Final Reporl

NASA Grant NAG 5-147

"Characterization of potential sources of magnetic anomalies

within the crust in a tectonically active region:

amphibolites and migmatites from Potrillo Maar, New Mexico

Principal Investigator, Frank S. Spear

Co-investigator, Elaine Padovanni

Objective of the study

The purpose of this study was to characterize the

oxide mineralogy and petrology of samples collected from

Potrillo Maar, New Mexico with the goal of explaining the

magnetic anamoly that is observed over this region from remote

sensing. (

Approach

Potrillo Maar is a diatreme that has brought rocks from all

depths in the crust to the surface almost instantaneously. The

samples are therefore thought to be representative of the crust

as it exists today below this portion of the Rio Grande Rift.

It is generally believed that oxide minerals (magnetite,

hematite, etc.) are responsible for the magnetic signature of

the crust. The samples from Portillo Maar therefore offer a

unique opportunity to examine the magnetic mineralogy of the

entire crust. The results from the petrology are to be

integrated with data on the magnetic properties of these rocks

collected by P. J. Wasilewski.

Results

The rocks recovered from Potrillo Maar are varied in

composition, mineralogy and history. These types include

granitic and mafic gneisses characteristic of the lower crust,

tonalitic and other intrusive rocks characteristic ,of the

intermediate crust which are associated with amphibolites and

greenschists. Characteristic of practically all of the rocks of

the suite are complex reaction histories and sequences of

mineral parageneses, which suggest multiple events of both

igneous and metamorphic activity have affected the crust of this

region. It was quickly realized that any interpretation of these

events would require analysis of the silicate mineralogy as well

as the oxide mineralogy, and the project immediately grew in

proportion to this additional objective.

The two most significant "events" are hydration and

mylonitization (not necessarily simultaneously). Many of the

samples contain multiple generations of hydrous silicates that

overgrow one another in complex fashions. For example, pyroxene

C (NASA-CB-176385) CHARACTERIZATION OF N86-2G994

POTENTIAL SODBCES, OF MAGNETIC ANOMALIES

HITHIN THE CROSI IN A TECTONICALLY ACTIVE

1 REGION: AHPHIBOLIIES AND MIGMATIIES FBOHH
is variably replaced by amphibole which is in turn replaced by

chlorite. The mylonitization appears to have affected some

samples extensively but not others suggesting that shear zones

are localized in the crust at specific depths.

The oxide mineralogy is similarly complex. Magnetite and

ilmenite are the principal oxide minerals, but hematite and

rutile are also present. Many of the reactions that resulted in

the hydration of the silicates also involved the oxide minerals

resulting in a great diversity of assemblages depending on the

original rock type and the degree of hydration.

By combining phase equilibria analysis of the silicate and

oxide mineralogies, a preliminary senario for the evolution of

the crust beneath Potrillo Maar can be presented. The lower

crustal rocks are largely unaffected by the hydration events but

the middle crustal rocks are almost prevasively hydrated and

mylonitized. This suggests a large scale fluid infiltration in

the middle crust, which we propose is at least in part, related

to the shear zones and mylonitization. The infiltration of

these fluids also resulted in the general oxidation of the

crust, which has the result of converting Fe-bearing silicate

minerals into Mg-bearing silicates + oxide minerals. These

oxides are dominantly hematite and magnetite and will enhance the

magnetic signature of the crust. Therefore, our preliminary

suggestion is that the magnetic anamoly observed over the Rio

Grande Rift may ultimately be a consequence of the tectonic

activity that caused mylonitization of the rocks and allowed the

infiltration of oxidizing fluids. Further verification of this

hypothesis will come from correlation with the magnetic data on

these samples.

Papers, abstracts and reports

Published abstracts (abstracts attatched)

Padovanni E. and Hart, S.R., Geochemical constraints on the

evolution of the lower crust beneath the Rio Grande Rift, in

Papers presented to the conference on the processes of

planetary rifting. LPI contribution no 457, p 149-152, 1981.

Wasilewski, P.J. and Padovanni, E. Magnetization in the lower

crust, EOS, v. 61, p 222, 1980.

Wasilewski P.J. and Padovanni, E., Crustal magnetization beneath

the Rio Grande Rift based on xenoliths from Kilbourne Hole

and Potrillo Maar, in Papers presented to the conference on

the processes of planetary rifting. LPI contribution no.

457, p 153-155, 1981. |

Invited lectures

Padovani E., Granulites and Migmatites from Kilbourne Hole and
Potrillo Maar, Harvard - M.I.T. symposium on deformation

mechanisms in the lower crust, 1982.

Papers in preparation

Padovanni, E. and Spear, F.S. Characterization of potential

sources of magnetic anamolies within the crust in a

tectonically active region: amphibolites and mylonites from

Potrillo Maar

Wasilewski, P.J. and Padovanni, E., Magnetic properties of the

lower crust beneath the Rio Grande Rift

Padovanni, E., Spear, F.S. and Wasilewski, P.J., Effects of

decompression melting on oxide mineralogy and physical

properties crustal xenoliths
GP -• EOS, vol. 61, no. 17. April 22, 1980, page 222 ORIGINAL PAGE IS

OF POOR QUALfTY

reveraal Individually disagree, yet the overall

•ean pole of the four sampled sites (49.6*11,

• 108. 3'E) agrees mil with the previous Lover .

Triaaaic polea for the Chugvatex Formation published by various author*.

Keaulta of thla study suggest: (1) Afcer the

1 removal of aecondary components by a«an5 of

thermal cleaning Che polarity transition exist*

and can be correlated between eltea, (2) The

dipole field which decaya substantially before

the polarity reveraal occurs, thereby perhapa

allowing the nondlpole field to predominate r

- during a portion of the transition. , „. \; •

GP 64 ^ :„:„-.;..--. ,;

- THE NON-DIPOLE FIELD DURING GEOMAGNETIC FIELD

REVERSALS . . _ . .,_-,..,, .-.„>

.

"ITTutierTBoth at: "Department of Geological

Sciences, University of California, Santa .

^'Barbara, CA 93106) . ;„ , ^. ^', ^,, ,,4

* Virtual geomagnetic pole'(VGP) paths from ^,

'various records of the last reversal show a

dependence on the observation site meridian. ..

This suggests that the transition field Is non-

" dipolar yet dominated by low order zonal harmonics (Hoffman 1977, Hoffman and Fuller 1978).

'Predictions from quadrupolar or octupolar

transition fields have been compared with the

available VGP data. However these VGP paths do

not determine which of the two geometries Is , ,

most Important. ' .".-. . • ^

As an alternative approach we have used the ,

low order zonal harmonics to model the change

of Inclination and Intensity during a transition. The dipole energy Is reduced and distributed amongst the other harmonics in a variety of

"reasonable" ways.

It 1s Interesting that neither a purely octupole nor quadrupole Intermediate field fits the

"observed behaviour of Inclination. A combination of the two fields models the data better. .

This analysis suggests that the decrease 1n

Intensity will not be symmetric about the change

' In Inclination. Both the Intensity change and

the apparent time taken for the transition, are

dependent upon the latitude of the observation

site. -4.i''.v~- i-.'~ ''^ ••-:*'- .!.-<• j*1 •<-• .'-**'« •&"

.

GP 65 "»•.• V »i i

ON THE DOMINANT GAUSSIAN COEFFICIENTS OF THE

- TRANSITION FIELD CURING THE MATUYAMA-BRUNHES :•

REVERSAL • _ t -t; - ^ - i

,.K. A. Hoffman, (Physics Pept., Calif. Polytech-

'-mic State Univ-^-San Luis Obispo 93*07) -c. ,-.

A model of the reversing geodynamo based on i

the assumptions (1) that reversals start in a -

localized region of the core and (2) that upon

its onset this reversed region extends, or -s

"floods" both north-south and east-west until -

the entire core is affected, has recently been

shown to successfully simulate existing paleomagnetic records of the reversing field during

the Matuyama-Brunhes transition (Hoffman, 1979).

An analysis of the modeled field during the <. i

transition reveals that the dominant components

at the time of total axial dipole decay include

an axial octupole (gj) and a non-axial quadrupole (gj, hj). Given the distribution of sites

corresponding to the available records of the

Matuyama-Brunhes, the existence of a significant

axial quadrupole field component cannot be ruled

out; however, the role of any equatorial dipole

component can be neglected. The modeled min- •

imum field intensity is found to be dependent _.

on both site latitude and longitude. For the

Matuyama-Brunhes the modeled minimum strength j

experienced, say, at aid-northern latitudes ia *

- about 10% of the full polarity strength, yet

such values differ from site to site by more

•than a factor of 3. . — * + c»" -".-

. .Although not a unique solution, the findings

from this analysis may be considered to be independent of the phenomenological model from which

they were derived. For example, whether the

above determined transitional field geometry is

modeled to be produced by configurational --^

changes of the reversing geodynamo or alteraa-'n

lively, to be of s stationary (non-reversing) ""„

-, origin has little effect on the calculated VGP ^ '

path localities for a given site. These two ^

possible situations, in principle, should be ",

distinguishable given sufficient transition /(J1

. data corresponding to back-to-back (R*N, and ~^^»

N-»R) reversals. ,

GP 66 -

STOCHASTIC HODELS FOR THE POLARITT BIAS OF GEOMAGNETIC REVERSALS

Allan Cox Dept. of Geophysics, Stanford University. Stanford, CA 94305) .

- The oaln statistical features of the geomagnetic reversal process are simulated quite well

by a variety of deterministic and stochastic •

•odels. However bias in the lengths of polarity

intervals such as Chat at the end of the Meso-

• zoic (normal polarity bias) and that at the end

of the Paleozoic (reversed polarity bias) has

not *» yet been modeled either deterministically

. or stochastically. Two classes of stochastic

models are presented to account for polarity ~

bias. In the first, instabilities in the dynamo

process which accompany reversals are assumed to ,

occur at a higher rate during one polarity than

during the other. In the second, the rate at

which Instabilities occur remains constant but a

different fraction of Instabilities are Infertile

In the two polarity states. The most likely *

cause of changes In reversal frequency and changes

in polarity bias is mass transfer in the lower

mantle capable of producing undulations and temperature variations at the core mantle Interface,

j the latter providing the boundary conditions for

the dynamo. Symmetry considerations require the

presence of odd zonal harmonics in these boundary conditions during times of polarity bias.

Moreover because changes in polarity bias occur

with a much longer characteristic time than do

changes in reversal frequency, the lower mantle

processes responsible for changes in bias may well

have longer time and length scales than do the

processes responsible for changes in frequency.

An interesting speculation is that changes in

reversal frequency may reflect shifts in the

location of individual lower mantle convective

cells whereas changes In polarity bias may reflect

changes In the entire pattern of mantle convection

such as might, for example, be associated with a

regrouping of the continents. - -•-

GEOMAGNETIC SECULAR- VARIATION IN * . ,.?J

"SOUTHERN ITALY AND ITS APPLICATION TO „" .

ARCHEOLOGICAL DATING ,

G.S. Hoye (Institute of Earth and "",*< -

''-^Planetary Physics, Department of -.

•""Physics, University of Alberta, " t

Edmonton, Alberta , Canada T6G 2J1) 5*-

-'M. E. Evans . r^;,-TM* ,"• *, -*. '/-: -*-^ tv ^f-v

-'The lavas from nine eruptions of . ^ a

Mount Vesuvius have been sampled at 20 i

sites, the dates of the eruptions .-

(1631-1944 A.D.) covering the complete

range of historically recorded lava

.flows. " In addition, the mud flow - "*

'resulting from the Plinlan eruption Vf^ ^

79 A.D. was sampled at Herculaneum. " ",,

Eleven archeological baked features "• J •*

- (kiln, hearth, or oven) of well- -'l^V

determined archeological date covering

the interval from the eighth century

B.C. "to the fourth century A.D. were 7*

also sampled. - — • . > ,*

' Site mean magnetic directions from "'•'*

"the lavas are precise (typically a,s

is < 2* for N-5) and yield an elliptical secular variation curve for the

last few centuries which is in agreement

with observed data for Rone after allowing for dipole variation In inclination.

The (detrital?) remanence of the 79 A.D.

deposit is stable and is not aligned ^.^

with the present field direction.

An archeomagnetic date (lying in the

second half of the first century A.D.)

for a kiln situated in a villa rustica

at San Giovanni di Ruotl has been * -

derived using data from this study and j

this date is consistent with the circumstantial archeological evidence r

relating to the kiln. „. Vr rffc*«?T*#W*t ~

-* GP 69 V^__:."^;'" '-''rVl

 tr'

^ AEROMACNETIC INTERPRETATION OF THE - * -ajS.*.V,r

" - ARCHEAN VOLCAHICS IN THE KIRKLAHD-LARDER r C>

' " LAKE AREA OF ONTARIO

% Stephen Letros, (Department of Physics and :

^' Geology, University of Toronto, Toronto, =

~ '• Ontario M5S lAl, Canada) .->,-. ,-„--•_ "

,/'D.W. Strangway • "-*_~ -"j " "'

^', (Toronto) , . ' r-. " Xj,-i,-_ *S " -.-^f 2<*

4« (Sponsor: D.W. Strangway) j^^r— ji?-, /--/*"*i' i

"t*f*S*"' " V "* *'^' '-— '-'i',~ f* • -L J'-^r-'Q v

Tj High resolution total field aeromagnetlc T-

' ^"« data for the 2.7 billion year old volcanic ;,

sequences north of Klrkland Lake are being r.-c

,, operated on by geophysical filtering tech- >

r niques. This is done to enhance the data •.—: -

V-y in order to derive geological Information. ~-

~ Magnetic properties such as susceptibilities,:

,- NRH directions and intensities and known -».-" >.

>^ geological information are being used as a _>f

.jj control over the interpretation. Data ^-^j-^

' ->-, enhancement techniques have focused on three -u

vi approaches; first vertical derivation, down- '

,^-ward continuation and apparent susceptibility

mapping. The latter technique which reduces

, anomaly overlap and allows for an accurate •

^ location of formation boundaries was the • ^

-i most useful for interpretative purposes. -^ ***

a; ..Five major rock types were found in this -»^

j,, area, these being magnesium and iron—rich • *

t tholeiitic flows, calc-alkallne volcanics svf

."•and various mafic and felsic intrusives. •*!•-/*•=-

^ The Iron-rich thole 11 tes are in general

 r- ***

> extremely magnetic and the calc-alkalines *r *-

>, only weakly aagnetlc. The paleomagnetic «'"*^.

*,results suggest that the remanent magnetiza- ^

vt-tlon of the Iron-rich tholeiites and the **$.**

-,• ultranafic intrusives are the main source of '*

-t^the anomalies while negative'susceptibility *

'•contrasts due to the magnesium-rich at-w*-*-*

tholeiites are observed. Thus detailed

- .geologic mapping of the thick atratigraphic ^

' sequences is possible. - -- /•* ' "c

GP 68 *' **??* *&**'*& i*~ •****-

, >- -"at1" •* ~V *> ^'f> ""-"i- *•.•"' '•OT

MAGNETIZATION IH THE LOWES C8UST

p.t.r waallewaltl (RASA/Goddard Space Flight

Center, Laboratory for Extraterrestrial

*'- rfcyalca, Creenbelt, Maryland 20771) jv.'-l'^.

'(Elaine Padovanl (Dept. of Earth 4 Plane Wry

^..Science, HIT, Cambridge, MA 02139) .»>»B».JII.

^ ,'ci. f'*,s •,•;-".' ?•*' 'i «*.-"*i;*.~ •'::s-5*>*«<

• *'-i- Deep crustal and Mantle xenoliths from nuaer-

'-ous aources along the Alo Grande rift and the

. Colorado plateau, granulltes froa high grade

• letanorphlo 'terrain in North laerlca, India,

Brazil, and Africa, and a pro file across the

Ivrea, xona have been studied in order to evaluate tb* distribution of magnetization in the

- deep crust." These preliminary studies provide

additional confirmation of our idea that the

Hobo ia a magnetic boundary. There la DO

•vldenoe of *xotio magnetic mineralogy or metal

In any of the rocks studied, naturally reduced

basaltlo roeka containing a range of titano-

< Magnetite from fe_ .Ti- fiO. to Pe.TIO* with

Increasing doailnancV of llaenlte and. finally

•«tall to iron are used to model what might be

expected magnetically with Increasing degrees of

reduction in the lover crust. The fO^-T

. • conditions at depth will determine the Curie

.point of the oxide minerals and the geotheraal

,cgradient will determine the depth at which this

Mineralogy is non-aiagnetic. Several preliminary

• „ Models of continental crust magnetization are

t> presented based on the magnetic measurements,

since no single model Is likely to bold for the

entire continental crust. - Ttie magnetic properties of deep crustal rocks *ay reside entirely along the present geomagnetic field. The

magnetization contrast is made up of three

,.; terms: ,x H-the induced component dependent an

•mount, composition and grain size of toe

•agnetic mineralogy, M_-the viscous magnetization which Is enhanced with depth because of

-'Increasing temperature at depth and X H * H -

presently not well defined contributions due to

^ pressure and the effect of susceptibility

I enhancement within 100°-150 C of the Curie

point. . __ .^v^.j^t- i*

GP 70 -~}\ ::;• ^ ' ^L^-l"

• AN ALTERNATIVE EXPLANATION FOR INTERMEDIATE^•J3 -

WAVELENGTH MAGNETIC ANOMALIES • - " ' ^'~<

'-"L. Shore (Institute of Geophysics and Planetary 't

*~.s. Physics, Scripps Institution of Oceanography,

**-*-* University of California, San Diego. La Jolla 1

r*X CA 92093) -t ' . ,•, „'<«• **~r\+» **{%<3 '-,

wR. L. Parker (Institute of Geophysics and ^ ' .,£

-*•* Planetary Physics, Scripps Institution of -(,

Vi

*.-H_ Oceanography, University of California, San , *^

• Diego, La Jolla, CA 92093) »-'ai ^

L. M. Dorman (Geological Research Division. 3 ^\

Scripps Institution of Oceanography, Unlver- '

:J.-*J&s1ty of California, San Diego, La Jolla. CA -;'*%

."- 92093) ,_ -,;-.;.• ' '\'^^a.-^S

-•*•*" Harrison and others have examined very long '-. '''"*;£.

« magnetic-field profiles and have calculated - - '•-•'*

.r their one-dimensional power spectra. In these --

. they expected to see, but did not find, a -_-" .

minimum 1n power at Intermediate wavelengths,

between 65 km and 1503 Ion (from sea-floor ^x-*J>

v Jhis page may be freely copied
149

GEOCHEMICAL CONSTRAINTS ON THE EVOLUTION OF THE LOWER CRUST BENEATH

THE RIO GRANDE RIFT, E.R. Padovani and S. R. Hart, Dept. Earth & Planetary

Sciences, Massachusetts Inst. of Technology., Cambridge, MA 02139

As part of a long-range objective to characterize the lower continental

crust, petrologic and geochemical studies have been undertaken on representative suites of lower crustal xenolith samples from Kilbourne Hole maar and

Potrillo maar in south central New Mexico. Major element analyses completed

on about 50 samples indicate a wide range of compositions among the orthogneisses and paragneisses, with Si02 contents ranging from 43% to 70%,

contents ranging from less than 0.5% to 6% and 1^0 contents ranging from 0.2%

to 6%. The data do not suggest any dramatic depletion of alkalies in these

granulite facies rocks. Rb/Sr and Sm/Nd systematics reveal mineralogic and

isotopic disequilibria over a 1-3 cm scale defined by compositional layering

within xenoliths. Although disequilibrium exists between layers, mineral

pairs (such as plagioclase-garnet, plagioclase-clinopyroxene and K feldsparplagioclase) are commonly in perfect equilibrium which must have occurred

within the past 10-20 m.y. Monotonic Sr isotopic gradients within paragneisses may hold the key to understanding a much larger scale diffusion gradient

(larger than individual xenoliths) at depth which may explain the difference

in behavior between the Rb/Sr and Sm/Nd systems. The "error chron" defined by

the xenolith suite is consistent with an age of 1.7 b.y. (see Fig. 1) for

the basement beneath the southern Rio Grande Rift. The combination of

mineral geothermometers-geobarometers and isotopic results allows construction of a tentative time- temperature history for the lower crust beneath the

rift and distance-scales of isotopic exchange.

Time-temperature history. The original age of cratonization in this area was

'about 1.7"b.y. Granulitei conditions" probably'prevailed in the lower -crust- —

_during this erogenic event. Following orogenesis, the crust cooled to perhaps

stable shield geothermal gradients (implying lower crust temperatures of less

than or equal to 500°C). Based on diffusion models, we believe the interlayer isotopic gradients would not survive continuous granulite conditions

for the whole period 1.7 b.y. to present. Other intrusive events have been

recognized in nearby regions of New Mexico, Texas and Mexico, with ages of

1.6, 1.2-1.7 and 0.5 b.y. (Muehleberger and Denison, 1964; Silver et al.»

1977; Loring and Armstrong, 1980)—their effect on Kilbourne Hole crust is

yet unknown. No significant vertical motion (and erosion) has effected Kilbourne Hole crust during the Precambrian as suggested by the presence (Franklin Mts.) of a shallow granite-rhyolite terrain, perhaps 1 b.y. old. During

the cooler, orogenically quiescent Precambrian times, mineral geothermometers

would be reset to sub-granulite temperatures. Starting about 30 m.y. ago,

with the beginning of extension of the Rio Grande Rift, magmatic activity and

crustal thinning increased temperatures again into the granulite range. Peak

temperatures of 1000-1100°C were reached,las evidenced by 2-pyroxene thermometry, and "bulk" K-feldspar-plagioclase thermometry. The present granulite

textures were probably formed at this stage, along with mobilization of Rb

and some Nd isotopic exchange between layers. This peak temperature was followed by some cooling as rifting slowed (about 5 m.y. ago), to bring the geotherm to its present value (Lachenbruch and Sass, 1977; Cook ^t^ al_., 1978);

heat flow models predict present lower crustal temperatures of 800-900°C. Kfeldspar "host"-plagioclase thermometry consistently records_teopera.tures__of __

800-900°C, in agreement with the heat flow models. These thermal conditions

were sufficient to keep adjacent minerals in continuing local isotopic equilibrium for Sr and Nd (see Fig. 2). Prior to eruption, probably by heating of
150

GEOCH. CONST. EVOL. LOW. CRUST BENEATH FIO GRANDE RIFT

Padovani, E. R. and Hart, S. R. •

the erupting magma, the xenolithlc material was brought to >1000°C, such that

subsequent decompression during eruption produced local "decompression meltting", especially of garnet (Padovani and Carter, 1977b). This heating did

not have any significant effect on the isotopic systems, or on mineral chemistry (as witnessed by only small 20-30;i diffusion gradients in plagioclase

adjacent to the decompression melt, and the very heterogeneous composition of

the melts).

Chemistry of the lower crust. At least under Kilbourne Hole, the lower crust

has not been massively depleted in alkalies, and paragneisses retain typically sedimentary isotopic signatures for Sr, Nd and S180 (James et_ al. , 1980).

Compositional heterogeneity is marked, with a whole spectrum of rock types

ranging from very basic orthogneisses to highly siliceous peraluminous paragneisses.

Distance-scales of isotopic exchange. The recent granulite-facies event under

Kilboume Hole, though admittedly not particularly long-lived (£30 m.y.), has

been insufficient to bring about Sr and Nd isotopic homogenization on anything but a rather local (0.1-1 cm) scale. Further thinking is needed to establish the relevance of this result to the question of isotopic heterogeneities in the mantle source-regions of basalts (see Hofmann and Hart, 1978).

Cook, F,A., Decker, E.R. and Smithson, S.B. (1979), Preliminary transient

heat flow model of the Rio Grande Rift in southern New Mexico, Earth Planet.

Sci. Lett.. 42, 332.

Hofmann, A.W. , and Hart, S.R. (1978), An assessment of local and regional isotopic equilibrium in the mantle, Earth Planet. Sci. Lett., 38, 44-62.

James, D.E., Padovani, E.R. and Hart, S.R. (1980), Preliminary~~results on the

oxygen isotopic composition of the lower crust, Kilbourne Hole Maar, New

Mexico, Geophys. Res. Lett.. _7, 321-324.

Lachenbruch, A.H. and Sass, J.HT (1977), Heat flow in the U.S. and the thermal-regime of the crust, In: The Earth's Crust, Heacock et al., _eds., Geo-

. physical Monograph 20, A.G.U., Washington, D.C., 626-673.

Loring, A.K. and Armstrong, D.G. (1980), Cambrian-Ordovician syenites of New ,

. _ Mexico, part of a regional alkali intrusive episode, Geology, 8_, 344-348.

Muehlberger, W.R. and Denison, R.E. (1964), Precambrian geology of south-central New Mexico, New Mexico Geol. Soc. 15th Field Conference Guidebook, 62-

69.

Padovani, g.R. and Carter, J.L. (1977a) , Aspects of the deep crustal evolution beneath south central New Mexico, In: The Earth's Crust; Its Nature

and. Physics Properties, Geophys. Monogr. Ser. , V. 20, Heacock, ed. , A.G.U.,

Washington, D.C., 19-55.

Padovani, E.R. and Carter, J.L. (1977b) , Non- equilibrium partial fusion due

..... to decompression and thermal effects in crustal xenoliths, In: Magma Genesis 1977_r Dick, ed., State of Oregon Dept. of Geology and Mineral Indus- "

tries, Bull. 96, 43-57. • . -. . - -

Silver, L.T., Anderson, C.A., Crittenden, M. , and Robertson, J.M. (1977), "

Chronostratigraphic elements of the Precambrian rocks of the southwestern

and far western United States, Geol. Soc. Am., abstracts with programs,

1176. I .

-
151

GEOCH. CONST. EVOL. LOW. CRUST BENEATH RIO GRANDE RIFT

Padovani, E. R. and Hart , S. R.

.800

.790

.780

.770 -

.760 -

•o

00

in

rv

09

I I I

SUMMED WHOLE ROCKS

\ 70 b.y.

REFERENCE ISOCHRON

.750 -

.740 -

.730 -

.700

0.2 1.4

Rb /Sr

Figure 1. Rb/Sr systematics of "whole rock" xenoliths, obtained either by

whole rock analysis of single xenoliths or by "summing" the layers of those

xenoliths for which multi-layer Rb/Sr studies were performed (e.g. Fig. 2).

Only xenolith 1975 falls significantly away from the 1.7 b.y. reference isochron. This data shows that the "original" age of cratonizatioh of Kilbourne

Hole lower crust was M..7 b.y. (though this point clearly needs further documentation). Furthermore, though the inter-layer data (such as in Fig. 2) suggests significant relative mobility of Rb versus Sr, the overall effect is

not consistent with any dramatic Rb depletion for the lower crust, and the

paragneiss samples (closed circles) in particular still show relatively"-high

Rb/Sr ratios. The orthogneiss xenoliths (open circles) are characterized by

uniformly low Rb/Sr ratios, but this is probably an original characteristic

inherited from their Igneous protolith, as opposed to an effect -due to -largescale Rb depletion during granulite metamorphism. Note the data for five amphibolite-facies xenoliths from nearby Potrillo Maar (triangles). -
152

GEOCH. CONST. EVOL. LOW. CRUST. BENEATH RIO GRANDE RIFT

Padovani, E. R. and Hart , S. R.

.715

X .710

.703

1-6-3 b.j.

ORIGINAL PAGE IS

OF POOR QUAU7!*

is «•

N

.02 JO*

Kb/S>

. FAHAGNEISS

2-pto,

Figure 2. Rb/Sr (top figure) and Sm/Nd (bottom figure) isotopic relationships

in minerals and layers of paragneiss 7K3. Minerals from within a given layer

are essentially perfectly equilibrated with respect to Sr (plagioclase-K-feldspar pairs from layers 2 and 6) and Nd (plagioclase-garnet pairs from layers -

2 and 6); allowing for the observed analytical errors, the maximum ages indicated for these mineral pairs are in the range 10-20 m.y. for both the Sr and

Nd systems. In contrast to this small-scale intra-layer isotopic homogeneity,

both Sr and Nd show significant isotopic disequilibria between layers. However, with respect to the presumed original age of metamorphism of this paragneiss Ovl.7 b.y.), both the Sr and Nd systematlcs appear to show open-system

behavior. The location of layers 1-4 to the left of the reference Sr isochron

suggest loss of up to 80% of Bb from these layers at a relatively recent time;

this Rb loss has produced a crude linear array of data points with an obviously meaningless slope age of ^6.5 b.y. In contrast, the Nd data suggests some

inter-layer isotopic equilibration, as layers 5 and 6 and layers 1-4 and 7

are essentially in isotopic equilibrium. Note the regular monotonic change in

8/Sr/°°Sr across the layers; this is a feature which has been noted in all of

the paragneiss samples studied thus far.
153

CRUSTAL MAGNETIZATION BENEATH THE RIO GRANDE RIFT BASED ON XENOLITHS

FROM KILBOURNE HOLE AND POTRILLO MAAR; P. J. Wasilewski, NASA/Goddard Space

Flight Center, Laboratory for Extraterrestrial Physios, Greenbelt, Maryland

20771; E. R. Padovani, Dept. Earth and Planetary Sciences, Massachusetts

Institute of Technology, Cambridge, MA 02139

Results of magnetic studies on xenoliths from the Colorado plateau and

Rio Grande Rift support the concept of the continental Mono as a magnetic

boundary (Wasilewski et al., 1979; Wasilewski and Padovani, 1981). Upper

mantle peridotites contain chromites which are nonmagnetic at Moho depths.

The xenolith results indicate that regional magnetic anomalies in the crust

are related to both the topology of the Curie isotherm and to petrologic

variations. These results combined with detailed petrologic and geochemical

studies on lower crustal xenoliths have revealed that in an area of steep

geothenaal gradient, the magnetic bottom is at considerably more shallow

depths (10-15 km) than is the case in an area with moderate geothermal

gradient such as the Colorado plateau where the magnetic bottom is deeper

(30-40 km). This is due to the steeper geothermal gradient as well as a

different magnetic mineralogy. Though the rift is of limited areal extent it

can be recognized in both POGO and Magsat magnetic anomaly maps due to the

contrast with surrounding regions.

Beneath the southern Rio Grande rift, the crust appears to be mere

reducing with increasing depth as reflected by the ilmenite dominated

anhydrous lowerocrustal xenoliths at Kilbourne Hole which have Curie points

less than 300 c and characteristically small saturation magnetization and

remanence. In contrast some of the xenoliths from Potrillo maar which are

considered to represent intermediate crustal depths between those defined by

exposed outcrop and wells drilled to basement and those defined by the

granulite facies xenoliths have 550 c Curie points and large values of

saturation magnetization and remanence. Granulite xenoliths ~fY6nr~ETephafvEButte and the Lucero Volcanic field (Wasilewski and Baldrich - unpublished

research) have magnetic characteristics that differ from both the Colorado

plateau and Kilbourne Hole and Potrillo maar granulites suggesting different

conditions of equilibration may exist at lower crustal depths along the rift.

It appears that active regions with high heat flow such as rifts may be

anomalous with respect to their magnetic properties.

Shown in Figure 1 are Curie point curves for garnet granulite (1983) and

garnet amphibolite (2266) from the Colorado plateau; pyroene granulties (79K7,

77K5, and 7K3) from Kilbourne Hole; and, an amphibolite grade rock from

mid-crustal depth (PMA, 8B) and a lower crust granulite (PMA, 5B) from

Potrillo maar.

All Colorado Plateau xenoliths have 550 C Curie points no matter what

lithologies were evaluated. Kilbougne granulites have Curie points <300 C and

Potrillo maar rocks have either 550 C Curie points (mid-crustal levels) or <

300 c Curie points (lower crust). The magnetic bottom beneath the Southern

part of the rift should be no deeper than about 15 km as indicated by point A

in Figure 2, which is the depth of the 550 c Curie point on a reasonable

geotherm for the rift. If ilmenite dominates as is the case for Kilbourne

Hole xenoliths then the magnetic bottom may be as shallow as 8-10 km (point B

on Figure 2). Therefore, at best we may have only half the crustal thickness

beneath the southern part of the rift made up of magnetic rocks.

Away from the central part of the rift where the geothermal gradient

becomes more shallow,.and.the magnetic mineralogy may be developed in a more,

oxidizing environment, the effective magnetic crustal thickness should

increase.

3.

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CRUSTAL MAGNETIZATION BENEATH THE RIO GRANDE RIFT

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Wasilewski, P. J., H. H. Thomas, and M. A. Mayhew, 1979, Tne Moho as a

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