The occurrences of megacrystic anorthosite and basalt in a variety of geologic settings were reviewed and it was found that these rock types occur in a variety of tectonic settings. Anorthosites and megacrystic basalts are petrogenetically related and are found in oceanic volcanic crust, cratons, and shelf environments. Although megacrystic basalts are most common in Archean terranes, similar occurrences are observed in rocks of early Proterozoic age, and even in young terranes such as the Galapagos hotspot. Based on inferences from experimental petrology, all of the occurrences are apparently associated with similar parental melts that are relatively Fe-rich tholeiites. The megacrystic rocks exhibit a two- (or more)-stage development of plagioclase, with the megacrysts having relatively uniform composition produced under nearly isothermal and isochemical conditions over substantial periods of time. The anorthosites appear to have intruded various crustal levels from very deep to very shallow. The petrogenetic indicators, however, suggest that conditions of formation of the Precambrian examples were different from Phanerozoic occurrences

Maczuga, D. E.

Morrison, D. A.

Phinney, W. C.

English

NASA Technical Reports Server

8p- 135 
TECTONIC IMPLICATIONS OF ARCHEAN ANORTHOSITE OCCURRENCES 
W.C. Phinney, D.A. Morrison, ,Johnson Space Center and D.E. Maczuga, LEMSCO, 
Houston, TX 77058. 
Introduction 
Anorthositic complexes occur in essentially all Archean cratons and contain large 
equidimensional plagioclase crystals (up to  30cm. diam.) with highly calcic compositiok 
(An,, t o  Ang0). Several occurrences have been described in India [ 11 ,[ 21 ,[ 31 ,[ 41. 
Because the anorthositic complexes represent cumulates, the composition and source of 
parental  melts has been a' longstanding problem. Plagioclase having t h e  same 
composition and texture as that in anorthosites also occurs as megacrysts in basaltic 
flows, dikes, and sills in which the crysts may be scattered or concentrated in lenses or 
trains. We suggest that  the anorthosites and megacrystic basalts are petrogenetically 
related. However, the tectonic settings for these occurrences appear to be quite variable 
suggesting that several environments may be represented. A brief outline of the regional 
settings of these anorthosites and petrogenetically related basalts follows. 
Archean Occurrences Megacrystic Anorthosites 
1. Cumulate crystal segregations in anorthositic to gabbroic complexes associated with 
volcanic sequences typical of low to middle metamorphic grade greenstone belts [5],[6]. 
2. Cumulate crystal segregations in thick anorthositic t o  gabbroic sills that  intrude 
volcanic sequences typical of greenstone belts [ 71. 
3. Cumulate crystal segregations in anorthositic to gabbroic complexes associated with 
high grade metamorphic terrains containing marbles, quartzites,  quartzofeldspathic 
gneisses, and amphibolites [3],[8]. 
Archean Occurrences Megacrystic Basalts 
1. Flows, dikes, and sills in volcanic sequences typical of greenstone belts [91,[101. 
2. Dike swarms in stable cratonic areas forming parallel t o  subparallel' pa t t i rns  over 
hundreds of thousands of square kilometers intruding both high grade granitic gneisses 
and low to middle grade supracrustal belts [lo]. 
Younger Occurrences 
Similar occurrences of megacrysts in basalts of early Proterozoic age are known in 
cratonic dikes of the Bighorn Mountains of Wyoming [ll] and the Beartooth Mountains 
of Montana [12) and in volcanic flows of the Bell Island Group of the  Northwest 
Territories [ 131. Recent occurrences of similarly calcic plagioclase phenocrysts are known 
in oceanic volcanic flows at spreading ridges, hotspots, aseismic ridges and fracture zones 
[14]. However, these normally involve only small phenocrysts up to a few millimeters in 
size and usually are more lathy than equidimensional in shape. In contrast t o  these 
common oceanic occurrences, volcanic flows over the Galapagos hotspot display more 
equidimensional calcic crysts up to 3cm. across [15]. In essentially all of the Archean 
and Proterozoic occurrences the distribution coefficients for REE's indicate equilibrium 
between the megacrysts and their matrices of Fe-rich tholeiites [le]. However, use of 
the same distribution coefficients in the more recent occurrences indicates substantial 
disequilibrium between the. crysts and their tholeiitic matrices. Thus, the more recent 
occurrences of calcic plagioclase crysts require an additional stage of evolution before 
reaching their current environment, thereby providing a bit more uncertainty about their 
petrogenesis than the older occurrences and making direct comparison with ancient 
tectonic environments untenable. 
Me!ts and Magma Chambers 
Utilizing experimental petrologic studies of the  basaltic matrices and distribution 
coefficients with trace element analyses of plagioclase i t  seems clear tha t  all of the 
above-listed megacryst occurrences are' associated -with similar parent melts for both the 
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TECTONIC IMPLICATIONS OF ARCHEAN ANORTHOSITE OCCURRENCES 
W. C. Phinney et al. 
136 
anorthosites and megacrystic basalts [le]. The melts are relatively Fe-rich, tholeiitic 
basalts that  exhibit a significant range of Fe-enrichment (50-70% on an  AFM plot) in 
association with the megacrysts. The basalts of the cratonic dikes are more enriched in 
K, Na, and light REE than  their greenstone counterparts but follow a parallel Fe- 
enrichment trend. Furthermore, the fractionation trends and formation of the crysts 
occurred a t  relatively shallow levels (<5Kb) [le]. The  megacrysts in  a l l  of t he  
occurrences are quite uniform in composition (*l to 2 An units) over several centimeters 
except for very thin rims (~100-200pm). This suggests nearly isothermal crystallization 
a t  a nearly constant melt composition over substantial periods of time. Geochemical 
modeling of trace elements and subtle cyclic compositional trends in the  plagioclase 
indicate multiple influxes of melts into the magma chambers during evolution of the 
melts and growth of the megacrysts. The widespread occurrences of the megacrystic 
units in both greenstone belts and huge cratonic dike swarms further suggests extensive 
development of magma chambers in which tholeiitic melts produce calcic plagioclase as a 
major liquidus phase under both cratonic and oceanic Archean crusts. In essentially all 
occurrences where adequate preservation of initial igneous textures and structures exists, 
there is evidence for at least two stages of plagioclase formation. In the anorthositic 
complexes there are bimodal units in which very large crysts are mixed with smaller, but 
still large, crysts. In the basalts the calcic megacrysts have thin sodic rims tha t  match 
the composition of the lathy plagioclase in the matrices. Both situations indicate  
formation of the large crysts at locations other than their final position of emplacement, 
probably indicating a complex series of magma chambers in the crust. 
Crustal Levels 
The anorthosites appear to have intruded at various crustal levels. In many of the low- 
--
grade supracrustal- -(greenstone) settings the preservation of primary sedimentary and 
volcanic structures and textures indicate that the regions have always been at low grade 
and  t h a t  t he  anorthosi tes  intruded at  very shallow levels. In the  higher grade  
occurrences it is not always clear whether the anorthosites intruded at the higher grades 
or at low grade and were la te r  upgraded. In Mani toba  there  is a clear case of 
anorthosites init ially intruding low grade supracrustal  uni ts  b u t  la te r  a regional 
metamorphic gradient produced a continuous sequence from low greenschist t o  granulite 
grades in all of the units [17]. In the granulite grade Shawmere anorthosite complex of 
Ontario [18), however, there are some nearly undeformed enclaves where the more mafic 
units contain well preserved olivines and pyoxenes with well preserved exsolution texture. 
Furthermore, some plagioclase contains well preserved polysynthetic twinning tha t  looks 
like original igneous twinning. Such preservation seems unlikely if the anorthosite were 
intruded at low grade and underwent progressive metamorphism to granulite grade, unless 
the system were essentially dry during metamorphism which also seems unlikely in view 
of the biotite- and amphibole-bearing units adjacent to  the intrusion and  amphibole- 
bearing pegmatitic zones within the complex. 
Effects of Fluids at High Grades 
Several petrographic observations in high-grade anorthositic complexes indicate  the  
infiltration of substantial  amounts of fluid. Recrystallized plagioclase ranging from 
strained patchy areas to polycrystalline areas may occur as irregularly distributed zones, 
vein-like stringers, or rims around relict cores. Generally these areas display elevated 
values of N a  and REE’s in the plagioclase. Inclusions of tiny amphibole needles are  
common in non-recrystallized plagioclase of upper greenschist and  higher grades. 
Concentration of the inclusions is highly variable even within a single crystal. Many 
plagioclase crysts contain 10% or more by volume of these inclusions. The initial FeO 
content of the plagioclase is in the .4-.6% range and the FeO content of inclusion-rich 
plagioclase is ~ . 1 % .  However, microprobe analyses of the amphiboles indicate 15-20% 
FeO which for 10% inclusions requires several times more FeO than was present in the 
initial plagioclase. Similarly the heavy REE contents of these plagioclase separates are 
TECTONIC IMPLICATIONS OF ARCHEAN ANORTHOSITE OCCURRENCES 
W. C. Phinney et al. 
137 
substantially increased over the initial values reflecting the heavy REE enrichment in 
amphiboles. Clearly there must be flow of fluids through the plagioclase in some manner 
to add Na, Fe, heavy REE, and H,O. 
Conclusions for Archean and early Proterozoic Megacrystic Units 
1. Segregations - -  of plagioclase may occur at  various depths  in the  crust  t o  form 
anorthosite. 
2. Anor thos i tes  may occur  in  oceanic volcanic c rus t  a n d  in  c ra ton ic  o r  shelf 
environments. 
3. Megacrystic basalts may form in oceanic or stable cratonic environments. 
4. Plagioclase megacrysts in Fe-rich tholeiites indicate relatively shallow magma chambers. 
5. Large uniform crysts require extensive periods of isothermal growth at nearly constant 
melt composition and almost certainly formed in fractionating magma chambers tha t  are 
periodically replenished. 
6. Megacrystic tholeiitic dike swarms indicate widespread replenishing magma chambers 
under stable cratons. 
7. It is not clear what oceanic environment is represented by megacrystic uni ts  in  
greenstones b u t  i t  does require magma chambers for subs tan t ia l  t ime at similar 
temperatures and melt compositions over extensive areas. 
8. Petrogenetic conditions for formation of megacrystic anorthosites and basalts in the 
Archean and early Proterozoic were not the same as in younger times. 
9. Substantial flow of fluids accompanied by exchange of components can occur in  
anorthosites a t  high grades of metamorphism with little more effect on the plagioclase 
than formation of amphibole inclusions and scattered recrystallization. 
I n  summary, megacrystic anorthosites and basalts can occur i n  a variety of geologic 
settings and by themselves are not definitive. Only with additional field, petrologic and 
geochemical data can the settings be understood. 
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Tectonic implications of Archean anorthosite occurrences

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