Petrology and Stratigraphy of the Fra Mauro Formation at the Apollo 14 Site GEOLOGICAL SURVEY PROFESSIONAL PAPER 785 Prepared on behalf of the National Aeronautics and Space Administration Petrology and Stratigraphy of the Fra Mauro Formation at the Apollo 14 Site By H. G. WILSHIRE and E. D. JACKSON GEOLOGICAL SURVEY PROFESSIONAL PAPER 785 Prepared on behalf of the National Aeronautics ancl SjJace Acl1ninistration Dish·ibution of Apollo 1-1 fragn1ental rocks, classified into four groups, suggests that the Fra Mau.ro Fonnation consists of sh·atifiecl ejecta UNITED STATES GOVERNl\1ENT PRINTING OFFICE, WASHINGTON 1972 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 72-600103 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Page Abstract ____________________________________________________________________________________ _ 1 Thin-section petrology- Continued Introduction ______________________________________________________________________________ _ 1 Matrix materials _____________________________________________________________ _ 15 Returned samples ____________________________________________________________________ _ 2 Matrix-clast reactions ____________________________________________________ _ 16 Megascopic structures ____________________________________________________________ _ 4 Partial fusion ___________________________________________________________________ _ 17 Hand-specimen petrology _______________________________________________________ _ 4 Deformation of mineral and lithic clasts ____________________ _ 17 Homogeneous crystalline rocks _____________________________________ _ 5 Compound clasts---·-------------------------------------------------------··-· 17 Fragmental rocks ____________________________________________________________ _ 5 Classification of the rocks ______________________________________________________ _ 19 Thin-section petrology ____________________________________________________________ _ 6 Rock distribution _____________________________________________________________________ _ 21 Glass clasts _______________________________________________________________________ _ 7 Fra Mauro Formation ____________________________________________________________ _ 22 Mineral clasts ___________________________________________________________________ _ 7 Conclusions _______________________________________________________________________________ _ 23 Lithic clasts ______________________________________________________________________ _ 9 Fra Mauro Formation ____________________________________________________ _ 23 Igneous rocks ____________________________________________________________ _ 9 Pre-Imbrium terrane ______________________________________________________ _ 24 Metamorphic rocks __________________________________________________ _ 11 References cited .. ·----·--··-·········------------······-······························· 25 ILLUSTRATIONS Page FIGURE 1. Regional geologic map of the area surrounding the Apollo 14 landing site __________________________________________________________ _ 2 2. Map of major geologic features in the Apollo 14 traverse area ________________________________________________________________________________ _ 3 3. Surface photograph and photomicrographs of Apollo 14 rocks _________________________________________________________________________________ _ 4 4. Scheme used for subdivision of Apollo 14 rocks ... ----------------------------------------------------------------------------------------------·-··--------· 5 5. Frequency distribution of clasts >1 mm in fragmental rocks of groups F 1, F::?, F3, and F4------------------------------- 6 6. Histograms showing relative abundance of glass, mineral, igneous, and metamorphic clasts and microclasts in the fundamental rocks of groups F 1, F 2 , F 3, and F4--------------------------------------------------------- 8 7. Triangular plot of plagioclase-pyroxene-olivine microclasts________________________________________________________________________________________ 9 8-11. Photomicrographs of Apollo 14 rocks·------------------------------------------------------------------------------------------------------------------·10, 13, 14, 18 12. Triangular plots of clasts and microclasts of glass, igneous, and metamorphic rocks___________________________________________ 20 13. Distribution of fragmental rock types at the Apollo 14 landing site_______________________________________________________________________ 22 14. Map showing outer mountain rings of major lunar basins, their relative ages, and the extent of their continuous ejecta blankets--------------------------------------------------------------------------------------------·------·-··-------- 25 TABLES Page TABLE 1. Classification of rocks larger than 1 gram.. -------------·----------------------------------------------------------------------------·--------·--·--·--------·--· 5 2. Principal types of clasts and microclasts---------------------------------------------------------------------------------------------··------····-·········---·-·· 7 3. Clast sizes and grain sizes of some igneous clasts.--------------------------------------------------------------------··------·-··-----·----------------· 9 4. Modal compositions of basalts·------------------------------------------------------------------------------------------------------------------------------------------· 9 5. Types of vitrophyres·------------------------------------------------------------------------------------------------------------·---------------------------------------·-----· 11 6. Modes and average grain sizes of common cumulates·------------------------------------------------------------------------------------------------··· 11 7. Distribution of mineral reactions.-----------------·---------------------------------------------------------------------------·-·------··-----------·-···--··--------· 16 8. Clasts and matrix types in compound clasts·------------------------------------------·-----------------------------------------------------------------------· 19 III PETROLOGY AND STRATIGRAPHY OF THE FRA MAURO FORMATION AT THE APOLLO 14 SITE By H. G. \VILSHIRE and E. D. JACKSON ABSTRACT weakly lithified regolith, derived either from disaggregated The Apollo 14 rocks returned to earth are divisible into two F1 type breccias or from originally unconsolidated material broad groups: relatively homogeneous crystalline rocks and at the top of the Fra Mauro Formation. This stratigraphic fragmental rocks (breccias). The crystalline rocks are, for sequence is consistent with the comparative abundance of the most part, metaclastic, but a few, including the second glass and the weakly developed reequilibration reactions in largest sample returned, are aphyric basalts. All of the crys­ F2 rocks and with the dominance of light-colored metaclastic talline rocks are considered to be clasts dislodged from frag­ rocks among the lithic fragments of the soils. It is presumed mental rocks. These fragmental rocks constitute the Fra that thermal blanketing of the lower dark-clast-dominant Mauro Formation, which is presumed to be the ejecta deposit breccias allowed time for reactions to proceed ami for anneal­ from the Imbrium basin. Megascopic properties of the frag­ ing and devitrification of glass. Thus, although we see that mental rocks suggest a fourfold subdivision: Ft, poorly con­ the Fra Mauro Formation is composed of three or four solidated breccias with light clasts dominant over dark ones, distinct units based on clast populations, its internal thermal with few clasts larger than 1 mm, and with comparatively metamorphism appears to be gradational. abundant fragmental glass; F2 , coherent breccias with light The compound clasts and great variety of lithic frag­ clasts dominant and with comparatively abundant fragmental ments indicate that the pre-Imbrium terrane, presumed to glass; Fs, moderately coherent breccias with dark clasts dom­ have existed prior to about 3.9-4.2 billion years ago, was inant and with no fragmental glass; and F 4 , coherent breccias already exceedingly complex. However, the similarity of with dark clasts dominant and with no fragmental glass. older breccias to the Fra Mauro breccias themselves in­ Systematic microscopic descriptions and clast counts of dicates that processessuch as extrusion of basalt, intrusion 19 different breccias revealed that (1) clasts of fine-grained and differentiation of basaltic magma, impact brecciation, thermally metamorphosed fragmental rock are by far the and metamorphism prior to the Imbrium event were not un­ dominant type in the breccias; (2) there are eight main like later events. The complex clasts of the Fra Mauro varieties of igneous clasts including basalt, anorthosite-gab­ breccias therefore reveal a long history of impact-related bro, plagioclase cumulates, and quartz-alkali feldspar inter­ events that probably record half a billion years of planetary growths; ( 3) a significantly high proportion of mineral accretion, of which the Imbrium impact was one of the last clasts occurs in the size range 0.1-1.0 mm; plagioclase is large events. dominant among the mineral clasts, but the proportion of INTRODUCTION pyroxene is as much as 50 percent; and (4) lithic and mineral clasts commonly are cataclastically deformed. Apollo 14 landed on an area of lunar terra composed Most of the rocks