9 8.0

87.8 004.0 025.5 007.5 072.0 061.8 063.5

—10.4 00.3 —04.8 —10.9 11.0 —04.2 —14.2

K1, K2, K3 = maximum, intermediate and minimum susceptibility, I = inclination in degrees, positive downward.

X

the peridotite. The grain size of the magnetite in the serpentine veinlets is in the micron range. Magnetite replacements adjacent to the serpentine veinlets are larger, approximately the same size as matrix magnetite (about 0.1 mm).

Chromite is present in the peridotite in amounts much less than 1%. Chromite grains typically are up to several millimeters in diameter and are elongate with ragged borders. Individual lenses up to 10 mm by 30 mm are also observed. Except where altered to magnetite along serpentine veinlets, chromite probably does not contribute significantly to either the RM or AMS fabric. Bleil and Petersen (1981) indicate that chromite with a Cr/Fe atomic ratio greaten than 0.5 has a Curie

~

~ 872

30

A!m~_~.~ 89~2

6

882 lOll

temperature less than 100°C, and thus probably

IRM 20 10,

0 0

2

02

04

A/rn

IRM

~0

06

A

does not contribute significantly to the RM.

50

‘

0

0-2

0-4

0-6

0

0

02

c

0-4

0-6

N/up

21)

02

(2

6B00 02 ° Magnetizing Field

N/up 92.2

tlii

~

00

iO~, in SI units. D = declination in degrees, east of north;

92.3

E,E

:~O~t~0

04

06

04

Fig. 6. Isothermal remanent moment versus magnetizing field. Sharp shoulders at low fields below 0.2 T indicate that moderate coercivity material, i.e. magnetite, is the principal magnetic mineral. (A) Pendotite with dominant E—W veinlet set, (B) Peridotite with dominant N—S veinlet set. (C) Host-rock amphibolite, with additional ramp at 0.4 T, indicating the presence of a higher coercivity phase, such as ilmenite or hematite.

~+2O

5i~~15

-~6 S/down

6 S/down

Fig. 7. Multiple generations of serpentine—magnetite veinlets explain the complex multiple-component moment directions in orthogonal plots such as these for sample 92.2 (left). Less common univectoral behavior is seen in a few samples, such as in sample 92.3 above 20 mT a.f. magnetic induction (right). Units along axes are 1o 2 Ain1. Peak a.f. magnetic induction is in mT.

309 TABLE III Site mean AMS and RM ratios and magnitudes Site

K

H

V

K

86 87 88 89 90 91 92

6.8 7.4 437.8 1.9 15.4 6.4 8.5

23.0 20.9 3.0 15.2 15.1 14.8 10.3

54.4 50.3 35.2 48.6 48.1 37.2 32.3

1.073 1.064 1.013 1.069 1.064 1.091 1.099

1/K2

K2/K3

K1/K3

M

1.283 1.111 1.005 1.092 1.072 1.089 1.033

1.377 1.182 1.018 1.167 1.141 1.188 1.136

1.3 6.0 170.8 2.0 1.5 2.5 2.2

= mass susceptibility x i0~ SI units (see text, eq. (3)); H = total anisotropy (see text, eq. (2)); V in degrees, after Graham (1966) (see text, eq. (1)); K1, K2, K3 as in TABLE I. M = moment X 10~A m~. All data here are site means.

3.4. Remanent magnetization Zijderveld (1967) diagrams (Fig. 7) indicate strong multi-component moments at many sites, consistent with thin section observations of multiple generations of magnetite. Therefore, it is not surprising that the RM directions (Fig. 11) are not well-grouped. Further comments on RM directions follow later.

4. Discussion This investigation has shown that for five of the six penidotite sites, the principal axis of magnetic susceptibility (K1) is aligned with the intersection between mylonitic foliation and serpentine veinlets. This indicates that both the veinlet fabric and the mylonite fabric contribute to the AMS ellipsoid. The principal AMS ellipsoid plane, contaming K1 and K2, is aligned parallel to the serpentine veinlets. This can be attributed to the formation of abundant secondary magnetite accompanying serpentinization along joints after emplacement and cooling of the peridotite. Previous studies on the AMS of peridotite and serpentinite are few. Bogue (1983) describes AMS results from a much less deformed peridotite in Alaska. There the elongation of the AMS ellipsoid is parallel to the intersection between a primary crystal layering in peridotite and a secondary planar fabric related to serpentinization. From another geologic setting, Rathore (1980) described the AMS of country rocks surrounding the senpentinite in the Lizard area, Cornwall, but not of the serpentinite itself.

Numerous studies have been made on bulk susceptibilities of peridotite and serpentinized peridotite, but without AMS or structural fabric comparisons (Frolich and Stiller, 1960; Griscom, 1964; Cox et al., 1964; Saad, 1969; Wagner, 1984). These studies find increased susceptibilities associated with the development of serpentine and associated magnetite. Because the foliation of Tinaquillo peridotite is mylonitic (i.e. deformational) in origin, comparisons with other mylonitic fabrics are pertinent. In a study of the Lake Char (Connecticut) mylonite zone, Goldstein (1980) found some oblate AMS ellipsoids in the plane of the mylonitic foliation. He deduced the presence of superimposed fabrics (mylonitic foliation over host-rock metamorphic foliation) in his analysis of the AMS patterns. His work (Goldstein, 1980, figs. 3b, 5, 6, and 10) suggests that the intersection between Lake Char mylonite foliation and the pre-mylonite metamorphic foliation corresponds to the AMS intermediate axis, K2. For the Tinaquillo peridotite, it is the K1 axis which corresponds to the intersection of foliation with veinlet surfaces. Bogue (1983) reported a marked anisotropy for the Duke Island, Alaska, serpentinized peridotite. The Tinaquillo peridotite similarly shows (Fig. 8) a high degree of total anisotropy H (Owens, 1974), where H= [(K1

—

K3)/K]

x 100

and K, the bulk susceptibility, is given by k= (K, + K2 + K3)/3

(2)

(3)

In comparison, the amphibolite country rock (site

310

and

TINAQUILLO PERIDOTITE

f

50

86 4 • •

•

S. •~9l

(5)

.

•

.

xlOO

This variation is clearly related to the orientation of the veinlets relative to each other and relative to the foliation. This can be illustrated with reference to two ‘end-members’. The most prolate ellipsoid (site 92, Fig. 10) corresponds to a

40

20

[(ic2—K3)/k]

•

8987

:

Fig. 8. High total anisotropy characterizes the Tinaquillo pendotite, as seen in this plot of total anisotropy (H) versus Graham’s V. Specimen (5) and site-mean (•) values are plotted,

single well-defined veinlet set which consequently intersects the mylonitic foliation in a singular way. These intersections are parallel to the prolate axis, K1. The most oblate ellipsoid (site 86, Fig. 10) corresponds to similarly abundant veinlets of which many fan out from a central maximum set. These veinlets intersect the mylonitic foliation in a great-circle streaking in the foliation plane and they intersect one another approximately along

88) exhibits a much lower H, although with much higher magnetite content. This is probably explained by the relatively uniform distribution of magnetite in the amphibolite compared with the peridotite. The plot (Fig. 9) of lineation 1 versus foliation f (after Khan, 1962) shows considerable variation, where

the K2 axis. Thus, at site 86, there are two well-developed intersection axes in the petrofabric: (1) veinlet with mylonite foliation (parallel to Kr); and (2) veinlet with veinlet (parallel to K2). At other sites, more random veinlet geometries reduce the axial-ratio anisotropies. Intersection AMS fabrics have been described from other geologic settings. As an example, Borradaile and Tarling (1981) studied the intersection

. ~o

92’ 10

.

10

20

•

•

88

.

0

.

.

•

30

6’~

~

~

Graham’s V (degrees)

xlOO

(4)

TINAQUILLO PERIDOTITE

::

Oblate

~9;91?8~:7~:.

N

N

•~~•

Fig. 9. Plot of AMS lineation parameter (1) versus foliation parameter, / showing approximately equal development of each, with extremes at site 86 (oblate) and site 92 (prolate). The latter two can be regarded as end-members of systems of veinlet geometry (see Fig. 10). Specimen (.) and site-mean (•)

Fig 10 Serpentine veinlet onentations in samples showing prolate (site 92) and oblate (site 86) AMS ellipsoids. A clearly defined NNW trending veinlet set (left) intersects the mylonitic foliation parallel to the K1 axis of the strongly prolate AMS ellipsoid. A ‘fanning’ pattern of veinlets (nght) radiates away from a NNW trending veinlet set with a veinlet—veinlet intersection along an axis approximately parallel to the AMS K2 axis. The veinlet—foliation intersection is parallel to the K1 axis. A strongly oblate AMS ellipsoid is associated with this

values are plotted,

pattern. S

88

1.00

S

1.04

1.12

I

1.20

1.28

I

1.57

FOLIATION

=

veinlet poles.

311

5. Conclusions

N

Two intersecting surfaces, defined by mylonitic foliation and serpentine veinlets, dominate the petrofabric of the Tinaquillo peridotite. In the area sampled, the mylonitic foliation trends generally east—west with southerly dips. Serpentine veinlets, of several generations and more variable orientation, post-date the mylonitic foliation. Serpentine veinlets, accompanied by secondary magnetite, form dominant sets with at least two orientations: (1) NNW with steep east dips; and

1 86

~

8

90 91

92

2e 87

Fig. 11. RM a.f. demagnetized directions (+) are not wellgrouped. Most lie within 260 of the associated K1 — K2 AMS

plane. Symbols as in Fig. 3.

of pressure solution cleavage with bedding in Devonian siltstones and related rocks in Great Britam and found K3 to lie parallel to the line of intersection of cleavage with bedding. They attributed their result to the interference of a magnetic fabric of sedimentary origin in the bedding plane with a magnetic fabric of tectonic origin in the cleavage plane. Finally, the pattern of RM directions is interesting but enigmatic. It shows no simple relationship, for example, to the regional rotational pattern of RM directions described by Skerlec and Hargraves (1980) for northern Venezuela. The mean directions selected here for each site represent that combination of a.f. demagnetized direclions which provides the optimum Fisher’s k statistic. Possibly the pattern which results from selecting directions in this fashion is without physical significance, because of the many generalions of secondary magnetite present. Nevertheless, it is interesting to compare the pattern of RM directions with the AMS fabric (Fig. 11). It is noteworthy that, for 5 of the 7 sites, the RM lies within 26°of the AMS K1—K2 plane.

(2) WNW with near-vertical dips. The axis of maximum magnetic susceptibility (Kr) is parallel to the veinlet—foliation intersection. The magnetic foliation plane (K1—K2) is parallel to the veinlet plane, while the minimum AMS axis (K3) is normal to the veinlet plane. The total anisotropy is typically large, with a maximum near 50%. Variations in the prolate/oblate aspects of the AMS ellipsoids reflect variations in the preferred orien.

.

.

tation of the veinlet sets and their interaction with the AMS of the foliation.

Acknowledgments The writers thank R. Jacynas, D. Tuttle, K. Holley and 0. Kurty at the State University of New York, Binghamton for technical support. Sampling was initially undertaken with the support of grant 040-0274A from the National Science Foundation. Comments by J.-J. Wagner and two anonymous reviewers are appreciated.

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MacKenzie, D.B., 1960. High-temperature alpine-type pendotite from Venezuela. Geol. Soc. Am. Bull., 71: 303—318. Mattson, P.H., 1985. Ultramafic and gabbroic rocks of Venezuela as possible ophiolite: Tinaquillo peridotite complex. Memoria, Sixth Venezuelan Geologic Congress, Caracas, 1985, 4: 2515—2539. Noltimier, H.C., 1964. Calibration of a spinner magnetometer with a wire loop. J. Sci. Instrum., 41: p. 55. Ostos-Rosales, M., 1985. Peridotita de Tinaquillo: ofiolita Paleozoica en el sistema montanoso del Caribe. Memoria, Sixth Venezuelan Geologic Congress, Caracas, 1985, 4: 2557—2602. Owens, W.H., 1974. Mathematical model studies on factors affecting the magnetic anisotropy of deformed rocks. Tectonophysics, 24: 115—131. Rathore, J.S., 1980. A study of secondary fabrics in rocks from the Lizard Peninsula and adjacent areas in southwest Cornwall, England. Tectonophysics, 68: 147—160. Saad, A.H., 1969. Magnetic properties of ultramafic rocks from Red Mountain, California. Geophys., 34: 974—987. Skerlec, G.M. and Hargraves, RB., 1980. Tectonic significance of paleomagnetic data from Northern Venezuela. J. Geophys. Res., 85: 5303—5315. Stone, G.B., 1963. Anisotropic magnetic susceptibility measurements on a phonolite and on a folded metamorphic rock. Geophys. J. R. Astron. Soc., 7: 375—390. Wagner, J.J., 1984. Petrophysical properties of the Ivrea zone and adjacent areas. In: J.J. Wagner and S. Mueller (Editors), Geomagnetic and Gravimetric Studies of the Ivrea Zone. Kummerly and Frey, Geographischer Verlag, Berne, Switzerland, pp. 31—37. Zijderveld, J.D.A., 1967. A.f. demagnetization of rocks: analysis of results. In: D.W. Collinson, R.M. Creer and 5K. Runcorn (Editors), Developments in Solid Earth Geophysics, 3: Methods in Palaeomagnetism. Elsevier, New York, pp. 254—286.