Alternate Author Name(s)

Vicente Mendoza Sánchez

Document Type

Dissertation

Date of Award

1974

Keywords

Geology, Venezuela, Suapure River Area, Petrology

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

Thomas W. Donnelly

Second Advisor

William D. MacDonald

Third Advisor

Marc W. Bodine, Jr.

Abstract

In the Suapure River area, NW Guiana Shield (Venezuela), crops out the Cedeño Supergroup which is a Precambrian lithologic volcanic-plutonic unit that includes the Cuchivero Group (Caicara Formation rhyolitic rocks, Santa Rosalía biotite granites and San Pedro leucogranites) and the Suapure Group, (Pijiguao leucogranites and Parguaza rapakivi granites). Radiometric ages by Rb-Sr whole rock isochron method (Hurley and others, 1973) for these rocks are as follows: 1490 m.y. for Parguaza rapakivis; 1875 m.y. for Santa Rosalía and San Pedro granites; and >1750 m.y. for Caicara rhyolitic rocks. Ortho-quartzites of Roraima Group and Cinaruco Formation unconformably overlie the Parguaza rapakivis.

The following features are shown by whole rock chemical analyses, biotites chemical analyses, trace element analyses and Sr87/Sr86 initial ratios:

a) High Si02, Na20; high oxidation ratios; Mg-rich biotites; and low Ni content were found in the Caicara-Santa Rosalía granitic suite, suggesting the calc-alkali chemical affinities.

b) High Fe0, Ti02; low Si02, Na20; low oxidation ratios; Fe0-rich biotites; and high Ni contents in Parguaza rapakivis indicates that these rocks show tholeiitic iron-rich chemical affinities.

c) Criteria for strong fractionation (e.g., high Rb, low K/Rb and Ba/Rb ratios; decrease of K/Rb and Na/K ratios with increasing silica) were observed in the Parguaza rapakivis, but not in the Caicara-Santa Rosalía suite.

d) High Fe0 (in whole rock and in biotites) and high Ni contents in Parguaza rapakivis suggest derivations of these rocks from a more basic and relatively “dry” magma. Low Fe0 (in whole rock and in biotites) and low Ni content in the Caicara-Santa Rosalía suite implies derivation of these rocks from a relatively more “hydrous” and intermediate magma.

e) Sr87/Sr86 initial ratios are relatively low for Parguaza rapakivis (0.703) and a little high for Caicara Santa Rosalía suite (0.706 - 0.705, respectively) as compared with basaltic mantle-derived rocks.

This data is discussed in the light of the present ideas about the generation of granitic magmas. It is believed that the Caicara-Santa Rosalía suite was derived from a parent magma produced by partial melting of older crustal rocks of the Guiana Shield (amphibolites and Na-rich granites of the Pastora Supergroup) or by a mixture of crustal rocks with mantle-derived melts. Parguaza rapakivis could have originated from lower “dry” crustal rocks (charnockites of the Imataca Complex) or by a mixture of charnockitic lower crustal material with basaltic mantle-derived melts. Heat from an upwelling zone in the upper mantle must have been necessary to melt the parent sources of granitic rocks of the Cedeño Supergroup, because such rocks are not related to either geosynclinals-anatexis orogenic cycle or to subduction processes.

Structurally, this area represents a Precambrian complex in which foliated Caicara rhyolites and massive to weakly foliated San Pedro and Santa Rosalía granites are in fault and intrusive contact with massive, non-tectonic Parguaza rapakivi granites. The Caicara rhyolites, Santa Rosalía and San Pedro granites show foliation trends (N 20-40 W) which are the same as the regional foliation trends of the Amazonas structural province. These foliation trends were probably developed during the Transamazonian orogeny (1.7 - 1.9 b.y.). The Caicara rhyolitic rocks also show a second foliation trend (N 20 E), which seems to be related to local faulting, or to a younger orogeny (Nickerian orogeny, 1.2 - 0.8 b.y.).

Regional metamorphism in the area was of minor importance and only reached the greenschists facies in the Cuchivero Group.

However, no specific hypothesis concerning the origin of granitic rocks of the NW Guiana Shield can be advanced, until more is known about the geochemistry of the probable source rocks (Imataca Complex and Pastora Supergroup), the results are consistent with the view that the Guiana Shield did not grow concentrically by sialic accretion (by tectonic causes), as postulated for other shields (Canada, Africa), but by lateral aggregations of granitic rocks, in part related to the weakly tectonic and strongly magmatic Transamazonian event (Cuchivero Group), and in part not related to any tectonic event (Parguaza rapakivi granites).

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